Vacuum Advance

[bales 1]

I know that this has been discussed a lot before, but everyone seems to say that it is there for part throttle economy but no-one actually explains how it does this.

 

My car currently doesn't have it connected and I have not really had any major mpg losses, my dad has always messed with cars from a young age and all the engines he has built (old engines mind you) he never bothers using the vacuum advance.

 

I know that a conventional distribitor is basically 2D as it simply works in relation to RPM, the vacuum advance is there to advance timing at times other than full throttle high revs situations. At full throttle anyway the VA isnt doing anything as the timing will be as advanced as it can be anyway. Hence why it doesn't effect peak power.

 

Race engines don't use them as supposedly the baseplate would vibrate at high RPM and cause the timing to be out, and I think this is where the the myth stems from that you don't need VA as race engines don't use it.

 

However in the actual sense of how the VA helps to increase MPG, this is what I struggle with. Is it because at light throttle low load the cylinder filling isnt as good and hence the burn is slower and so needs more advance to burn completely. I can understand that if that is the case.

 

I have also heard it mentioned that the VA helps snap acceleration say at mid revs when you just floor it, is this true as I would have thought that as soon as you open the throttle fully you lose vacuum unitl the engine starts to speed up and pull more air in. This is also saying I suppose that you need more advance at heavy load sites as in lower revs and high load, but this contradicts the fact that at poor cylinder filling you need more advance as it is slower burning. Surely in the low revs high load situation you are getting better cylinder filling and hence it burns quicker and doesnt require the same amount of advance, until at least the engine speed reaches a point where the time the piston stays around TDC is more critical than the time it takes for the mixture to burn. If that makes sense which I am not sure it does from how I have typed it.

 

Maybe I can put that better, if the engine is at 3500rpm at light throttle and at full throttle, which requires more advance?

 

Sorry for the long post but it one of those things that everyone quotes "increase mpg" but it is never fully explained why. Hope someone can help me with this, cheers.

 

Alex

Edited August 25, 2006 by bales

[Rob_the_Sparky 9]

You seem to have already answered your own question, e.g. you need more advance at light throttle openings due to the longer burn times due to the partial cylinder filling. Without this extra advance it is less efficient hence fewer mpg.

 

FYI, I noticed about 2mpg between no vacuum advance and with vacuum advance. Small enough to not be noticed unless you are paying attention.

 

Rob

 

P.S. Of little use on a race car because you are never going to be crusing on part throttle! Just because it isn't helpful on a race car does not mean it is not helpful.