Quote:
Originally Posted by Execut1ve
It is said that the best fuel econ occurs at 80% of max torque rpm, which would be 1920 rpm.
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That sounds like a garbled version of what I have seen around here. That best economy (for acceleration) occurs at about 80% load (roughly equal to producing 80% of rated torque at the RPM) at around 2000 RPM.
For cams: Lobe center and separation are pretty much different ways of saying the same thing.
Overlap (where the intake and exhaust valves are open at the same time) is good for high RPMs and bad for low RPMs.
More duration (how long the valve is open) tends to be better for high RPMs versus low, in large part because high duration usually means more overlap.
Lift, I'm not so sure about. I believe that if you go with too much lift you wind up with less velocity, similar to if you use too large-diameter a pipe (e.g., as an exhaust primary). But obviously if you have very little lift you won't let enough air into or out of the combustion chamber.
And these sorts of things also depend on the rest of the configuration of the engine; things like valve shrouding (how close the ports are to the edges of the chamber), chamber size and shape, piston crown or dish, spark plug location, cooling, yadda yadda yadda yadda.
Some of this stuff really is still a "black art". You want to promote velocity and mixture quality at some RPM/throttle combination. The best engine guys can figure out an approximation of the different things to do with the engine to promote that, but then they have to experiment and test to get things right.
-soD