Thread: basic cam questions,`

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Cams design is perhaps the most complex aspect of engine design. There is no exact science that says a cam with these numbers is going to perform exactly this way. I will try to explain what the numbers mean and how they should effect the performance of the enigne. The gross lift is the amount the cam opens the valve. More lift typically means the valve is open more which means more fuel/air can enter the cylinder which means more power. That is fairly simple. Why does every cam not have maximum lift? It's hard on the valve gear. I'll have to explain duration before I can continue. Duration is the number of degrees of crankshaft rotation that the valve is open. There are 720 degrees of crankshaft rotation in a complete cycle. You have probably noticed that cams with short durations also have smaller valve lifts. This is because there are fewer degrees of crankshaft rotation for the valve open and close and the acceleration required to gain such a lift in such a short time becomes too great. You have also probably noticed that cams with large durations have very high working rpm ranges. An engine has an ideal air/fuel velocity in the intake port of about 2500 feet per minute. A long duration cam keeps the valve open for a long time so the fuel/air mixture has plenty of time to meander into the cylinder. So much time, that it doesn't need to move at the idea 2500 feet per minute to fill the cylinder by the time the valve closes. On the other hand the air has to rush in with a short duration cam. As rpm increases, the amount of time available to fill the cylinder decreases and this is when the long duration cam becomes an advantage. You may also see lobe seperation angle or LSA on a cam spec. LSA is the number of degrees between maximum lift of the exhaust valve and maximum lift of the intake valve, typically 110 degrees. At low rpms, as the piston travels downwards on the intake stroke, the exhaust valve will still be open and the cylinder will draw air in from both the exhaust and intake side of the engine. Not very efficient. At higher speeds, the fast moving exhaust pulse moving down the header actually creates a small vacuum that can be used to draw fresh air into the cylinder as long as the exhaust valve remains open. The sciency term for this is exhaust pulse scavenging. Wider LSA's, greater than 110 degrees, mean more torque but at a cost of high rpm efficiency. Tight LSA's can also be used to bleed off cylinder pressures in high compression engines that have problems with detonation at low rpms. I hope this helps, it's a lot to learn at once and it's very difficult to explain.