Vacuum advance - Team Camaro Tech
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post #1 of 6 (permalink) Old Jun 21st, 02, 01:33 PM Thread Starter
Tech Team
Join Date: Dec 1999
Location: Salem,Oregon USA
Posts: 60
What source do you use for vacuum advance?
Ported or manifold.
I'm presently using the port on the side of the front metering block of my Holley.
Is there any reason to go to manifold?
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post #2 of 6 (permalink) Old Jun 21st, 02, 02:29 PM
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Join Date: Dec 2000
Location: Kentwood, MI
Posts: 8,097
I use ported for my 383. In my opinion you only want to go with a manifold source if you have a large cam and have trouble getting the engine to idle decent. If you have a mild cam and go to manifold vac you can wind up turning the idle screw down so low to get a normal idle speed that you get surging at idle - the extra advance at idle makes idle speed go up and you wind up trying to compensate by turning the idle screw down.

The ONLY difference is at idle. A ported source will have no vacuum at idle and no vacuum at full throttle, so the only time vacuum advance is added is at part throttle. Manifold vacuum pulls at both at idle AND at part throttle, with no vacuum at WOT.

Hope this helps.

68 Camaro with 383 small block. ET - 11.9's at 112 mph and never trailered.

The combo:

10.3:1 KB pistons, Eagle cast crank, 5.7" SIR rods.
Trick Flow 23* heads ported 2.055 intake, 1.6 exhaust.
Comp Cams 282s mechanical cam with 1.6 rockers.
Victor Jr intake and Holley O-3310 carb.
1-3/4" Hedman Headers, full length Flowmaster exhaust.
TH350 trans, 3000 stall, 3.55 gear.
CE subframe connectors, drag shocks, SSM lift bars.
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post #3 of 6 (permalink) Old Jun 21st, 02, 02:39 PM
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Join Date: Mar 2001
Location: Tucson, AZ, USA
Posts: 122
Your fine running the ported vacuum at the carb....this allows the vacuum advance to be used durring cruise mode..this will give a snappier feel at cruise mode as well as provide fuel economy... this also allows you to set base timing without the influence of vacuum advance, which I happen to like..
The main difference with using the manifold vacuum is that you get vacuum advance at cruise mode and at idle, which I don't prefer having at idle...and a reverse advance curve action at take-off...this means that you start off at idle with full vacuum advance then as you accelerate from idle, the diaphram is relaxing from reduced vacuum, thus reducing the advance....this can also induce pops from the carb, since there is typically a delay in the system response....
So the vacuum advance curve is actually retarding from it's full advance position while at the same time the mechanical advance is truly you get a convolution of the two curves as a resultant curve that needs to be properly analyzed and can have discontinuities in the curve and it may not look to desirable when you plot the curve...
So I would suggest using the ported vacuum for the advance...

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post #4 of 6 (permalink) Old Jun 21st, 02, 03:28 PM Thread Starter
Tech Team
Join Date: Dec 1999
Location: Salem,Oregon USA
Posts: 60
Is it possible to be getting vacuum from the front metering block port?
At idle
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post #5 of 6 (permalink) Old Jun 21st, 02, 03:34 PM
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Join Date: Feb 2002
Location: LR AR
Posts: 2,372
Many engines were set up with manifold vacuum before emissions came into effect. I am trying to set up my curve like a 64-65 327 SHP Vette engine as it has almost the same specs as the 302 in my car with the exception of .25" less stroke in the crank.

I have a lot of vacuum advance data from various sources and I am trying to optimize a manifold vacuum setup on my DZ 302. It is working fairly well at this point but that off idle "hole" is annoying. If I crack the throttle slightly it can quickly loose vacuum advance and the RPM's drop which causes the centrifugal advance to reduce and now the idle is half what it should be, the vacuum is way low and the engine can not pick itself back up to normal idle speed and it dies. This is not the drivabillity I was aiming for

I will try reducing the available travel in the autocam slot and see if that helps to prevent the advance from backing off at RPM below idle. I need to break the motor in more before I can realy tear into it so it might be a week or two but I will report back sucess or failure. If reducing the slot does not do it I will probably go back to ported with a custom distributor advance can and curve. I KNOW that will work

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post #6 of 6 (permalink) Old Jun 21st, 02, 06:34 PM
Join Date: Dec 2001
Posts: 56
You should search this forum for past posts on vacuum advance, for which the source (ported or manifold) has created significant discussions. I recall several posts by Ignition Man, who heavily promotes manifold vacuum unless you have EGR. These past posts are all there for your review. I have become a believer in manifold vacuum ever since I changed over and eliminated my engine pinging problem. Keep an open mind and check the old posts.....Scott
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