I am a fan of big camshafts. By "big cams", I mean duration and overlap as
well as lift. There are lots of misconceptions about the role of a
camshaft in performance. The purpose of a camshaft is, of course, to open,
hold open, and allow the the valves to close in a set pattern and at
certain degrees of crankshaft rotaton.
If a cam opens sooner and closes later, more time is allowed for cylinder
filling. Given certain limitations, larger overlap at higher rpms is
always better.
Most will say (erroneously) that a bigger cam merely moves the rpm point
of the horsepower peak. This is certainly true of torque, but horsepower
increases with the movement of the torque peak upwards. Horsepower is
simply torque adjusted for rpm. Thus, as rpm increases for a given torque
figure, horsepower increases as a factor of the rpm increase.
What is the downside to a big cam? There are a couple of them. the first
is the loss of torque at lower rpm. In a street car this is a bad thing,
but in competition, it is easily overcome by the use of lower rear end
gears and closer spaced transmission gears. The other downside is valve
train instability. This has greatly handicapped engine development in the
past, but, with modern technology, this handicap has been largely
overcome. For example, F1 cars run at a staggering 19,000 rpm. That is
why they are able to get so much horsepower per liter.
I will discuss carburetion next.
Marcus A. Pryor
For a new age of reason
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