Glen wrote:Then can't the jetting just be changed to suit??

I must have misunderstood.. What exactly does the 'power' part of the graph refer too??

I take it some may have heard about Nology leads and silver spark plugs.. Would folks also say that these would be bad news for the motor as they create more spark and in turn more heat?? But would it not come back to jetting to suit??

I think the main factor that affects cylinder temperatures IS the jetting, and as you will know, lean = hot, rich = cool as the excess fuel helps to remove heat from the combustion chamber. You can run different heat range plugs, but I don't think this makes the engine run any hotter or cooler, it just dictates the operating temperature of the plug itself. Too cold and it won't get to it's self cleaning temperature, too hot and it will melt and possibly cause pre-ignition.

Sound is pressure waves travelling through air, a mechanical vibration, and there is energy associated with it (Joules, J), and hence power (Joules/Second, W). So that's what the power part is referring to. A -3 dB drop in sound level corresponds to a halving of the intensity of the source.

I would have thought that a stronger spark could only be good, faster burn > bigger bang > more power!

Hopefully I have my facts straight here.