vacuum advance

[philminotti]

I haven't read much on this forum about the merits or demerits of vacuum advance. Does a street driven FE benefit from vacuum advance? Is the potential fuel economy gain worth it? Is there any other advantage other than fuel economy?

Just wondering.

[mylesdw]

Whether you see a benefit or not depends upon your driving style: blasting about with your foot on and off the throttle you would see little difference but in a long steady cruise the economy gain is worth having. There is no disadvantage to having it as long as it is set up right.

Another possible advantage (and I'm guessing here!) might be for an engine that tended to oil up plugs at light throttle and tickover where the extra advance might help to stop this occurring.

[Excaliber]

I recently posted on a thread this response to a similiar question.
Myles, I think your onto something about the plug fouling idea!

"Direct intake manifold vacuum is a good call, in fact "back in the day" it was an old drag racing trick to get the vacuum signal respone time just a little quicker (didn't really work). But be advised your "base" timing will include base + vacuum advance + mechanical advance when using this method.

Manufuacturers (old school stuff) used a vacuum port placed just ABOVE the lower throttle plates. That way the vacuum advance would not come into play until the throttle was opened just slightly. Avoiding potential "to fast an idle" condition due to vacuum signal advancing base timing.

At WOT (wide open throttle) there is low or no vacuum present so your running on base + mechanical advance only. You should set your base timing and FULL mechanical advance with the vacuum port plugged and the vac advance NOT active at the time.

So WHY do some engines use vacuum advance anyway? Better "cruise rpm" fuel mileage is the MAIN reason. Say your total advance (base + mechanical) doesn't come fully in until 3200 rpm, but your cruise rpm is only 2800. Under partial throttle and load conditions (such as cruise) engines GENERALLY respond better to more advance. The vacuum signal is fairly strong at cruise conditions, thus advancing the timing using the vac signal is a "good thing" in that case.

Stepping on the gas to pass or accellerate lowers the vac signal and your back to base + mechanical advance. That keeps the timing from potentially causing "ping" or "knock" under those conditions.

Many people simply ignore the vacuum advance and set up their engine for base + mechanical and let it go at that. That works fine. But again, MANY engines respond favorably to a crusie rpm vacuum signal approach to timing control."

A link to the other thread:
vaccum advance question...

[Carnut427]

Ported vacuum for vacuum advance (vacuum port placed above the throttle plate) originated to lower pollutants from engines. Back in "the day", before smog equipment, engines used full manifold vacuum. You merely adjust the idle screw to raise or lower the idle when changing vacuum source. Some engines run better with ported vacuum, many have better off-idle response with full manifold vacuum. Try them both and take your choice.