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Old 03-27-2005, 06:56 PM   #1
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timed vacuum vs full vacuum, what's the difference?

There are two vac ports on the edel 1405/6 and I don't know which to use for my distributor. It's a vac advance, and I'd assume this is "full vac" and I'd assume the mechanical advance with vacuum is "timed"

no idea though.
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Old 03-27-2005, 10:48 PM   #2
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What you are refering to is manifold (full) and ported (timed) vacuum. Here is the difference:

Manifold vacuum- low pressure in the intake manifold below the throttle plates. At idle usually around 17-21 in/hg. As the throttle plates open, vacuum drops. This is the vacuum that is made by the piston moving down in the cylinder on the intake stroke.

Ported Vacuum- This is vacuum that is not present until the throttle is open. This is taken above the throttle plates. THis vacuum increases as throttle opens up. This is usually used to control vacuum advance on Chevy distributors, and various other vacuum controlled devices. Usually not brake boosters cause you will not be opening the throttle plates while applying the brakes.

Not all cars use ported vacuum for advance though. Chevy HEI and such do. So connect it to a vacuum port that is above the throttle plates.

Hope that helps ya out
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Old 03-28-2005, 12:34 AM   #3
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I hate to give you an obvious answer, but do a search. There was a thorough article posted here not to long ago. The only difference between the two is that ported vacuum has no vacuum at idle. Part throttle and beyond they will both function the same. I prefer to use full vacuum for my vacuum advance for a better idle. But try both and see what you like better.
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Old 03-28-2005, 07:31 PM   #4
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I have my HEI hooked up to manifold vacuum. Works great, never had a problem with it. Either one will work, just try both and see which one your engine seems to like.
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Old 03-29-2005, 03:24 PM   #5
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Just a side note... most (all?) pre-emitions high-perf SBC's had their vac advance hooked up to manifold vacuum, not ported vacuum.
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Old 03-29-2005, 08:05 PM   #6
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I seem to recall that I started seeing ported vacuum used on everything right about the same time PCV appeared; maybe 66 or 67 or so. By about 69 it was universal.

Timed vacuum gets its vacuum from right above where the throttle blades are at idle. So there's none there at idle, but whenever the car is in cruise conditions (throttle open a little, light load) there's lots of vacuum on it. In other words, it more nearly responds to the exact set of circumstances that it's intended for: light load, relatively high (significantly off-idle) RPM, somewhat open throttle. In other words, cruise conditions. It enables the engine to burn a leaner mixture more safely when lightly loaded.

A street car will almost always run better with ported (timed) vacuum to the dist. It makes the idle ALOT less sensitive to load conditions, especially in a car with auto trans. It also makes it easier to run plenty of mech advance, without getting that big shot of pinging right off idle that you get with modern watered-down-cat-pee fuel and a timing curve designed for 103 octane REAL GASOLINE.

Once you get past idle and off-idle transition, there's no real difference between the 2 vacuum sources.
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Old 03-31-2005, 01:22 PM   #7
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I just asked mallory because i installed a new unilight distributor and they said to use ported vacuum with it.
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Old 03-31-2005, 04:06 PM   #8
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Use manifold!!!! It will help your idle, your stop light performance, and keep you from overheating as easily. If you use ported make sure you use a timing light and don't time the engine by ear. Doing that will cause bad detonation at part throttle.

My engine idles much better with manifold vacuum because the timing is advanced at idle somewhat and the rpms come up slightly. On ported I get camming idle no matter what and a flat spot out of a light. On manifold the engine was eager to pull at 1,000 rpm and in general ran much better and got better fuel economy.
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Old 03-31-2005, 05:10 PM   #9
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Quote:
Originally posted by Fast355
Use manifold!!!! It will help your idle, your stop light performance, and keep you from overheating as easily....
My engine idles much better with manifold vacuum because the timing is advanced at idle somewhat and the rpms come up slightly...On manifold the engine was eager to pull at 1,000 rpm and in general ran much better and got better fuel economy.
On mine it also brought up idle speed, so I could close the throttle plates some more to keep the transfer slots from being exposed too much. The only time I think you can get into trouble is if you have a rowdy cam that doesn't make much vacuum at idle.
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Old 04-02-2005, 12:17 PM   #10
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Just because something was done in the past doesent make it better.
If you have a high compression engine with a big cam you will notice better idle with full vacuum and have better starting since you will have base timing retarted to run full vacuum. I'd rather have more base timing and let the mechanical and vacuum advance features do thier job.
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Old 04-02-2005, 03:20 PM   #11
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so I should put it to the one under the throttle blades you think?

my buddy said on top of the blades, but hey.
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Old 04-02-2005, 03:38 PM   #12
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Try it one way and see how it behaves. If you don't like it, it takes a minute to change it.
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Old 04-03-2005, 07:24 AM   #13
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What was the original reason behind ported vacuum
to the distributor?

also, am I correct in assuming that, in normal practice,
with manifold vacuum, at idle, the vacuum advance
was at max?

