If your are running a non comp carburator what are you running ported or manifold vacuum? I am running manifold vacuum cuz I noticed my car responded way better and my engines temp also went down a bit.

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Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,2500 stall converter. Will install 3.73's later on and might go with a more aggresive cam.As of 4/01 added Hooker Headers,2.5inchpipes with 40 s. Flowmasters.Ignition upgrade include: MSD Coil and MSD module.

 

I'm not sure on your question but:
I'm using both. Ported is for the vacuum advance. Manifold is for most everything else.


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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

I run full manifold vaccuum to the distributer, the extra advance gives it a much nicer idle.

[This message has been edited by Apeiron (edited April 22, 2001).]

 

Yes, full manifold vacuum is best unless you have OEM/mellow cam. You'll need/appreciate the extra advance at idle big time. Big cams actually NEED the extra advance or will not run properly, could get hot and could have bad emissions.

 

Sorry 82Camaro I forgot to mention vacuum routed to the distributor(vac. advance)

------------------
Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,2500 stall converter. Will install 3.73's later on and might go with a more aggresive cam.As of 4/01 added Hooker Headers,2.5inchpipes with 40 s. Flowmasters.Ignition upgrade include: MSD Coil and MSD module.

[This message has been edited by Big454blockchevy (edited April 22, 2001).]

 

Hmm, I run ported vacuum to mine. Adds in a ****load of advance on mine just at idle if I run it off of the mani vacuum (seems like too much to me)...

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Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.
www.inter-scape.com/Ray

 

Vacuum advance systems are designed to be used off of ported vacuum. I for one think it's a good idea it feels snappier because of the extra advance down low...but at WOT where it matters you actually get less. Besides, when I tried it for a bit I had problems with it over advancing the distributer when I was trying to start it..and it wouldn't turn over at all.

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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
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ICON Motorsports

 

Two things at work here: 1) Your carb's idle circuit design, and 2) you distributor's advance curve.

Way back when (like the original 283 in my '57), the factory designed those two to work with manifold vacuum to the vacuum advance. In the mid-60's, they went to ported vacuum, primarily for emissions, and redesigned the idle circuits and advance curves accordingly.

If you're running all aftermarket pieces and tune properly, you can probably run manifold vacuum. If, however, you try that on a carb or distributor intended for ported vacuum advance, your idle will likely be too high, and in trying to compensate for that, you'll mess up the geometry between the throttle plates and idle & transition circuits. You can probably tweak things to get around it, but unless you know what you're doing, you just might make things worse.

It certainly can't hurt to try both and see which you like better. But, if you need to modify plates & circuits to make the manifold hookup work, that's a whole different story.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam and intake, World 305 heads, Hooker headers & y-pipe, 3" Catco cat).
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If you run it on manifold vacuum you have to back the base timing up to compensate. Example, 10 degrees advanced at idle using ported vacuum will end up being 25 degrees advanced if you have vacuum advance(15 degrees from the vacuum advance). That's too much timing at idle. Then you have to back up the timing 15 degrees(to 10 degrees)so it will start easy and idle good. At that point you have a good idle with the vacuum advance hooked to manifold vacuum. But, the total advance will then suffer by 15 degrees. At full throttle under load, vacuum is totally gone. If your mechanical advance brings in 25 degrees + 10 initial - 15 vacuum that is gone you get 20 degrees total. Not enough. Using ported vacuum, you end up with 35 degrees total. Just about right. The vacuum advance is meant to be a part throttle advance that helps mileage and part throttle driveability.


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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

I run manifold for everything but the distributor. If you recurve the dist and add an ajustable vac. advance pot you wont have any issues with too much/too little advance for a given RPM. The extra advance at low rpms gives better performance, like Jester said though, it sacrifices upper rpm performance. By the same token, if you retard your timing you can get by with a lower octane fuel. You'll also notice a difference in operating temp with the different settings.

In the combo I'm running I can dial in the exact progression of advance I need for the rpm range I drive in. If I had to give a hp estimate a properly done recurve, adjustable pot and good coil is worth about 20hp over running manifold vacuum, and any hillbilly with a screwdriver and a 4th grade edukation can do it.

 

I always run ported vacuum to the distrubutor. Yes, you can run full vacuum to it but I've found that causes 2 typical problems:

1. If you have the inital advance set where it makes the best WOT power, you will have way way way too much additional advance at idle and light thottle. Possibly unstable idle and definitely part throttle detonation will occur.

2. (For carbureted engines) High levels of advance at idle will cause the combustion chamber to stay very very hot. You know what that means- RUN ON! You'll knocking, thumpin' and wheezin' long after you turn the key off!

