Can't Get Enough
Join Date: Mar 2000
Location: So. Jersey
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Re: ported or manifold vac. for vac adv.?
The way I understand it is that manifold vacuum is highest at idle and tapers off the faster the engine turns. Spark ported, or just ported, is very low at idle and increases as engine speed goes up. I'm saying engine speed here but in actuality it's throttle plate position. Both vacuums act differently when you punch it, when engine speed does not correspond to throttle plate position-manifold has a big drop and ported has a mild increase. This is why ported seems like it should be used because with a vacuum advance distributor when you mash the gas pedal, to try to accelerate, is when you need the vacuum put to the distributor to give you some spark advance for more power. Ive tried both vacuum sources and did not notice any difference with my stock engine and carb. The vacuum advance is working in my stock distributor, I just don't think you get much of an advance to matter, maybe because ported vacuum at the point where it has its highest sucking action is still less than manifold at its highest. Now, manifold vacuum at its highest has more of a suck than ported at its highest. Manifold is highest at idle. If your engine wants to run hot at idle or very low speeds, try manifold vacuum to advance the distributor spark. This seems to work.