To the beginner, cam timing figures and terminology can seem a bit strange, this page will hopefully explain what all the figures etc. mean.
I run a Piper 285 cam in my 2.1. The figures quoted by Piper read as follows: Inlet Duration: 270 degrees Exhaust Duration: 290 degrees Valve lift: 11.81mm Timing (open/close) inlet: 27/63 exhaust: 73/37 Full lift: 108 degrees So... what does all this actally mean? Lift: This is the amount that the valve is lifted off it's seat by the cam and rocker assembly. Cam lift is the amount the cam moves the valve, valve lift is the cam lift multiplied by the rocker ratio. Most cam manufacturers list lift as cam lift. Duration: This is the length of time, measured in crankshaft degrees that the valve is off it's seat. Cam manufacturers will include this information in the cam spec sheet. The duration of a cam can be calculated from other timing figures given by the manufacturer. for example a cam with timing figures of 27degrees lead and 63degrees lag has a duration of 270 degrees 27+63+180. Lead: This is the point, measured in crankshaft degrees before top dead centre, when the inlet valve is first lifted off it's seat. For the exhaust valve, this figure is measured before botom dead centre. Lag: This is the point, measured in crankshaft degrees after top dead centre, when the inlet valve returns to it's seat. For the exhaust valve, this figure is measured after top dead centre. Overlap: Quite simply, this is the amount of time, measured in crankshaft degrees that both inlet and exhaust valves are open together. To calculate the overlap, simply add the numbers together for inlet opening and exhaust closing. Typical cam timing diagram: These diagrams can be a bit confusing, but the only thing to really remember about them is that they show 2 full revolutions of the crankshaft, Cam Timing: The position of the camshaft in relation to the crankshaft. This is expressed as the number of degrees that full lift occurs after top dead centre of the inlet valve and before top dead centre for the exhaust valve. This figure can be calculated from the figures given by the cam manufacturer. Here, I'll use the figures for the cam in the diagram above. Simply divide the duration figure by 2 and subtract the lead figure. For the Piper 285 this works works as follows: Duration = 27+63+180 = 270 degrees Lead = 27 degrees Full lift = (270/2)-27= 108 degrees Valve Timing: The opening and closing position of the inlet and exhaust valves relative to the crankshaft as figures before and after top dead centre and bottom dead centre. Rocker Ratio: The ratio between valve motion and cam follower motion. Push rod engines typically use a rocker ratio of between 1.1:1 and 2.0:1. Over head cam, direct operating engines obviously have no rocker ratio as the cam follower motion is exactly the same as the valve motion. There are more technical terms for cams, but these can get a bit too technical for here. More in depth info is available from most cam manufacturers websites. 
