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Old 06-12-2006, 02:13 PM
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ported vacuum vs. manifold vacuum

hi everyone, i've been doing alot of reading on ported vs manifold vacuum. i've also checked out some of the old posts here on the site. from what i understand, the only difference is at idle. is this correct? i've always used manifold, however i keep finding fuel in the hose on manifold. i was thinking of changing to ported until i can find out how the fuel is getting in the mainfold port.

i've recently tried to re-curve my distributor, but haven't really been getting the numbers i expected. i've set my initial timing at 20* which the car seems to like. and i've adjusted the vacuum canister to limit my vac advance. however, my mechanical timing has 2 issues. 1: no matter what combo of weights and springs i use, timing is all in by 2000 rpm. from what i've read, this is too early. i was shooting for 2800-3000 rpm. 2: it doesn't appear as though i am getting any more that about 16* mechanical adv. again, i was expecting to see 18*-20* mechanical. the only real difference i've seen trying different combos with the springs and weights has been the actual numbers between idle and full advance, which is really only about 1000 rpm. any thoughts on this?
thanks for your time.

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Old 06-12-2006, 08:00 PM
acr acr is offline
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You seem to have done your research. As you prbly know, every engine is different. All you can do now is "Trail & Error" testing (that you can verify), changing only one thing at a time.
Good Luck! acr
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Old 06-12-2006, 10:19 PM
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You haven't indicated the type if dist. I've found many aftermarket perf dist are set up that way with a relatively small amount of mechanical advance. They expect that you have a longer duration cam and will run alot of initial advance.
One thing ported or manifold the initial timing is the same with the vac hose plugged. Manifold is fine for a cooler idle and simpler set up. It's stock on a 283.
I find that with any amount of cam duration, it makes the idle a pain cold and the engine wants to hunt (rev up and down). I prefer the ported. Take your maximum available mechanical advance in degrees and subtract that from 32. That should be your initial to start . If you get knocking in any med or heavy accelleration back off the initial 4 degrees at a time until it stops. Any light throttle knocking is the vac advance not backing off soon enough and requires an adjustable vac advance to get the vac pot to advance at a higher vac reading.

Rule of thumb : short duration cam or tall gears mechanical advance comes in slowly. all in at 3000

long duration cam or low gears ie 411/1. mechanical advance comes in quick .all in by 2000

good luck
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Old 06-13-2006, 07:43 AM
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I experimented with my mechanical advance by buying a Moroso advance kit (weights and springs). I found the Moroso weights were lighter and the springs were not as strong as on the original stock HEI distributor.

If you have the original heavier weights and try using weaker after market springs the mechanical will come in very fast like you are experiencing. Do you have the original stock springs?
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Old 06-13-2006, 01:53 PM
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i'll try to reply to 2 posts here. the distributor is an hei in a 76 350 sbc. i have a retro fit roller cam 228 @ .050. 288 advertised. and i also have 4.10 rear. so maybe this setup will work. i do not think that i have the old springs, if i can recall, many years ago we played with the springs on my old distributor. i just bought this new distributor, it was set up with an adjustable vac advance. i will continue to test performance using both ported and manifold vacuum. it does not seem to idle bad, the cooler temps would be nice, but i don't expect to do a whole lot of idling. i think it's time to get things back together and let her loose!!! hopefully, i can answer the problem of gas getting into the manifold vac tube.
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