{for 'ported vac' arrangement} I find it hard
to believe that best idle is at 8 degrees of advance.
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Old 04-03-2005, 11:08 AM   #14
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Quote:
Originally posted by contact
What was the original reason behind ported vacuum
to the distributor?

also, am I correct in assuming that, in normal practice,
with manifold vacuum, at idle, the vacuum advance
was at max?

{for 'ported vac' arrangement} I find it hard
to believe that best idle is at 8 degrees of advance.
Until the early 70s their wasn't such a thing as ported vacuum. All vacuum came off the manifold period. It wasn't untill emissions controls that ported vacuum came around. Ever notice how cars diesel, get poorer fuel economy, and lack low speed driveability when on ported vacuum. Manifold vacuum makes the vacuum advance work as intended. When their isn't much of a load on the engine the vacuum is high and timing is advanced. That would be during idle, cruise, and decel (ever notice all the popping some cars with exhaust make during decel on manifold vacuum it is basically eliminated, I had no popping even with glass packs with it on manifold vacuum, care to guess what ported did. BANG, BANG, BANG, etc. At heavy throttle such as accelerating or climbing a hill vacuum is low, the air/fuel is burning much quicker so less advance is needed and the vacuum advance backs down on the timing. Put into contrast ported vacuum. The signal is much stronger off idle and still has a signal most of the time at WOT. Combine that with centrifical advance that advances with engine speed you get way too much advance. Ported vacuum advances the timing momentarily when you romp on the pedal that can cause alot of detonation. With manifold vacuum the timing will back off in response to the load, which is exactly what you want. Also manifold vacuum will change the timing at idle in response to loads that effect the engine, which gives you a more stable steady idle.
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Old 04-03-2005, 08:10 PM   #15
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Quote:
Originally posted by SSC
Just because something was done in the past doesent make it better.
True... all I was saying is that GM used that method, and it worked just fine for many years.
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Old 04-04-2005, 05:36 AM   #16
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Here is a great info on vaccum advance from one of the nova sites I use .. I copied and pasted so you don't have to log in to the site. It came from a corvette site originally, two guys contributed to parts of the article John Z and Lars Grimsrud .

3 parts...

Part 1


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
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Old 04-04-2005, 05:37 AM   #17
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Vacuum can part 1

Vaccum Can info:

Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).
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Old 04-04-2005, 05:38 AM   #18
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Vacuum part 2

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

--------------------------------------------------------------------------------------------------------------------------------------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14

The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8

--------------------------------------------------------------------------------------------------------------------------------------------

VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
1979 305 El Camino Calif.
1978-79 350 Blazer & Suburban
1979 Buick 305/350

VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
1975 454 Caprice, Chevelle, Monte, Suburban

VC1826 AR2 5-7 12 @ 10-13

VC1827 AR3 5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
1976 350 Pontiac

VC1831 AR7 6-8 12 @ 14-16

VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14

VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16

VC1834 AR9 4-6 13 @ 13-16

VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
1977 350 All exc. Calif.
1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305 All Exc. Calif
1976 350 All Exc. Vette, Exc. Calif
1976 350 Vette Exc. Calif., Exc. Hi Perf
1975 262, 350 All w/2-bbl carb
1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
1977 305 Chev Truck Light Duty
1975-76 350 El Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl

VC1839 AR13 4-6 12 @ 11-13

VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14

VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15

VC1844 AR16 3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
1977 301 Pontiac

VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
1977-79 350/403 Pontiac Hi Alt

VC1848 AR18 4-6 12 @ 9-12

VC1849 AR19 4-6 12 @ 7-10

VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11

VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
1978-79 350 Pontiac

VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty

VC1854 AR24 3-5 13 @ 10-13

VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12

VC1856 AR25 3-6 15 @ 10-14

VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
1978 350 Camaro
1978 305 Chev Truck, M/T, Light Duty
1978 350 Chev Truck Hi Alt
1978 305/350 Buick & Olds
1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
1978-79 305 w/1103282 Distr., Incl. El Camino A/T
1979 350 Camaro, Impala, Nova, Malibu, Monte
1979 350 Suburban
1979 350 Buick Century
1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi Alt.

VC1860 AR29 3-6 12 @ 10-13

VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
1979 301 Olds
1978-79 301 Pontiac

VC1863 AR32 2-4 10 @ 11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13

VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15

VC1867 AS9 2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
1979 305 Buick & Olds
1979 305 Pontiac A/T

VC1869 AS10 2-4 12 @ 8-11
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Old 04-04-2005, 11:25 AM   #19
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novass, thanks for posting that stuff.

>is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like).<

Where do those numbers, come from?
My understanding is, that timing is 'set' by:
{ignore the issue of tailpipe emmissions}
at part throttle, advance is set to maximize output,
at WOT, max output {is normally}not gettable, advance is set
at the maximum of tolerable knock.

This may not apply much to carbs, but today,
people with FI, might have some type of
injection...water, methanol, race gas, propane
tolulene, etc, at WOT, so that 'knock issue'
is less of a limitation.

