ported or full manifold vacume - Chevelle Tech
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  • 4 Post By Aaron
  • 1 Post By oldcutlass
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post #1 of 14 (permalink) Old Oct 11th, 19, 4:50 PM Thread Starter
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butch
 
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ported or full manifold vacume

hey all, i have a bone stock 305 chevy and would like to know witch would be better for gas milage ported or full manifold vacume its got the hei dist
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post #2 of 14 (permalink) Old Oct 11th, 19, 5:01 PM
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Re: ported or full manifold vacume

manifold....read the ignition 101 thread in the ignition forum.
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post #3 of 14 (permalink) Old Oct 11th, 19, 6:00 PM
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Re: ported or full manifold vacume

Either, once the rpms are increased and you are out of the idle circuit they both act the same.
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post #4 of 14 (permalink) Old Oct 11th, 19, 6:23 PM
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Re: ported or full manifold vacume

Manifold for smoother idle, Ported for everything else.
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post #5 of 14 (permalink) Old Oct 11th, 19, 7:30 PM
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Re: ported or full manifold vacume

Full manifold vacuum.
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post #6 of 14 (permalink) Old Oct 11th, 19, 9:57 PM
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Re: ported or full manifold vacume

There is another thing you MUST do to use full manifold vacuum for the vacuum advance, set up a degrees stop for the vacuum advance, not hard to do, not expensive. If you don't do some sort of degrees stop, expect problems. And, if the engine is NOT an emissions engine, that is, if there is NO EGR valve in use, you definitely want to do the degrees stop, AND, use full manifold vacuum for that vacuum advance.

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post #7 of 14 (permalink) Old Oct 11th, 19, 10:28 PM
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Re: ported or full manifold vacume

GM engineers say full manifold vacuum
Attached Files
File Type: pdf Timing 101.pdf (2.88 MB, 42 views)

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post #8 of 14 (permalink) Old Oct 12th, 19, 6:50 AM
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Re: ported or full manifold vacume

Someone once posted a chart/image that showed manifold and ported vac advance were exactly the same once the throttle was cracked open. It just comes down to whether or not you want more advance at idle.
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post #9 of 14 (permalink) Old Oct 12th, 19, 6:54 AM
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Re: ported or full manifold vacume

So we all dont overthink it:

http://hutch.smugmug.com/Other/Just-...uumchart-L.jpg
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post #10 of 14 (permalink) Old Oct 12th, 19, 9:28 AM
 
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Re: ported or full manifold vacume

My Chevelle with a 396 runs better on manifold vacuum so I tried it on my 1979 C30 with a 454 and it ran horribly. I couldn't stop and switch it back to ported fast enough. I would just try it both ways and see what you think if it were me.
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post #11 of 14 (permalink) Old Oct 13th, 19, 4:09 AM
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Re: ported or full manifold vacume

Quote:
Originally Posted by 69-CHVL View Post
I've read that ignition 101 before and in my opinion, it overstates the benefits of running manifold vacuum. It states you'll get better fuel economy, performance, the engine will run cooler, better engine efficiency, and better throttle response. That's all a little bit of a stretch really if you see 69-CHVL's chart above. Unless you really idle your car a lot and your cooling system is borderline and you consider better idling a "performance" upgrade. I suppose maybe then.
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post #12 of 14 (permalink) Old Oct 13th, 19, 9:21 AM
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Re: ported or full manifold vacume

It would be great if that chart actually had a 4th line that showed engine RPM.

You could then use the data from the chart to plot out the entire timing curve combining initial, mechanical and vacuum at any point and see where the crossovers are with different curves and see what the engine total timing would be at any point with manifold vs vacuum. Of course you also need to know the vacuum advance canister curve and mechanical curves, but it can be done.

For example, with manifold vacuum, from idle, when you crack the throttle and start opening it up, at some point the timing starts to retard until the mechanical catches up as the vacuum advance drops off (it may or may not ever cross, again, depends on the curves.) With ported, from idle, when you crack the throttle and start opening it up, the timing suddenly advances as the vacuum advance kicks in and then starts to retard until the mechanical catches up (again need to know the exact curves to know how much and crossover points, if any.)

Thus, having the RPM plot points could tell you those crossover points and you could see how the total timing behaves at any given RPM.

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post #13 of 14 (permalink) Old Oct 13th, 19, 3:46 PM
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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 attack...me 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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post #14 of 14 (permalink) Old Oct 14th, 19, 12:37 PM
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Re: ported or full manifold vacume

Butch, checkout Hot Rod Magazine 1960 January Page 42. They show you how the Mallory dual point system works, you bring on a second set of points connected to a (full manifold) vacuum actuated limit switch, this retards the timing during heavy load conditions.

I started to make one of these and never finished it. I might put it on the single points triggered CD system I am building now for my 454.

https://www.hotrod.com/articles/hrdp...-covers-1960s/
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