V8 engines fire every 90 degrees. It would take a 16-cylinder to get a cylinder firing at the 45-degree point. The ability to handle very early iginition timing is a factor of engine load, as well as speed. The idea is to have the flame front from the burning fuel meet the piston crown just as the piston rocks over TDC, and get the maximum amount of time to push the piston down the bore. As speed increases, the piston's dwell time at the top of the stroke is very short, and the flame front travels across the combustion chamber at almost a constant speed, regardless of RPM. Based on hundreds of dyno runs I've done on small block race engines, 36 to 37 degrees of timing advance is where these engines run best at full load, 2 inches of manifold vacuum or less.
As the vacuum increases, (light throttle) the timing can be advanced another 8 to 10 degrees for fuel economy reasons, as long as the timing is retarded again as soon as the throttle is opened and vacuum drops during acceleration, passing, pulling uphill, etc. A vacuum advance unit does this on older engines, or electronic timing advance accomplishes the same result on computerized engines. I've seen timing advance of up to 55 degrees on light-throttle coast conditions on a scan tool during road tests.
Jerry
Edit: The timing advance makes the plug fire before top dead center, not after. The plug fires, and starts the fuel burning BEFORE the piston reaches the top, not after. As speed increases, the plug must fire sooner, to get the flame and the piston together at the right time.
Jerry
Last edited by Hotrod Lincoln; 06/28/2010 5:03 PM.
