Camshaft Lift and Duration Theory QUESTION: Could someone please explain what the advantage to having short duration like my 270 over say 320 would be? or the advantage a 320 would have over a 270? ANSWER / EXPLANATION: One would think that the ideal camshaft would have 180 degrees of duration and zero degrees of overlap. That is, the intake would open precisely at TDC, close at TDC, and cycle through the compression and power strokes until you were at BDC again, where the exhaust valve would open for another 180 degrees until you reached TDC at the beginning of the intake stroke. Some have even commented that the ideal valvetrain would open the intake valves instantaneously at TDC, leave them open for 180 degrees, and then slam them closed instantaneously at BDC (likewise for the exhaust valves). This would work fine on an engine up to about 2000 RPM, after which power would be seriously compromised. The main problem is that air has momentum. Because of this, the intake valve must open before TDC in order to make sure that the valve is open far enough to allow the incoming air/fuel charge in with the least amount of restriction once the piston begins to move downward. Likewise, exhausted gases that leave the combustion chamber create a vacuum behind them that is used to assist the flow of the intake charge into the combustion chamber, hence the need for a certain amount of overlap (ie: when both the intake and exhaust valves are open between the end of the exhaust stroke and the beginning of the intake stroke). This technique is called "scavenging" and is present it properly designed exhaust manifolds as well (which time each exhaust pulse so that each one helps draw the next ordered pulse out of the engine). The opposite is also true... long overlap allows the intake charge to help push the exhaust charge out of the cylinder if it is moving fast enough. Finally, there is funny physics going on between the crankshaft's circular motion and the piston's linear up & down motion. Around TDC and BDC, the crankshaft can rotate almost 20 degrees in either direction while moving the piston downward very, very little. The piston does not move at a fixed, linear velocity inside the cylinder. Rather, it follows the path of a sin wave (eg: accelerating from TDC to a point exactly halfway down, where it decelerates to a stop at BDC). It all has to do with a factor called "volumetric efficiency", or "VE". VE is a measurement of how efficient an engine is at drawing in air into the cylinders. A cylinder has a fixed volume while the piston is at BDC, and an engine with a VE of 100% should be able to cram in exactly the same volume of air as the calculated volume of the cylinder. Because of restrictions in the intake, however (primarily the venturis on carburetored engines), most stock engines have a maximum VE of around 80%, while most race engines hover around 95-100%. With the proper cam and/or ram air