Is 36 degrees, really best for power, always?
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Old 04-04-2005, 01:19 PM   #20
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Quote:
Originally posted by contact

Is 36 degrees, really best for power, always?
I think that you would have to determine what works best for your individual setup, using something like a g-tech etc. Some engines can run more timing, some like the fastburn heads do better with less.

I would think that the 34-36 is a ball park for the standard SBC.
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Old 04-04-2005, 01:46 PM   #21
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Ok I read all of excluding the part numbers.

My engines like ported vacuum. Maybe its an altitude thing but they tend to have better manors with ported. Last time I ran full manifold vacuum for an extended period I had dieseling issues and wasent about to drop my base timing below 8* to eliminate the problem.

I say try both ways see what it does, the guy that wrote the artical wrote it based on his experiance and with mostly stock pre emmision engines with stock timing tables. He writes alot so I wouldent expect more details on the setups he decribes, they sound failry stock to me.

I do think Token's engine with the Comp 305 cam will like full vacuum but It doesent hurt to try both and see which source respoinds better.
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Old 03-06-2013, 05:41 AM   #22
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Re: timed vacuum vs full vacuum, what's the difference?

I don't like the manifold vacuum, it idles smoother and has 2 extra inches of vacuum but i have to close the throttle blades too much this gives me poor of idle performance and as soon ans a little lowd is put on the engine the engine dies or almost die all it takes is the e fan to kick in. I have the vacuum advance limited to 12" cause of the 19* initial advance I have. Ported vacuum gives me a strong loppy idle and great off idle response and no overheating problems
.
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Old 03-06-2013, 08:08 AM   #23
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by novass View Post
I think that you would have to determine what works best for your individual setup, using something like a g-tech etc. Some engines can run more timing, some like the fastburn heads do better with less.

I would think that the 34-36 is a ball park for the standard SBC.
I agree with above. I've dragged raced for many years using only SBC motors and each one reacts differently. Our flat topped piston 388 motor gave us the best runs with only 34* total timing and our domed top piston 355 preferred 36* total timing. I haven't had a motor yet that has ran better down the track with more than 36* total timing.
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Old 03-06-2013, 04:27 PM   #24
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Holy 7 year old thread brought back from the dead, Batman!
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Old 03-06-2013, 06:04 PM   #25
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Re: timed vacuum vs full vacuum, what's the difference?

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Originally Posted by five7kid View Post
Holy 7 year old thread brought back from the dead, Batman!
It has a lot to do with this thread.

http://www.thirdgen.org/techboard/ca...ptoms-but.html (Vapor Lock symptoms but floats full of cool fuel)

I'm still not 100% on board with the full manifold vacuum for the vacuum advance. The flutuations in available idle vacuum with a cam that has significant overlap makes it difficult to establish a consistent and steady idle. That's my experience but that's limited to only about a dozen or so hot rod Gen 1 SBCs.
Interesting bit of info regarding the fuel economy vs maximum available timing though. At cruise rpm, I'm still about 500 rpm or so from the full available mechanical advance. (Cruise at 2500, all in at 3000) and with my vacuum advance all with the available cruise vacuum ( IIRC, 8-10 dist. degrees in by 12-15") I'm leaving a few degrees on the table. I can see with a little tuning, I may be to squeeze out a few more mpg.

Last edited by skinny z; 03-06-2013 at 06:17 PM.
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Old 03-06-2013, 07:37 PM   #26
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Re: timed vacuum vs full vacuum, what's the difference?

Yeah, I dug this thread up over in that thread

I'd have thought that the fluctuations in idle vac present with a lumpy cam - ie the reversion - are themselves responsible for the unsteady idle, with timing being a lesser side casualty when using full vac advance.

You have way more experience than me, which is limited to one sbc with 2 very mild cams, with about 45 and 70 deg overlap iirc. In both cases, the idle actually smoothed out a tiny bit with using full vac, in line with discussions earlier in the thread.

I've found also that vac advance, whatever the source, has a major effect on mpg. I've run with it disconnected for a couple of tanks just to see the effect. No change in performace, but noted about 30% increase in fuel consumption - quite stark. Get as much cruise timing as the motor will tolerate.
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Old 03-06-2013, 08:03 PM   #27
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by TreeFiddy View Post
Yeah, I dug this thread up over in that thread

I'd have thought that the fluctuations in idle vac present with a lumpy cam - ie the reversion - are themselves responsible for the unsteady idle, with timing being a lesser side casualty when using full vac advance.

You have way more experience than me, which is limited to one sbc with 2 very mild cams, with about 45 and 70 deg overlap iirc. In both cases, the idle actually smoothed out a tiny bit with using full vac, in line with discussions earlier in the thread.

I've found also that vac advance, whatever the source, has a major effect on mpg. I've run with it disconnected for a couple of tanks just to see the effect. No change in performace, but noted about 30% increase in fuel consumption - quite stark. Get as much cruise timing as the motor will tolerate.
Thanks for the link to the thread. I was a good read. It's in my library now.
My point regarding the unsteady idle and the full vacuum was more directed to not having the proper vacuum can. With the naturally fluctuating idle from a high overlap cam, having a vacuum can that's moving the timing as the vacuum varies can cause quite the tuning problem. The answer is to have exactly the right spec vacuum can which is probably going to tough one for the average enthusiast. Of course the way around this is to use ported vacuum, lots of initial timing and the proper amount of mechanical advance built into the distributor to satisfy the engine's needs.
This is my experience but as pointed out, it's not everyone's preference.
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Old 03-07-2013, 08:35 AM   #28
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Re: timed vacuum vs full vacuum, what's the difference?

