Experimenting between manifold and ported vacuum - Chevelle Tech

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post #1 of 6 (permalink) Old Apr 7th, 08, 6:56 PM Thread Starter
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Experimenting between manifold and ported vacuum

I have a '69 Malibu with an '84 GMC 461, stock compression, Isky 270 cam, high rise intake, headers, 3" exhaust, stock converter, 2.73 open, and GM HEI.

I just put an Edelbrock 650 cfm Thunder AVS on the car and love it. However, the guy I put it on with hooked the distributor to manifold vacuum. It works great, but Edelbrock recommends using ported vacuum.

I've read a ton of posts about ported vs. manifold vacuum and it seems to come down to personal preference and individual engines. I'm going to experiment and would like to compare apples-to-apples. Beside switching the vacuum line, is there anything else to adjust when trying out each vacuum port? I assume I'm going to have to adjust the mixture and idle screws. Anything else to play with to get a level field of comparison?
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post #2 of 6 (permalink) Old Apr 7th, 08, 10:01 PM
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Re: Experimenting between manifold and ported vacuum

The key to using manifold vacuum is having the right vacuum canister. With many (probably most) canisters you can have a problem maintaining a consistant idle speed. As you increase rpm, vacuum increases and so does advance. When you let off the gas the added advance causes the idle rpm to be higher than it was initially, which in turn causes centrifugal to be added to initial, further raising rpm even though the trottle plates are in idle position. The cure for this is a B-26 canister (Napa VC1810) which fully deploys by about 8" of vacuum so you can set idle rpm with vacuum advance already in. I've heard that this canister has been discontinued (but there may still be some in stock out there). If unable to find one, an adjustable canister is another option. If you're not having any issues with it hooked to manifold with your current canister, just leave it as it is. If you are subject to emissions testing, you may need to switch to ported for the test.
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post #3 of 6 (permalink) Old Apr 8th, 08, 3:09 AM
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Re: Experimenting between manifold and ported vacuum

First it may be worth your time to read the "ignition 101" thread.

Next as Jerry suggests, often ported vacuum is used to cover a mismatched vacuum advance can. This can should provide full vacuum advance at 1-2" less than your idle vacuum (in gear if an automatic). If the can has the wrong range of operation, then you can be in the middle of the range at idle and all kinds of unpredictable and unstable things can start to happen. Things like an unstable idle or an idle that is either way too fast or way too slow (dying). So a lot of people just try and shutoff the vacuum advance to keep any of those things from occurring rather than fix the real problem.

The problem is that you also wind up with a crummy tune at idle this way.

It is much better, and with more aggressive cams it is critical, to understand how things are really supposed to work and configure them appropriately. And this included running manifold vacuum with the proper ranged vacuum advance can. The more aggressive the cam the more critical it is to have the extra ignition advance at idle that can be provided by a fully activated vacuum advance can.

Edelbrock recommends ported so they will receive fewer support phone calls.

You should not really need to touch the idle mixture screws when switching. You may very well need to adjust the idle speed though. But note that if the engine drops flat and dies as you slow down the idle, you likely have a vacuum advance can that pulls out advance at too high an vacuum level.

Steve
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post #4 of 6 (permalink) Old Apr 8th, 08, 4:10 AM Thread Starter
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Re: Experimenting between manifold and ported vacuum

"Edelbrock recommends ported so they will receive fewer support phone calls."

Beautiful. I thought as much.

Thanks for the replies. I'm going to play around a bit, but from the responses and the fact that the engine runs well as is, I suspect I'll end up sticking with manifold vacuum.
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post #5 of 6 (permalink) Old Apr 8th, 08, 7:45 AM
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Re: Experimenting between manifold and ported vacuum

Quote:
Originally Posted by 658Chevy View Post
"Edelbrock recommends ported so they will receive fewer support phone calls."

Beautiful. I thought as much.

Thanks for the replies. I'm going to play around a bit, but from the responses and the fact that the engine runs well as is, I suspect I'll end up sticking with manifold vacuum.
They also have to deal with the C.A.R.B. and the approval of their products, so they would naturally recommend the most smog friendly setup.
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post #6 of 6 (permalink) Old Apr 8th, 08, 3:05 PM
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Re: Experimenting between manifold and ported vacuum

Use manifold vacume and get the correct vacume adv canister. Other wise you are just perpetuating a very bad thing done by GM to help emmisions/ hurt performance. The ignition was designed to use manifold vacume and was band aided by using ported vacume. Spend a little time setting it up correctly and enjoy the increase in throttle response and performance. If not you are just trying to polish a turd.
Bill

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