Cam duration info

[Phred]

I've been reading/hearing about some confusion many home engine builders are having concerning camshaft timing-duration numbers. Such as,"my 300 degree cam dosn't have as much lift as my buddies 280 degree cam, and his seems to make more power. Whats up?" First, understand there are two different methods cam duration is measured. One is the absolute, or advertised duration. The distance in degrees measured from a point where the lobe physically starts to rise from the base circle, to another point where the lobe blends back into the base circle. The second is a standard "checking height" of .050 lifter rise, or in the case of the L series engines, valve lift. Naturally, if you take a degree reading .050 after the the valve starts to rise, and another .050 before the valve closes, the total duration will be less than the "advertised" duration. So, the first rule in describing a cam is to state, " 300 degrees @ .050" or "300 degrees total advertised duration". Without that distinction for comparison, degree numbers are useless.

Lastly, valve lift is a seperate function of cam design. Valve lift is also measured two different ways. Advertised lift, is the total physical size of the lobe, measured in thousandths of an inch. This is somtimes refered to as "gross valve lift". Then there is the actual running valve lift which is known as "net valve lift.

This is a lesser number, (which is why they don't advertise it) because of the required subtraction of valve clearance values and the amount lost (or gained) because of the rocker ratio.

I hope I didn't confuse the issue any more than it already is.

Simply be aware of the many ways of the black art of the bump stick.

Phred, the Engine Shaman