I hear what you're saying - with full manifold advance, a high-reversion cam is going to be jerking the timing around via the fluctuating vacuum. Have you seen this with a timing light, and is it enough to cause unacceptable surging or even stalling?

I did quite a bit of research on full-vs-ported out of interest a few years back. I reached several conclusions based on articles like the GM tech piece above, and lots of anecdotes:

* Full vac should result in better mpg, lower temps, smoother idle, or at worst have no effect on any of these.
* Ported vac was an early emission reduction tactic, as outline above.
* What works best for one motor is unlikely to be best for the next.
* For some reason, it's one of those highly emotive topics with no real absolute correct answer, like GM vs Ford type arguements.
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Old 03-07-2013, 11:14 AM   #29
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by TreeFiddy View Post
I hear what you're saying - with full manifold advance, a high-reversion cam is going to be jerking the timing around via the fluctuating vacuum. Have you seen this with a timing light, and is it enough to cause unacceptable surging or even stalling?

I haven't tried full manifold vacuum for any of my engines in quite a while. I can say that my idle vacuum is around 11" at 750 rpm. The 59 degrees of overlap (measured at .050" lift) doesn't put too much strain on vacuum production and as such my vacuum is relatively steady. Maybe a 1/2" fluctuation. Having said that, if full manifold vacuum was selected and the proper advance can installed, I can't see how the timing would bounce appreciably if the can is all in by 9" (in my case). It would be a case of reducing the initial timing to offest the amount added by the vacuum advance. Using ported vacuum, I don't have that concern.
Perhaps this engine would enjoy more advnace at idle however with 14-16 initial (and 200 psi of cranking pressure) it seems quite happy.

I did quite a bit of research on full-vs-ported out of interest a few years back. I reached several conclusions based on articles like the GM tech piece above, and lots of anecdotes:

* Full vac should result in better mpg, lower temps, smoother idle, or at worst have no effect on any of these.

Regarding the mpg, it should be remembered that vacuum advance, ported or otherwise will still (or should be ) all in at cruise. I made a note to check that that's the case in my application as I know that I don't have full mechanical advance at cruise rpm which is around 2500. Vacuum at cruise is approaching 20 degrees so I'm reasonably sure that I'm getting the full amount however I need to double check. Presently I'm 20+ mpg hiway however the 350 previous to this, with roughly the same cam and heads was getting slightly better.

* Ported vac was an early emission reduction tactic, as outline above.

So was an 8:1 compression ratio.

* What works best for one motor is unlikely to be best for the next.

I agree with that whole heartedly.


* For some reason, it's one of those highly emotive topics with no real absolute correct answer, like GM vs Ford type arguements.

It'll be interesting to see if the resurrection of this thread elicits any of that emotion.
.

Last edited by skinny z; 03-07-2013 at 04:12 PM.
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Old 03-07-2013, 11:39 AM   #30
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by skinny z View Post

It'll be inteeresting to see if the resurrection of this thread elicits any of that emotion
.
Heh, I think Camarito's already leading the charge for ported in the other thread
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Old 03-08-2013, 09:34 AM   #31
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Re: Vacuum can part 1

Quote:
Originally Posted by TreeFiddy View Post
I'd have thought that the fluctuations in idle vac present with a lumpy cam - ie the reversion - are themselves responsible for the unsteady idle, with timing being a lesser side casualty when using full vac advance.
That's the reason for the authors statements below. You want to select a full in advanced vacuum level below the choppy vacuum. Example, if the vacuum advance is full in at 10"Hg it doesn't matter if the manifold vacuum is jumping from 12" to 14"Hg because the pot is still able to hold the minimum of 10" it needs to stay pulled in. This is a good argument for getting an adjustable vacuum advance pot.

Quote:
Originally Posted by novass View Post
How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm.

Last edited by bestracing; 03-08-2013 at 09:49 AM.
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Old 03-08-2013, 01:20 PM   #32
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Re: timed vacuum vs full vacuum, what's the difference?

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Heh, I think Camarito's already leading the charge for ported in the other thread
lol
I just don't get it why i need 31 degrees advance at 750 rpm with the same afr i have at 6500.
The minimum AFR i can get away with at idle with 17" manifold vacuum (31* advance VA limited to 12* in my case) is 12.5:1 any less fuel and the engine dies as soon as i put it in drive, if i add more air it idles at over 1k, now if i use 15" ported vacuum (19* advance in my case) it runs fine at 13.5:1, it might not be a problem with a manual transmission or a stock cam but an automatic the TC puts a load on the engine, at 31* advance the engine is not making enough torque to handle the TC load plus the Mark VIII fan (why the lower temperatures) because the mixture it's being ignited way before the right time...
This is what i came to conclude after a few attempts and tests with my setup to run full manifold vacuum
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Old 03-08-2013, 10:37 PM   #33
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by bestracing View Post
This is a good argument for getting an adjustable vacuum advance pot.
That's why I did . Its also required to dial out any pinging you might get with the extra low rpm timing at throttle tip-in.

