Vacuum advance units VC31? VC25? VC221? What does this mean?

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Geoff2
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by Geoff2 »

Not correct. Vac adv at idle can be run off any closed vacuum port. Except the PCV line [ which of course is open ]. Many cars like my GTO came with man vac adv from the factory & had a 'tree' at the carb rear with vac ports for vac adv, power brakes & trans modulator.
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by hoodeng »

The advice to run ignition vacuum pickup location and it's independence was advice given to me by a chasis dyno driver way back when, the same guy also had very definite ideas on isolating ignition power supplies and fuel system configurations.

Might have been a lot of irrelevant old wives tales, then again, he may have been bitten by experience? or outdated.
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by skinny z »

FishFry wrote: Tue Jan 31, 2023 10:54 am TIMING AND VACUUM ADVANCE 101
Excellent article. I've had in my tech files for years.
This is part of why I'm in the full manifold vacuum camp (at least for today). A low compression, over-cammed 355 SBC's drivability came alive after incorporating full manifold vacuum and painstakingly tuning the tip-in, cruise and idle characteristics.
Kevin
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by skinny z »

rgalajda wrote: Tue Jan 31, 2023 11:58 am "Of course the advance retards the timing the split second that the butterflies start to open so it is no big deal. "
This can lead to a tip in or "torque sag" and I've found it to be a common complaint when experimenting with full manifold vacuum advance. It can be a problem.
In my case (as unrepresentative as it might be) it took a combination of adjustable vacuum cans and timing limiter plates along with a handheld vacuum pump plus disabling the mechanical advance to get things dialed in. It certainly makes the case for a programmable distributor.
That said, the car responded with higher idle vacuum (with further fiddling as a result), reduced engine idle temperatures as well as increased fuel economy.
Kevin
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by Geoff2 »

^^^^^^^^^^
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by rgalajda »

FishFry wrote
"Of course the advance retards the timing the slit second that the butterflies start to open so it is no big deal. "


This is not true. The vacuum advance will not retard ( drop out ) until engine vacuum drops below the set rating of the vacuum advance unit.
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by jsgarage »

Another subtle thing I stumbled on with regard to vac-advance is, using shiny precision-appearing contour-bent metal tubing- of any size, often gives a different answer than just hooking it up with a couple of hose bibs and rubber windshield washer hose.

The rubber hose seems to damp some of the truly wild vacuum pulsations you get from IR intake Webers or certain other carbs & intakes. So two similar engines plumbed with metal vac tube or rubber hose can wind up with two radically different tunes. And then we get arguments in posts on the subject.
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by pitts64 »

Remember, the mechanical advance action also figures into the overall advance degree number when you have a distribuitor with both vacuum and mechanical..
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Re: Vacuum advance units VC31? VC25? VC221? What does this mean?

Post by skinny z »

pitts64 wrote: Tue Apr 11, 2023 7:46 pm Remember, the mechanical advance action also figures into the overall advance degree number when you have a distribuitor with both vacuum and mechanical..
This is in part why it's difficult to setup the vacuum advance curve using a hand held vacuum pump (Mighty Vac). As more vacuum is applied to the idling engine, the RPM increases. Some mechanical curves may start to bring the RPM based advance into play. I've physically disabled the centrifugal mechanism and concentrate solely of the vacuum side of things. It's a PITA.
Kevin
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