The typical cure to this is to back the initial advance setting down but that causes the engien to run like dog doo doo at WOT. There's an article on how to dial in a good GM HEI distributor advance curve in the tech articles section. Even if you don't have an HEI, everything it says about what a good performance advance curve should look like still applies to other distributors/ignition systems.

 

I run manifold vacuum only. No problems.




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*I do custom performance mods on Edlebrock Performer carburetors (dualplane intake mods in the works),
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I use manifold vacuum on my car. In my experience when you have a fairly radical cam manifold vacuum seems to work best. Ported seems to work better on stock engines.

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best 1/4 mile so far: 13.4 @ 101mph

84 Camaro 305 ported & milled heads, 3 angle valve job, Edelbrock Performer RPM intake, Hedman full length headers, Accell ignition, Crane 274H cam, B&M 700R-4, 3000 stall speed, 4.10 gear,
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On a fairly radical engine, I just run without it--it will just add advance at idle or higher rpm not underload. How would that help performance?

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

because you don't want more timing under heavy load at lower RPM's.

 

Yesterday I tried changing to ported vacuum to see if the engine would respond better . It did not. I went back to manifold vacuum. My car accelerates faster and I have not noticed any problems at high rpms.

------------------
Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,2500 stall converter. Will install 3.73's later on and might go with a more aggresive cam.As of 4/01 added Hooker Headers,2.5inchpipes with 40 s. Flowmasters.Ignition upgrade include: MSD Coil and MSD module.

 

If you switch to ported vacuum, you need to readjust the timing(advance it) to see the benefit. At WOT both won't function because the engine has no vacuum(unless you intake/aircleaner system is restricted).Underload at low rpm, ported and manifold will give you about the same advance because the vacuum is limited to the vacuum of the engine. If you lose manifold vacuum underload at low rpm, ported vacuum will be gone as well because the whole engine has no vacuum.

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

82camaro,
Ported vacuum is created by airflow moving through the throttle bore.
Manifold vacuum is created by the restriction posed by the throttle blade.

the two are completely different.

ODB

 

In many carbs and TBI's, manifold vacuum is derived from a tiny hole in the throttle bores JUST BELOW the throttle plates at idle, thus producing vacuum all the time it is present in the engine.

ported vacuum is derived from another tiny hole JUST ABOVE THE OTHER HOLES and just above the throttle plates at idle, so NO vacuum is shown at idle. When the throttle is cracked slightly, the plate starts to "go over" the ported hole and VIOLA, now it is a manifold vacuum source as it is below the throttle plate restiction.

Make sense? So, in theory, as the throttle is cracked, the ported and manny vacuum readings in this scenario should be VERY close to the same. EDIT: except fr possible velocity/venturi effects, I suppose, but I have found that they measure quite closely.

[This message has been edited by FastBroker (edited May 02, 2001).]

 

82Camaro the timing was adjusted with each ported and manifold vacuum. LIke I stated my engine liked manifold vacuum way better.

------------------
Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,2500 stall converter. Will install 3.73's later on and might go with a more aggresive cam.As of 4/01 added Hooker Headers,2.5inchpipes with 40 s. Flowmasters.Ignition upgrade include: MSD Coil and MSD module.

 

big cams DO like the heavy initial advance gained with full vac, for sure...

 

The base timing should be adjusted with the vacuum line disconnected and plugged at the carb, whether using manifold or ported vacuum. The advanced timing, both mechanical and vacuum, are what you tailor to make it run properly w/o pinging.

 

I'm not on this board to argue with anyone. Just trying to explain why vacuum advance was developed and why it is typically used with ported vacuum. Every engine is different. The only way to test it is run back to back runs adjusting the advance using manifold vacuum until you run the quickest(finding the best timing). Then do the same using ported vacuum.

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

I agree the way to test it is to get back to back runs . I really only went with the feeling I got but I will try it out at the track. Although I still say manifold vacuum will be best.

------------------
Big 454 72 Chevy Nova
383 86 Camaro Z28
10.5:1compression 186 casting Heads,234dur.488lift exh&int,Edelbrock performer intake, Holley750 vac.sec 400 crank,flat top pistons,edelbrock valve springs and lifters,tranny shift kit,2500 stall converter. Will install 3.73's later on and might go with a more aggresive cam.As of 4/01 added Hooker Headers,2.5inchpipes with 40 s. Flowmasters.Ignition upgrade include: MSD Coil and MSD module.

 

Here's a reference off the net about ignition settings. May be informative for some.

http://www.centuryperformance.com/timing.htm