Quote:
Originally Posted by camarito View Post
lol
This is what i came to conclude after a few attempts and tests with my setup to run full manifold vacuum
Fair enough - this is what I concluded from reading - different engines are going to like or dislike different setups to various degrees. It's just a simple thing to pick one or the other, so just go with what your motor runs best with. Matter of personal preference - in my case, the pro's came down on the side of full vac. You and Skinny have found the opposite.
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Old 03-09-2013, 08:39 AM   #34
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Re: timed vacuum vs full vacuum, what's the difference?

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Originally Posted by camarito View Post
I just don't get it why i need 31 degrees advance at 750 rpm with the same afr i have at 6500.
Advance is about how long the mixture takes to burn, and AFR is only a very small part of that.
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Old 03-09-2013, 03:13 PM   #35
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Re: timed vacuum vs full vacuum, what's the difference?

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Advance is about how long the mixture takes to burn, and AFR is only a very small part of that.
Yes but since leaner mixture takes longer to burn and in my case the afr at idle is the same as wot, at idle the piston travels at much lower speed giving the mixture plenty of time to burn with half the advance you need at 6500rpm...
if the mixture was being ignited at the right moment at idle with 31* it should be able to handle the load of the converter and electric fan at 750rpm and that's my problem, it doesn't, with 19* it only drops 200rpm and holds fine, I've had this engine for 3 years now and the carb/distributor in every configuration possible (i like to mess with that) i drive it at least once a week to work (160 miles round trip) lot's of testing done...lol, and it's been the same since the beginning, if i was doing something wrong i would have figure it out by now...i think...
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Old 03-09-2013, 03:26 PM   #36
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Re: timed vacuum vs full vacuum, what's the difference?

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Usually not brake boosters cause you will not be opening the throttle plates while applying the brakes.
Why not? it's fun...lol
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Old 03-11-2013, 07:46 AM   #37
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Re: timed vacuum vs full vacuum, what's the difference?

I've had some slight problems with throttle tip in when I first installed my 570 carb on my motor and with 8* initial timing I could not get the idle up fast enough while keeping the slot from getting exposed more then 0.040". I know what my problem is and I'm just getting around to fix it. With racing I didn't worry about vacuum advance, just mechanical advance so now I get to play with the vacuum portion a lot more.
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Old 03-11-2013, 01:41 PM   #38
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Re: timed vacuum vs full vacuum, what's the difference?

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Yes but since leaner mixture takes longer to burn and in my case the afr at idle is the same as wot, at idle the piston travels at much lower speed giving the mixture plenty of time to burn with half the advance you need at 6500rpm...
At idle there isn't the turbulence in the chamber that there is at speed.
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Old 03-11-2013, 07:26 PM   #39
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Re: timed vacuum vs full vacuum, what's the difference?

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At idle there isn't the turbulence in the chamber that there is at speed.
Exactely, the turbulence produced by the 0.038 quench my engine have i can bump the advance to 39* total (93 octane) with no detonation but this is not a detonation case it's a peak pressure case.
"The period between the spark firing and the complete combustion of the fuel/air mix is very short - on average only about 2 milliseconds. Ignition of the fuel/air mix must take place sufficiently early for the peak pressure caused by the combustion to occur just as the piston has passed Top Dead Centre, and so is on its way down the cylinder bore. If the ignition occurs a little too early, the piston will be slowed in its upward movement, and if it occurs too late then the piston will already be moving downwards, so reducing the work done on it."
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Old 03-11-2013, 11:31 PM   #40
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Re: timed vacuum vs full vacuum, what's the difference?

From Taylor, Vol 2, Ch 1:

Quote:
...the variation in crank angle occupied by flame travel as engine speed changes by a factor of nearly 3:1 is very small. This means that flame speed must increase nearly in proportion to engine speed. As a matter of experience, the highest-speed engines require only slightly greater spark advance than those running at normal or even quite low speed, and even this is mostly due to the increase in combustion time before 10 per cent flame travel.

The increase of flame speed with increasing engine speed is due to the marked effect of turbulence, as has been noted in many bomb experiments and in engines where the turbulence was varied independently of speed...

The nearly constant crank angle occupied by the major portion of combustion when speed is varied is one of the most important facts in relation to the question of combustion time. Without this characteristic, spark-ignition engines could not run at the very high piston speeds used in some present-day engines.
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Old 03-12-2013, 07:29 AM   #41
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Re: timed vacuum vs full vacuum, what's the difference?

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Originally Posted by five7kid View Post
Holy 7 year old thread brought back from the dead, Batman!
It may be an oldie but it's a goodie.
Interesting information and a good read.
Certainly it builds a case for another layer of tuning when the new season arrives.
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Old 03-12-2013, 12:22 PM   #42
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Re: timed vacuum vs full vacuum, what's the difference?

