Vacuum advance confusion

[dodj]

I'm having a difficult time understanding how the vacuum advance is supposed to work with mechanical advance. Right now I run 36* @ 3000rpm and the vacuum advance is disconnected. That turns out to be 13-14* initial. I have a little surging issue at highway speeds but other than that, the car runs fine. Now if the vacuum advance is connected, won't that increase total advance at 3000rpm to over 50*? I know it is adjustable, but if you dial it out, as I have, what is the point of it? How much total advance can an engine actually take @ 3000rpm?

[Talkwrench]

I am in the same boat as you, I dont have any surging though, only slight pinging under load set at 35* total no vac. look forward to the answers...

[Bullitt-]

I have been told that total advance with vacuum could be as high as 50 degrees 

[dodj]

I have been told that total advance with vacuum could be as high as 50 degrees

But you set your mechanical @ 35*-38* , because more than that you get detonation. So if you hook up the vacuum and get 50*, shouldn't it detonate like crazy?

[Bullitt-]

I can't say I understand how/why but that is what it is there for...to add advance after the mechanical advance runs out at cruising speed.  This improves fuel economy, not performance & will not be a factor during heavy acceleration due to the fact that the vacuum drops off. 

[Strawdawg]

You get detonation under high load conditions (wide open throttle).  At cruise, there is very little load on the engine, combustion temperatures are low, and the addtional advance should not create detonation since conditions do not favor its creation.  The additional advance should make a noticiable improvement in fuel mileage as the burn will be more complete due to the early flame start.

oh, yeah...the additional advance goes away when you go full throttle as the vacuum goes away under that condition

[Chryco Psycho]

actually no , at higher |Rpm the vacuum advance is higher if it is connected properly to ported vacuum Not manifold vacuum , I leave the vacuum advance disconnected in most cases & run around 14* at idle & a total of 34-38 total , closer to 38* unless it pings

[Supercuda]

Vacuum advance exists only to improve driveability at low-load, high-vacuum running conditions (part-throttle cruise is the best example). It functions by pulling advance to a much higher amount, to allow for better combustion of the lean mixture that should exist under these conditions. On a properly-tuned engine that is running full circuitry in its fuel delivery system (injection or carb), lean mixtures should naturally happen under cruise conditions. The additional advance allows fo more efficient engine operation, and consequently, higher fuel economy and better power production during lean cruise. Without this feature, there is insufficient spark advance to complete the burn in the alotted time. This leads to lower fuel economy, and higher emissions, in addition to the lower power produced. If your car spends most of its time accelerating, or otherwise under throttle, vacuum advance is not going to be missed. If you drive your car like they were for most of their former lives (shopping, cruising, commuting), you will find some benefit in properly utilizing this distributor feature. Basically, if you only beat on your toy, don't worry about vacuum advance; if you actually drive it like a car now and then, use it.

[Aussie Challenger]

   Supercuda's explaination, if your car is surging then it is over advanced and you might need to pull the max advance back to 34* or it could be just slightly lean on light throttle, I assume that it is a stick and not an auto.   

[Strawdawg]

actually no , at higher |Rpm the vacuum advance is higher if it is connected properly to ported vacuum Not manifold vacuum , I leave the vacuum advance disconnected in most cases & run around 14* at idle & a total of 34-38 total , closer to 38* unless it pings

Actually, yes... 

Ported vacuum is exactly the same as manifold vacuum once the throttleblade is cracked enuf to uncover the port that the ported vacuum is connected to. 

The only difference between ported vacuum and manifold vacuum is the location of the two ports.  The ported vacuum point is above the throttleblade so that it does not see manifold vacuum until the throttle is opened a bit..then the manifold vacuum is able to see this port and supplies the same vacuum for all practical reasons to it as it does to the manifold vacuum port which is located beneath the throttleblades. 

From this point onward, as the blades are opened, both ports are producing manifold vacuum.  If the carb is adequately large enuf, then both drop to the same minimal vacuum number at wide open throttle under load and the vacuum induced advance goes way.

People generally do away with the vacuum advance mechanism on race engines to remove any potential jitter in the timing curve that might result from the vacuum mechanism not being fixed in place.

 

[Chryco Psycho]

no the ported vacuum is under the venturi in the carb & creates more vacuum as air speed through the carb increases so it will increase not drop at WOT

[Strawdawg]

the port is above the throttle blades, or under the venturi, as you like, but, it is exposed to more vacuum as the blades are opened...it does not draw its source of vacuum from the venturi effect, but from its exposure to manifold vacuum as the throttle is opened. 

This is well documented by Urich and Fisher in the Holley book and by numerous others. 

Ported (timed) vacuum vs full time manifold vacuum has been debated for years.  Ported vacuum was a crude means of improving emissions in the mid '70s. 

I am a proponent of using full time vacuum straight from the lower side of the throttle blades....but, either way, both sources of vacuum-ported, or manifold, go away at high load wide open throttle conditions. 

[dodj]

My 'ported vacuum source on the pass side (holley 3310) has noticeable vacuum @ idle. If I read the above posts correctly, there shouldn't be any vacuum there with the throttle plates closed?

[Strawdawg]

if the blade is set in the normal setting with relation to the transfer port, it should be virtually zero...if the blade is open enuf at idle, it will see a little

should note, when I saying transfer port, I am referring to the idle transfer port on Holley style carbs

[Chryco Psycho]

OK so if the ported vacuum in the venturi drops to Zero @ WOT as you suggest what is pulling the fuel out of the boosters