manifold vacuum on straight 8

[39CENT]

Hey! anybody running MANIFOLD VACUUM on non original distributor on straight 8? And any info if you can give, on performance??

[Jyrki]

I would not tap the vacuum advance system into straight manifold vacuum. Some hot rodders do this because it helps the engine idle and generate some vacuum with a radical camshaft. The S-8 doesn't have problems with idle or poor vacuum. And it doesn't like too much advance at idle (it'll start popping in the exhaust)

[Bob]

Remember, your vacuum advance is not there for performance, it is only there to increase mileage and manifold vacuum is often used to reduce emissions.

[39CENT]

I've heard all kinds of views on this problem and there seems to be lots of confusion out there. I dont, know why the straight 8 would be any different than other engines in this area. The only thing i noticed was a lot of people call manifold vac. 'full time vac.', and it certainly isnt full time, as vac comes and goes according to the the throttle actioin, when it opens the vac goes away, only to return at cruise and closed according to fuel needs. thanx guys, anybody running manifold vac?

[39CENT]

I would not tap the vacuum advance system into straight manifold vacuum. Some hot rodders do this because it helps the engine idle and generate some vacuum with a radical camshaft. The S-8 doesn't have problems with idle or poor vacuum. And it doesn't like too much advance at idle (it'll start popping in the exhaust)

Yes Jyrki, you are right as the stock dist. is designed for ported manifold vac. I should have made clear use a dist that uses manifold vac 39c

[Jyrki]

I still stand behind me statement: if you are using manifold vacuum on your distributor's vacuum advance, it will advance your timing also at idle, which is not good for a S-8. Advancing the idle beyond 10 degrees or so, forces you to close the throttle blades further to bring idle rpm down. That, in turn, makes your fuel distribution even worse than it already is, manifesting as a "popping" sound in the exhaust. The sound is not loud, but more like "spitting".

I have used manifold vacuum in all my V-8's, but for the S-8 with siamesed intake ports, it's not a good idea.

[Bob]

If you are concerned about full throttle performance, as in a race or whatever, manifold vacuum is at 0, therefore you have no vacuum advance. This is why many race only distributors do not have manifold vacuum. At part throttle, under light load an engine can use more advance than it can under maximum throttle, the vacuum advance causes this to happen. The benefit is significant improvements in gas mileage. Burning your fuel better under low loads is the only benefit of the vacuum advance whether it is take off ported or manifold and in either case is not a benefit under heavy load performance applications when the vacuum disappears.

[39CENT]

If you are concerned about full throttle performance, as in a race or whatever, manifold vacuum is at 0, therefore you have no vacuum advance. This is why many race only distributors do not have manifold vacuum. At part throttle, under light load an engine can use more advance than it can under maximum throttle, the vacuum advance causes this to happen. The benefit is significant improvements in gas mileage. Burning your fuel better under low loads is the only benefit of the vacuum advance whether it is take off ported or manifold and in either case is not a benefit under heavy load performance applications when the vacuum disappears.

Yes thanx Bob,

heres an article that clears it up for me. works on Buicks too.

http://www.chevytalk.org/fusionbb/sh...hp?tid/239321/

[Jyrki]

Yes thanx Bob,

heres an article that clears it up for me. works on Buicks too.

http://www.chevytalk.org/fusionbb/sh...hp?tid/239321/

Very good article. Buick used ported vacuum advance on these straight-eights for a reason, and that was not emissions. Like I said, I use manifold vacuum advance in everything but not the straight-8. You just need to sacrifice some fuel at idle in order to make it idle well.