cam degrees?

[Foxfan88]

ok you know how a cam has say 480 degree duration, degree of what??
so its like the intake valve is open 480degrees of a crank revolution?

a circle is 360 degrees so the valve is open even longer after the cranks turn??

hard to understand....

 

[cobradvm]

Where did you come up with 480 degrees of duration - that's quite the cam! Typically duration is simply the time that the valve is open, measured in degrees of crankshaft rotation. The crankshaft will rotate twice for every camshaft rotation. Also duration is usually measured at 0.050" of lift. So put a degree wheel on the crank, find TDC. Put a dial indicator on the intake lifter. Turn the engine over until the dial reads 0.050" of lift and see where the crank is at with the degree wheel. Lets say it is at 2* BTDC. Continue to turn the crank until the valve has opened all the way and has closed to the point where the dial is again reading 0.050" of lift. Lets say this is now 32* ABDC. The duration of the intake valve is the 2* BTDC, plus the 180* to go from TDC to BDC, plus the 32* ABDC for a total of 214*.
If you actually had 480* of duration of an intake valve, the engine would probably not run, since you would never build compression - this would mean that the camshaft lobe kept the valve open 240* of its rotation.

HTH

Howard

 

[Michael Yount]

Example - stock HO cam

Peak lift w/1.6 rockers - .444" - means the peak lift the VALVE sees is .444"; peak lobe lift is .444/1.6= .278". Means the peak of the lobe is .278" taller than the base circle.

The cam has 220 degrees of duration at .050" lift. That means the crankshaft rotates 220 degrees during the time it takes the cam lobe to move from being .050" open to .050" from being closed. It has 282 degrees of advertised duration -- duration at .006" of lift. Means the crank rotates 282 degrees during the time it takes the cam lobe from being .006" open to .006" from being closed.

Remember - the cam turns 1/2 speed of the crank. So it takes 2 crank revolutions for the cam to turn over once. So 220 degrees of crank rotation is only 110 degrees of cam rotation.