Originally Posted by
Fiesta_Jed
when you say "make peak power to soon" do you mean the mixture will explode befour, 15 degrees after tdc?
and det being a second explosion after the spark plug has fired.
Det is an explosion, no "second" bit about it, there should not be an explosion in normal operation of a petrol engine, it should be a burn only, and yes around 15 ATDC is about the optimum I believe.
I was always under the impression that because you were filling the cylinder more it would take longer to burn and theoretically would need more ignition agvance.
With a dense mix (ie a over 100% ve filled cylinder) the risk is great of exceeding the maximum peak cylinder pressure that the fuel can withstand before detonation, so you have to fire later so that the piston is well an truely on its way back down again before that peak is reached.
If you fire too late though, you lose efficiency, in which case your timing is too high for that scenario.