ok, i know the basics.

the idle of my motor i set at about 700rpm, i know how to set the base timing, but im not sure what to set it AT.

also what is this centrigual timing, what is it and how and what should i set it at?

and how do i set vacuum advance?

 

First off its "cenrifugal" timing or more commonly known as mechanical advance. To tinker with this you will need to go out and pick up a relatively cheap kit with different sized weights and springs.

For the vacume advance the stock GM ones are not adjustable and usually give way too much advance so once again you will need to go pick up an aftermarket adjustable one.

Still running a cc dizzy?

 

aren't the GM units a factory retard system rather then a rather then advance system?

it seems most the units I found for the vac can work off ported vacuum so when the motor is off or under light load there is no vacuum going to the vac can but once you open the throttle a little more it lets some vacuum hit the can and then it retards the timing from there.

 

i have an aftermarket HEI distributor, its a procomp, its brand new.

 

A distributor has vacuum advance. That is, if you disconnect it, the distributor will behave as if there is no vacuum advance, and the timing will not be advanced under the conditions that the vacuum advance advances it under.

Its purpose is to start the burn earlier when the mixture is very lean and relatively "thin" and at relatively high speeds; i.e. while cruising down the highway, with high vacuum and low load.

There is exactly one correct setting for your car's timing. It's exactly the same as every other car's optimum setting. That setting is:

Wherever it runs the best

About all that light is good for on a modified engine, is putting it back there, once you find it, if you have to disturb it.

No 2 engines will want the same timing curve. Finding that curve is part of what we call "tuning". People with computer controls do all of that with the programming; before computers were used, it was all accomplished mechanically somehow. You, the builder, are obligated to match the timing to the engine's use; different gears, car weights, cams, compression ratio, converter, fuel, weather, altitude, driver preference, and on and on, all affect the "ideal" setting. The notion that it's something you look up in a book and set to the "magic number" on the page, is wrong. For a modified motor, that is.

A typical small block will want about 36 degrees of "total" timing; which is, static (what you set by twiddling the distributor), plus the centrifugal (the RPM dependent part), without vacuum advance. Vacuum advance is not active while accelerating or otherwise under high load anyway, only while cruising, at low load. A typical timing curve for a warmed-over street engine would be around 12-14 degrees at idle; then starting at about 1200 RPM, it begins to advance (this is the centrifugal part, controlled by the springs and weights) until it reaches 36 degrees, which should be set to occur at about 2500-2800 RPM; and then, there should be about 12-15 degrees of vacuum advance on top of that, which should only be active when the throttle is off idle, and of course only when the vacuum is high, above 15" or so. This is a rough outline, your exact optimum curve will vary, but that will start you down the right path.

 

alright that helped.

just to get the motor started the timing is at like 28 for base timing, way to fast im my opinion, ill drop it down to 12-14 like you said. onc ethe base timing is set, how do i set the vacuum advance, and know what it is at? take off the cap and adjust the vacuum advance piece itself?

any help of that would be great

 

thats not always the case
yes if I disconnect the vacuum then the timing doesn't change all it works off of then is mechanical advanve but not how you say it will.

it all depends on how the vacuum can is setup.
some go to ported vacuum some go to manifold vacuum. where does yours hook up?
manifold vacuum sees highest vacuum under no load and the vacuum goes down as the load goes up.
what happens in this situation is at high vacuum it pulls on the vacuum can advancing timing. when the load goes up vacuum goes down and it doesn't pull on the vacuum can anymore therefore the timing goes back to it's base setting.


on many cars as far as I understand they run to ported vacuum.
it works a little differently. under no load there is no vacuum so no pull on the vacuum can. when the load goes up there is pull on the vacuum can only thing is this pull RETARDS the timings. under this one there is no advance really at the vacuum cans nuetral setting it is at max timing. so pulling the vacuum line off at idle or low load conditions should make much differenece as long as you plug the line and not make a vacuum leak.

just out of curiousity where does your vacuum can hook up? ported or manifold. if it's ported you have more of a vacuum retard

 

umm im not sure what you mean, where does the hose go from the vacuum advance on the dizzy? it goes to the carburetor

 

No, it doesn't depend on where it hooks up to.

The application of vacuum to the can in the distributor ALWAYS causes it to ADVANCE. The only thing you change by going from ported to manifold or vice-versa, is WHEN it advances; specifically, hard manifold vacuum will advance it at idle, while ported will not. Both will cause advance during the conditions under which advance is desirable; i.e. high-speed, low load operation, like cruising down the highway.

