My setup is a little more agressive than most. I'm running 12 initial and 26 centrifugal for a total of 38, all in by 2250 rpms. I could get away without a vacuam advance if I had to. I really need for my vacuam advance to drop quickly, and I need the earlier timing at idle to keep it from getting too hot of EGTs. My cam is a solid on 106 cenrterline 110 LDA with 224d @.050. Without manifold ported advance it lopes and runs hot. I could put a 10L arm in it and increase the initial to 18, and it would probably tolerate ported advance better.

My engine needs the advance at idle and less on the road.

 

Chuck, manifold vacuum does work the way you describe, and it is good that it does. Using your example, if we push on the gas the vacuum drops and the dist retards the timing. This is good and exactly what you want, suppose we are hitting the gas because we are climbing a hill or passing someone. The engine has to have the timing backed off when under load, as the engine speed climbs enough the vacuum will come back and pull the advance ahead as soon as the engine needs it. Once again, ported vacuum on the dist can is WRONG! DF

 

 

There is really no good point in arguing with you about this, other than the fact that others may be misinformed if they believe what you think. Vacuum advance was designed to be used off of ported vacuum. It was soley designed for better gas mileage and not for retarding the timing when going up a hill. As an automotive engineer, I know this to be true. There may be some instances where you would want the timing to be retarded when the throttle is opened, but for the most part, the timing needs to be advanced as the engine speed increases. It needs to be a smooth advance curve. Manifold vacuum drops as the throttle is opened. This happens every time, even if you are not going up a hill. This would cause your timing to be retarded each time you pressed the gas. Again, your engine may work fine this way, but it is not optimum. It would not be fair to others to let them think that manifold vacuum is the right way to hook it up. Look in any old car manual and you'll see the routing of the vacuum advance line. There is even a special place on many carburetors just for the vacuum advance (ported). Anyway, there is no sense in arguing over this. Please look in any manual that describes the function of a vacuum advance and read about it yourself.