One thing I have seen with dealing with LS1 and LS6 motors is that one of the differences in idle tuning is that the LS6 has more timing advance than the smaller cam LS1. A guy I know built up his LS1 and had some tuning issues with his AFR readings and idle surge. After downloading an LS6 tune he looked up the differences between the two and found that advancing his timing at idle corrected the majority of his problems and became more driver friendly.

Now I know that this is EFI vs Carb and GenI vs GenIII but the principle is still the same when it comes to spark advance. Just one uses electronics to control it and the other uses mechanical and vacuum to control it.

Here's all the things you can adjust on a distributor:

Weights and springs for when mechanical advance comes in and how fast is comes in.
How much mechanical timing advance is available
Adjustable vacuum on vacuum pot, to what vacuum level the pot is full advanced.
Vacuum pots with different amounts of advance available
Initial timing or base timing.

Lots of playing around that can be done here to fine tune your ignition system.
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Old 03-12-2013, 12:38 PM   #43
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Re: timed vacuum vs full vacuum, what's the difference?

Should also mention that cylinder filling isn't nearly as complete at idle with the throttle closed as it is at WOT at high speed.
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Old 07-20-2013, 07:46 AM   #44
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Re: timed vacuum vs full vacuum, what's the difference?

I thought I'd revisit this thread as I'm deep into the ignition and carb tune on my new engine (355). By new I mean everything from the carb (750 Speed Demon), heads (RHS Pro Torker Vortec), cam (custom grind along the lines of an XR276HR but with XFI lobes) and compression ratio (about 10.25:1 with iron heads).
I've always been a staunch believer in ported vacuum as a vacuum advance source. I've had excellent results both from an idle perspective as well as cruise and mpg. However with some of the difficulty I'm having getting to where I want to be I'll be experimenting with full manifold vacuum.
My MSD distributor has been outfitted with a fully adjustable vacuum can both for timing of the advance as well as amount. I've come across a couple of well written articles on ignition timing. I think they're well written anyway.

http://www.hotrodders.com/forum/dist...1-a-59033.html

http://classicinlines.com/spark.asp

Maybe others will find this a helpful read.
I might be struggling more with the carb tune than I am ignition as this Speed Demon carb, despite being outfitted with replaceable IFR, IAB and HSAB jets, the carb may simply be a little on the large size for my application (if the Barry Grant/Demon Carbs literature has any real meaning other than advertising). That said, perhaps a change in the overall idle advance, which is the significant difference between ported and manifold vacuum advance sources, might give me some freedom in the areas of the carb tune I'm working through.

Last edited by skinny z; 07-20-2013 at 07:53 AM.
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Old 08-01-2013, 12:15 PM   #45
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Re: timed vacuum vs full vacuum, what's the difference?

Skinny -

I have a 650 Speed Demon carb, and I've NEVER gotten it right from the git-go. I'm no carb expert by any means, but I've experimented enough with all the more common adjustments (jet changes, PV changes, timed vs. full vacuum, adj. can etc.) and none seem to keep my car from idling pig rich.

The biggest difference I have from most others here is that the carb is sitting on top of a small roots blower (Magna-charger MP-122) - but I'm only pushing 5 pounds of boost at the moment at WOT.

My issue was mentioned by someone else earlier in this post...I've tried the ported or "timed" vacuum source for a while on mine, and even though I can set my timing at around 16-19 degrees advanced at idle (runs smoother there) at a reasonable rpm (around 800) it still idles rich and if I try to lean it out using the 4 corner adjustments it'll stall out before it leans out.

I end up using a vacuum gage and adjusting each screw in until I get highest vacuum - and of course the rpm increases way above 1K rpm's. Then the ONLY way to bring it down is to adjust the primary and secondary blades down - which screws up the transfer slot adjustment. Then it's running pig rich again.

Option 2 is to back down on timing, which causes the engine to run hotter and overheat. I can't win on ported vacuum.

So - I turn to full vacuum....if I have my advance at the same 18-19 degrees advanced that I used before and then connect the advance can to full vacuum, the timing shoots up to around 38 degrees advanced and idle shoots up to 1500 plus rpms if I have the transfer slots set properly (I see a square).

So in this case, I end up backing the timing down (with hose disconnected) to about 12-14 degrees and when I plug the hose back on, I'm still idling way high (about 1400 rpms).....and my timing is still around 35 degrees advanced....leaning it out again using the 4 screws makes it idle even higher and I end up doing the same thing...backing the transfer slots (closing the blades) down until I can get it to idle still higher than I want it (about 1100 rpms)....

BUT - using full manifold vacuum, the off-idle response is much stronger and it runs overall much better than timed vacuum. It runs much cooler too.

The only next step I can think of doing is to back off on my adjustable vacuum can to get the thing to idle down and allow me to set the idle down below 1100 and lean it out a bit...but I think that's going to effect negatively while cruising at part throttle..and I bet it will still idle pig rich...

