CAM TIMING FORMULA
The formula below is intended for professional engine builders and will calculate the timing events in your engine. You are required to fill out the camshaft specifications boxes in yellow and the formula will calculate the correct timing events. Please note: This formula will calculate timing events at any duration.
|Intake Valve Opens||BTDC||Intake Valve Closes||ABDC||In Duration|
|Exhaust Valve Opens||BBDC||Exhaust Valve Closes||ATDC||Ex Duration|
|Time Intake Valve Fully Open||ATDC|
|Time Exhaust Valve Fully Open||BTDC|
Lobe Separation: is the amount of degrees the cam turns from the center of the exhaust lobe to the center of the intake lobe on the same cylinder. Also known as lobe centre angle (LCA).
Wide lobe separations (114-118 degrees) gives reduced valve overlap and produces higher manifold vacuum. Commonly used in engines where economy and exhaust emissions are important. Supercharged and Turbo charged engines can also benefit from wider lobe separation due to reduced valve overlap.
Closer lobe separations (104-108) degrees) will increase valve overlap and will give better mid to high rpm power when used in performance engines.
Duration: is the length of time that the valve is held open by the cam. This is measured by the degrees that the crankshaft rotates. More degrees of duration will make the engine operate in a higher rpm range. There are two ways of rating duration.
Advertised Duration was originally the S.A.E. automotive standard and was measured from .006” of valve lift. Over the years this has been altered by most performance cam makers to make their cams look hotter, or different, than the specs of their competitors. Valve lift points as low as .002” up to .008” are used and this can add up to thirty degrees to the advertised figure. Even when the cams being compared are all measured the same way the figures can still be misleading if you don’t know what the cams were designed for. Cams designed for quiet street operation can sometimes show higher .006” duration numbers than performance cams of the same rpm range.
Duration measured from .050” of cam lift is the best for comparison of cam specifications because most of the variations in cam design are reduced and the valves are open enough to start getting adequate flow past them. Most cam makers give accurate .050 ratings and good comparisons are possible between cams of the same type.