Some carbs have 2 ports, for example the Edelbrock Carters, where you get to choose which one you want. It's not impossible that your car will run WAY better with one than the other, or run like crap with one. Most carbs only have one, ported; if you want manifpld vacuum on those, you just hook the dist to the manifold instead of the carb.

Try "ported" first. Most likely, that's what your sitributor is set up for.

There is no such thing as "vacuum retard". Except of course, for people like me. That's what I am one of. But not on cars.

 

ok so lets look at my car then you say under vacuum ALWAYS causes the vacuum can to advance
I put a vacuum gauge on a ported vacuum line. at idle I see just about 0 vacuum. under full load you know WOT I see around 10"hg if I remember right. with manifold vacuum it works a little differently. under idle I see around 22" while at WOT I see around 0"
this doesn't make sense to me because it counteracts what you say about it advancing under light load high speed sitations as when cruising I maybe see 1-2"
so in a nutshell ported vacuum and manifold vacuum are going going to advance my timing at high speed low load situations AND the vacuum can is always going to advance the timing when under vacuum.
and while running ported vacuum the vacuum goes up as the load goes up while manifold vacuum the vacuum goes down while the load goes up.
does this about sum it up?

 

I think you do not remember right.

At WOT, you have basically ZERO vacuum; the throttle is wide open, so air is rushing into the manifold as fast as the engine is pumping it out. Therefore, at WOT, vacuum advance is a non-issue. Its purpose has to do with low-load, high-vacuum, high-speed (above idle speed anyway) conditions; nothing to do with WOT.

"Ported" vacuum is obtained from a little "port" right above where the throttle blades sit at idle. Hence the name "ported". It is above the throttle blade when idling; then as the throttle is opened, the blade passes the port, and the port becomes below the blade, and thereby exposed to manifold vacuum. Those statements are both correct. That statement is incorrect; vacuum ALWAYS goes down as load goes up. The ONLY difference between "ported" and "full manifold" vacuum, for this use, is that "ported" is obtained as mentioned above from a "port" that is not exposed to manifold vacuum until the throttles pass a certain amount of opening; and then once the throttles are open that far, that "port" gives the same vacuum as a "full manifold" connection would.

Neither "ported" nor "full manifold" vacuum will advance the distributor at WOT.

 

I could of sworn that I saw the vacuum gauge move higher as the car was given more of a load though which was different then when it was setup under manifold vacuum.

 

Combustion Engines 101, lesson 1:

Vacuum decreases with load; and reaches 0 (atmospheric pressure in the manifold) at full load and WOT. It is at a maximum at high speed with the throttles fully closed (decelerating from highway speed for example) because then the motor is pumping the air out of the manifold much much faster than it can squeek back in around the closed throttles.

 

understand how it works but I could of sworn when the vacuum gauge was hooked up that vacuum went up as load went up
but then again it was some time ago.



btw your not that sofaretarted guy are you?

 

i never noticed squeeking in my carb on decel

 

I'm not real sure... I might be sofakingretarted, now that you mention it, in addition to being merely sofakingdom but I've never used that screen name.

 

ok i understand what you guys are saying, but HOW do i set it, and tell what its at for an advance?

i set base timing at 12, then i plug in vacuum advance, how do i know what its at?

 

If you want to know the advance under any given conditions, get yourself a "dial-back" timing light.

Beyond that, you know what your distributor's curve is, since it's brand-new; set your static timing so that the static timing, plus however much the curve that MSD put in it gives it, adds up to 36 degrees; and then just hook up the vacuum advance and don't worry about how much it is, until you get everything else dialed in. That'll get you pletny close enough to start out.

 

Centrifugal advance: plates, weights, and springs located under rotor inside distributor. Inertia from increased acceleration causes them to stretch outward, thus creating the advance.

I'd get the full kit with the adjustable vacuum if you go the adjustable route. There are also a lot of great distributor timing and rebuilding articles to be found by google.

 

btw as I said I would admit if I was wrong and so far it seems I might be.

though I have heard they have vacuum retard unit on some jaguars

 

the new HEI distributor is ProComp, it has adjustable vacuum advance on it

im just wondering if since i already bought it, then would have set it to performance level to just drop in and be fine?