I had the car on a Mustang Dyno, and had the jetting set to maintain 12:1 at WOT...and that's where everyone says it should be. I know roots blown motors like a lot of advance at idle....and I think this is making it that much more difficult to at idle and cruise. I've installed a bushing in my mechanical advance to limit it's total to 12 degrees...otherwise I'd end up well over the 32-34 total mechanical when I set my initial advance at 18-19 degrees.

I've been told in the past that you need to be more conservative on carb size with Demon carbs...i.e. if you would normally run a 750 Holley carb on it, you should pick a 650 instead if it's a Demon carb. Of course that may be just hear-say because I've yet to hear an explanation as to why.

Guess I'm just sharing my own frustrations because I want you to know you're not alone....

Last edited by Confuzed1; 08-01-2013 at 12:19 PM.
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Old 08-01-2013, 12:26 PM   #46
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Re: timed vacuum vs full vacuum, what's the difference?

The Speed Demon 650 is set to 12:1 all over ther pm range to lean down the idle without driveability problems you have to mess with the idle feed restrictors, they are not replaceable so I used some thin cooper wire to restrict the fuel passage at idle, ended up with 13.5:1 idle in park and 14:1 in drive runs great
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Old 08-01-2013, 01:36 PM   #47
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by Confuzed1 View Post
Skinny -

I have a 650 Speed Demon carb, and I've NEVER gotten it right from the git-go. I'm no carb expert by any means, but I've experimented enough with all the more common adjustments (jet changes, PV changes, timed vs. full vacuum, adj. can etc.) and none seem to keep my car from idling pig rich.

The biggest difference I have from most others here is that the carb is sitting on top of a small roots blower (Magna-charger MP-122) - but I'm only pushing 5 pounds of boost at the moment at WOT.

My issue was mentioned by someone else earlier in this post...I've tried the ported or "timed" vacuum source for a while on mine, and even though I can set my timing at around 16-19 degrees advanced at idle (runs smoother there) at a reasonable rpm (around 800) it still idles rich and if I try to lean it out using the 4 corner adjustments it'll stall out before it leans out.

I end up using a vacuum gage and adjusting each screw in until I get highest vacuum - and of course the rpm increases way above 1K rpm's. Then the ONLY way to bring it down is to adjust the primary and secondary blades down - which screws up the transfer slot adjustment. Then it's running pig rich again.

Option 2 is to back down on timing, which causes the engine to run hotter and overheat. I can't win on ported vacuum.

So - I turn to full vacuum....if I have my advance at the same 18-19 degrees advanced that I used before and then connect the advance can to full vacuum, the timing shoots up to around 38 degrees advanced and idle shoots up to 1500 plus rpms if I have the transfer slots set properly (I see a square).

So in this case, I end up backing the timing down (with hose disconnected) to about 12-14 degrees and when I plug the hose back on, I'm still idling way high (about 1400 rpms).....and my timing is still around 35 degrees advanced....leaning it out again using the 4 screws makes it idle even higher and I end up doing the same thing...backing the transfer slots (closing the blades) down until I can get it to idle still higher than I want it (about 1100 rpms)....

BUT - using full manifold vacuum, the off-idle response is much stronger and it runs overall much better than timed vacuum. It runs much cooler too.

The only next step I can think of doing is to back off on my adjustable vacuum can to get the thing to idle down and allow me to set the idle down below 1100 and lean it out a bit...but I think that's going to effect negatively while cruising at part throttle..and I bet it will still idle pig rich...

I had the car on a Mustang Dyno, and had the jetting set to maintain 12:1 at WOT...and that's where everyone says it should be. I know roots blown motors like a lot of advance at idle....and I think this is making it that much more difficult to at idle and cruise. I've installed a bushing in my mechanical advance to limit it's total to 12 degrees...otherwise I'd end up well over the 32-34 total mechanical when I set my initial advance at 18-19 degrees.

I've been told in the past that you need to be more conservative on carb size with Demon carbs...i.e. if you would normally run a 750 Holley carb on it, you should pick a 650 instead if it's a Demon carb. Of course that may be just hear-say because I've yet to hear an explanation as to why.

Guess I'm just sharing my own frustrations because I want you to know you're not alone....
The idle transfer slot thing is more for the front throttle blades so you have good off throttle response and to also make sure it's not open so much it's not idling on the idle circuit.
You can close the front blades a little bit to fine tune the idle just not to much. The back transfer slots aren't as important and you can use the secondary blade stop screw ( or get one of these to make it easier http://www.quickfueltechnology.com/t...m-bracket.html )to lower the idle down. It's not as big of a deal if the rear transfer slot is smaller than a square. Just use the rear to lower the idle.

And I agree about the demon carbs. I have never had good luck with them.
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Old 08-01-2013, 01:39 PM   #48
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Re: timed vacuum vs full vacuum, what's the difference?

Quote:
Originally Posted by Confuzed1 View Post
Skinny -

I have a 650 Speed Demon carb, and I've NEVER gotten it right from the git-go. I'm no carb expert by any means, but I've experimented enough with all the more common adjustments (jet changes, PV changes, timed vs. full vacuum, adj. can etc.) and none seem to keep my car from idling pig rich.

