Originally posted by DanM
Thanks a lot man.. I appreciate all the answers. More...
How does retarding ignition timing help to reduce detonation? Logically, you would think that if the mixture is going to detonate, it would do so at the point of greatest compression (exact TDC) .. so why does moving the spark later after TDC reduce detonation?
spark ALWAYS happens after TDC. when you set your ignition to 12 degrees before top dead center, by the time fuel ignites, the piston has passed TDC. i stated that in my post.
if you retard the spark, the piston has moved away from TDC... therefore being less compression on the mixture compared to when the piston is at TDC. if you have less compression, you have less chance of detonation. when air is compressed, it heats up due to friction in the air molecules. if you over compress it, it will auto-ignite. this is how a diesel engine works... they have no spark plugs as they rely on auto-ignition for combustion.
so, if there's less compression due to the location of the piston during ignition, then your octane requirement is lowered. advancing your ignition increases your chances of pre-ignition, so octane levels must be higher to control the combustibility of the mixture as it's compressed.
Regarding the valve timing: Understaning that since it's a SOHC, and I can't change the intake/exhaust timing relative to eachother, what would be the effects of advancing or retarding valve timing? I understand what you were saying about the benefits of opening the intake valves earlier .. however that also means you're closing them earlier, and opening the exhaust valves earlier too. What overall effect does this have versus opening everything later? I understand that a dyno is really the only way to properly tell, but are there any general rules of thumb?
for single cam motors, advance for more power. retard if you're retarded or boosting.