This is the number of crankshaft degrees that the valve is off its seat. In these data sheets, Piper give you this figure as well as the timing figures. To calculate the duration, add the timing numbers together and add 180.

EXAMPLE: A cam with timing of 23/67 added, totals 90, plus 180, gives 270 deg.

The number of degrees crankshaft that the inlet and exhaust are open at the same time. Calculate overlap, add the opening number of the inlet cam to the closing number of the exhaust cam, ie the first and last numbers of the cam timing. Using our same example of the 23/67 inlet and 67/23 exhaust (usually referred to as 23/67 - 67/23), add together the first and last numbers (23 and 23) and the total (46) is the overlap. In general terms the larger this number or the greater the overlap, the hotter the cam.

The position of the camshaft relative to the crankshaft. This is expressed as the number of degrees that full lift occurs after top dead centre (tdc) in the case of the inlet and before tdc for the exhaust. This figure is included in the Piper technical data, but to calculate this, take the duration figure and divide by 2.

EXAMPLE: with an inlet cam of 23/76, the duration is the addition of these two numbers, plus 180, equals 270. Then divide by 2 resulting in 135. Deduct the number of degrees before tdc that the valve started to open, ie 23 degrees - the result 112. The valve is correctly timed with full lift 112 degrees after tdc.

The opening and closing position of inlet and exhaust valves relative to the crankshaft. As figures before and after tdc and bdc.