The biggest difference I have from most others here is that the carb is sitting on top of a small roots blower (Magna-charger MP-122) - but I'm only pushing 5 pounds of boost at the moment at WOT.

My issue was mentioned by someone else earlier in this post...I've tried the ported or "timed" vacuum source for a while on mine, and even though I can set my timing at around 16-19 degrees advanced at idle (runs smoother there) at a reasonable rpm (around 800) it still idles rich and if I try to lean it out using the 4 corner adjustments it'll stall out before it leans out.

I end up using a vacuum gage and adjusting each screw in until I get highest vacuum - and of course the rpm increases way above 1K rpm's. Then the ONLY way to bring it down is to adjust the primary and secondary blades down - which screws up the transfer slot adjustment. Then it's running pig rich again.

Option 2 is to back down on timing, which causes the engine to run hotter and overheat. I can't win on ported vacuum.

So - I turn to full vacuum....if I have my advance at the same 18-19 degrees advanced that I used before and then connect the advance can to full vacuum, the timing shoots up to around 38 degrees advanced and idle shoots up to 1500 plus rpms if I have the transfer slots set properly (I see a square).

So in this case, I end up backing the timing down (with hose disconnected) to about 12-14 degrees and when I plug the hose back on, I'm still idling way high (about 1400 rpms).....and my timing is still around 35 degrees advanced....leaning it out again using the 4 screws makes it idle even higher and I end up doing the same thing...backing the transfer slots (closing the blades) down until I can get it to idle still higher than I want it (about 1100 rpms)....

BUT - using full manifold vacuum, the off-idle response is much stronger and it runs overall much better than timed vacuum. It runs much cooler too.

The only next step I can think of doing is to back off on my adjustable vacuum can to get the thing to idle down and allow me to set the idle down below 1100 and lean it out a bit...but I think that's going to effect negatively while cruising at part throttle..and I bet it will still idle pig rich...

I had the car on a Mustang Dyno, and had the jetting set to maintain 12:1 at WOT...and that's where everyone says it should be. I know roots blown motors like a lot of advance at idle....and I think this is making it that much more difficult to at idle and cruise. I've installed a bushing in my mechanical advance to limit it's total to 12 degrees...otherwise I'd end up well over the 32-34 total mechanical when I set my initial advance at 18-19 degrees.

I've been told in the past that you need to be more conservative on carb size with Demon carbs...i.e. if you would normally run a 750 Holley carb on it, you should pick a 650 instead if it's a Demon carb. Of course that may be just hear-say because I've yet to hear an explanation as to why.

Guess I'm just sharing my own frustrations because I want you to know you're not alone....
Thanks for that Confuzed. Tuning a carb properly and THOROUGHLY takes enough time and I can't imagine compounding the issue with a supercharger (although I'd like to).
My last carb was a 670 Holley Street Avenger. With the PVCR enlarged to .059", I could keep it lean everywhere in the fuel curve with respect to idle and cruise but it would fly under part throttle once the PV kicked. I had also drilled the primary butterflies about 3/32" which really helped the idle. It was an excellent carb.
I mentioned earlier about how BG sizes their carbs a different way and yes, I feel that I could benefit from a smaller unit but I made my bed and now I have to sleep in it.
I've always been a ported vacuum guy only because I've had good luck using it. My hiway mileage is pretty good at 20+ mpg. My past experiences with full manifold vacuum have resulted in unstable idle characteristics mostly regarding how the rpms would hang up as I coasted from crusie and waited at a stoplight. I can see how I can work on that with an fully adjustable vacuum unit and some test equipment.
Thanks again for the post.
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Old 08-01-2013, 01:42 PM   #49
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Re: timed vacuum vs full vacuum, what's the difference?

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The Speed Demon 650 is set to 12:1 all over ther pm range to lean down the idle without driveability problems you have to mess with the idle feed restrictors, they are not replaceable so I used some thin cooper wire to restrict the fuel passage at idle, ended up with 13.5:1 idle in park and 14:1 in drive runs great
I've modified mine with adjustable IFRs, HSABs and IABs. I should be able to duplicate a test like yours without any trouble. I've already spent hours doing an earlier tune with the bleeds but I've made a few changes since and will to do it all again.
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Old 08-01-2013, 01:45 PM   #50
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Re: timed vacuum vs full vacuum, what's the difference?

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The idle transfer slot thing is more for the front throttle blades so you have good off throttle response and to also make sure it's not open so much it's not idling on the idle circuit.
You can close the front blades a little bit to fine tune the idle just not to much. The back transfer slots aren't as important and you can use the secondary blade stop screw ( or get one of these to make it easier http://www.quickfueltechnology.com/t...m-bracket.html )to lower the idle down. It's not as big of a deal if the rear transfer slot is smaller than a square. Just use the rear to lower the idle.

And I agree about the demon carbs. I have never had good luck with them.
The Demon instructions show different settings for the secondary slot for cars that idle above 1000 rpm and those that idle below. I've tried both. Yes, it obviously affects both idle speed and mixture.
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