Re: hot rod sound
The lumpy idle sound typical to most high performance engines comes from having a camshaft with a long duration wherein the timing events of the intake (valve) and the exhaust (valve) have a fair amount of overlap.
That means that the intake valve is opening and 'overlapping' during the exhaust stroke where the exhaust valve is also open, etc.
The reason for this is as the engine speeds up to high rpm the overlap is required to produce an efficient (thus powerful) result by having the events work in good unison for both filling and evacuating the cylinder....but
only really at high speed (ie: above 5000 rpm generally)
The downside is that at low rpm you have very little engine vacuum for things like power brakes and you are more likely to suffer from plug fowling, etc.
The other downside is that low speed performance and particularly torque suffer greatly with a very long duration camshaft which is a disaster in a heavier vehicle like a truck, etc or anything over 2 tons.
The camshaft is one of the most mis-understood and, more importantly, most incorrectly spec'd parts that people choose and put into their engines with the results often being very disappointing for most of their driving needs.
Always talk to the tech people at the camshaft manufacturer (Comp Cams, Lunatti, Crane, etc) to discuss what would work well with your application.
My two bits.
BP
