ported or manifold vac. for vac adv.? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 04-17-2002, 07:57 PM
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ported or manifold vac. for vac adv.?

Best to hook up vaccuum advance to manifold or ported vaccuum?

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post #2 of (permalink) Old 04-17-2002, 09:15 PM
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Re: ported or manifold vac. for vac adv.?

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post #3 of (permalink) Old 04-17-2002, 09:20 PM
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Re: ported or manifold vac. for vac adv.?

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post #4 of (permalink) Old 04-18-2002, 01:57 AM
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Re: ported or manifold vac. for vac adv.?

There was a thread about 6 or 7 pages long on this topic about 1 1/2 years ago that never really did answer the question. I think the consensis was "it depends." Most cars use ported but your Jeep came from the factory with Manifold vacuum for a reason, I just can't remember what it was right now. I switched mine over to ported once a while back and it seemed to run better for about a week and then it ran crappy so I put it back. I just did the nutter bypass finally and it says to switch to ported after the removing the computer but mine ran awful on ported so I left it on manifold. A lot of guys have switched over the 258 to ported vacuum and report good results. Try it for a while and see how it does.
post #5 of (permalink) Old 04-18-2002, 07:01 AM
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Re: ported or manifold vac. for vac adv.?

There are more recent posts on this subject as I have been doing some research along the same lines.

Check out:
Vacuum advance options for Duraspark and
carbureted 4.0 head conversion - timing questions

My specifics don't apply to you, but there is a lot of good information in these threads. Basically, in my research I found that originally the Jeep had a solenoid controlled (by the computer) mix of manifold and ported vacuum.

Before I did my 4.0 head conversion, my specifics very much applied to you. I had the stock 258 with a Weber 32/36. I ran manifold vacuum and it did fine.

Like the others have said, try it each way and see which one does better.
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post #6 of (permalink) Old 04-18-2002, 07:20 AM
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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.
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post #7 of (permalink) Old 04-18-2002, 09:58 AM
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Re: ported or manifold vac. for vac adv.?

The way I understand it, is most cars and trucks use ported vacuum from the factory, but not Jeeps. Most people will tell you to use ported, but others will say that ported AND manifold increase through the RPM's and both produce enough pull to accomplish the amount of advance a low-revving, torquey engine needs.

One thing to keep in mind, manifold vac to control advance will help with knocking when a load's on the engine. For example, when you're going up a hill or accelerating in too low of a gear the engine will begin to knock, if you step on the gas a little, the throttle blades open and manifold vacuum goes down thus decreasing advance and knocking.
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