Vacuum Advance

Vacuum Advance What does it do?

ok... lets simplify things.. start with the theory of ignition...

It take X amount of time for Y air/fuel mixture to get to full burning at Z pressure.

Ignoring the efficiency of the intake system. If you accelerate at full throttle from 2krpm to 6krpm then the time taken to burn stays the same.

As the engine goes faster you need to ignite it earlier so that it gets to full burn at the same point in the cycle. It is this that the springs and weights control in the dizzy.

As the dizzy spins faster the weights fly out against the springs and change the point it fires the spark. "max advance" is, on mini dizzies at about 4krpm beyond that they don't advance any more.

To measure it you run the engine at 4krpm and use your timing light in the normal way... ish. now that's all find and dandy at WOT but at part throttle we have a problem.

The pressure drops because of the restriction of the throttle valve. This means that the mixture takes longer too burn at wot.

This is what the vac advance for. It senses pressure in inlet manifold and advances timing as needed. does that make sense now?

Article by Ade added 29 12 02