Chevelle Tech - View Single Post - ported or full manifold vacume
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post #13 of (permalink) Old Oct 13th, 19, 3:46 PM
Dave Ray
Senior Tech Team
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 1,840
Re: ported or full manifold vacume

The biggest problem with running full manifold vacuum is outlined in the package I send out to anyone that wants it. Venting here, just read it.

Clearly, not many people are aware there are many different stock vacuum advances for either points type, or large HEI, and ALL of them have two different parameters that MUST be properly applied to EVERY engine they are used on,

1, vacuum advance vacuum level must be compatible with engine operating vacuum parameters, so it works correctly for the vacuum created

2, the amount of degrees given MUST be within a certain parameter, so that added vacuum timing degrees does not over time the engine.

Now, does anyone here actually know why some plug their vacuum advances in to full manifold vacuum, and get 15, 20, 30 more degrees of crankshaft timing, when the vacuum advance should not give that many degrees?????

Well, it is dead simple. EVERY STOCK POINTS type GM vacuum advance came from the factory with a short section of rubber vacuum hose placed over the pull pin in the advance, to stop down that pin travel, and DEGREES GIVEN on every advance, so they would not over time on full manifold vacuum.

Since large HEI WAS NEVER set up as a performance, or early NON-EMISSIONS engine until the ZZ crate engine series, those large STOCK HEI vacuum advances were NEVER run on full manifold vacuum, ONLY EMISSIONS RELATED PORTED VACUUM, which does not draw as many degrees of timing into an engine, because the vacuum level is too low to do so on ported (timed) vacuum, soo, no rubbr stop bushing was ever installed, factory stock, or after market replacement.

All this stuff is outlined in the package, always has been, always will be.

So, what I did when I got into modifying large ZZ4 HEI's for one car building speed shop in Whittier, California that used brand new ZZ4 crate engines for all the cars they built, was to design a set distance stop to modify those 69120 vacuum advances to give 10 crankshaft degrees out of metal, so they would not rot and fall away from the pull pin. It evolved into an adjustable scroll stop when I realized it was even easier to mount a stand alone Crane stop in place of the home made stop I was advising made. This gave the 10 degrees I wanted for any advance, and every other setting, in 2 degree increments. Dead simple, one 8/32 drilled and tapped hole in the vacuum advance mounting bar, the Crane stand alone 99619-1 plate, one each hardware store hex head stainless machine screw and flat washer, done, run full manifold vacuum to supplement the IDLE timing to the point it worked correctly on full manifold vacuum. Simple, 15 minutes, done.

Come to find out most ignition system manufacturers have techs that have no clue as well, and they get their info from the resident dyno hero's that have no clue, because dyno's LOVE too high a timing specification, and too rich jetting, and they also have NO idea as to how to properly set up a vacuum advance to work, either way, full manifold, or ported. Most are so far away from understanding how it works right, they insist on ripping the one device that makes, or breaks tuning, right off the distributor, giant error. Some are even worse, they insist on locking timing curves out on street driven milder engines as well, and that never works right.

It was so very simple, those that refused to even try to understand it, made trouble about it, "wouldn't work", "not the way it should be done", and on, and on. Well, it does work, and many have done it, way past me. And, I decided it was something that needed to be given absolutely freely, not charged for, FREE, NO strings, NO ads, just get it done and go drive the car, have fun. I still get beat up for it, including right now, 5 people on a Pontiac website that just refuse to understand it, so, they for helping.

Yes, there will be those that will now come on this topic and call me a moron for what I have written above, that they did what I said won't ever work, and, those that will come on and recount how they did it the way I outlined, and it fixed the problems. YOU decide, the parts are less than 5 bucks (I don't sell them), and it can be left completely off, not engaged.
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