Originally posted by Moore-VARIO
Many different factors affect the "timing" of the ignition of the fuel / air mix (i.e. fuel quality and mixture setting, ambient air temperature, engine temperature, fuel temperature actual compression ratio, inlet & exhaust timing, muffler/type, etc.). This is in contrast to a gasoline spark engine where the ignition timing is influenced predominantly by the spark timing, which can always be optimized for best engine running at any rpm.
I know from running diesels that smaller props allow the engine to turn faster, but require timing advance (more compression, leaner mix, or some other source of heat), so ignition starts in time for the fire to be in progress when the piston passes top-dead-center. Similarly, larger props will turn slower, so the timing must be retarded (reduce compression or run a richer mix).
Can anyone lay out the related equation for glow engines?
My guess is smaller/lower pitched props need timing advance (more heat; so add nitro, remove head shims to increase compression, run leaner, or use a hotter plug). Larger or higher-pitch props need retarded timing (less heat; so reduce the nitro content, add head shims, run richer, or use a cooler plug).
Have I got it right? Especially, have I got the nitro change right, or do I have it exactly backward?
Thanks in advance to those more knowledgeable,
Marc