The Ford Barn

Ol' Ron · 03-13-2016, 12:22 PM

Richard wrote a tuning post a few weeks back that explains everything. Remember the Carburetors and ignition systems are tuned for cruise conditions. Most street driven cars spend 90% of the time at part throttle, where the fuel mixture is lean. This requires more advance, which is supplied by the Vac adv. On acceleration the Power valve opens creating a rich mixture and the vacuum advance drops out and only the mechanical advance fires the plugs. Testing as shown that between 18-22 degs advance provides the best powser and 2428 the best economy. You have to consider the Air/fuel ratio as well. 14.7 is the norm in cruise bit 15.5 works very well in a higher compression engine with a lean mixture. However all this depends on the combustion chamber design, where piston to head clearance is the critical factor.
Now aren't you glad you asked??

03-13-2016, 12:22 PM	#3
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,985	Re: Vacuum Advance Richard wrote a tuning post a few weeks back that explains everything. Remember the Carburetors and ignition systems are tuned for cruise conditions. Most street driven cars spend 90% of the time at part throttle, where the fuel mixture is lean. This requires more advance, which is supplied by the Vac adv. On acceleration the Power valve opens creating a rich mixture and the vacuum advance drops out and only the mechanical advance fires the plugs. Testing as shown that between 18-22 degs advance provides the best powser and 2428 the best economy. You have to consider the Air/fuel ratio as well. 14.7 is the norm in cruise bit 15.5 works very well in a higher compression engine with a lean mixture. However all this depends on the combustion chamber design, where piston to head clearance is the critical factor. Now aren't you glad you asked??