Originally Posted by boogaloodude

Great questions, some are easily answered, others are a little more complex. I'm no expert, just an old hot rodder and hobbyist.





There's 2 types of compression, static and dynamic. Static compression is simply the ratio how much the volume of air inside the cylinder can be compressed with the valves closed. In other words, the difference in volume with the piston at the bottom of the stroke and the top of the stroke with the cylinder sealed. But since the intake valves close after the bottom of the piston stroke, and whle the piston is traveling upwards towards the heads, some of the trapped air will be pushed back out the intake valve while the piston is on it's compression stroke. Closing the intake valve earlier builds dynamic compression sooner by sealing the combustion chamber earlier. Close it early and you'll build torque early. Later, and the torque curve will have to wait for the motor to spin faster. Increase the static compression with domed pistons or a smaller combustion chamber, and you can run later closing cams without suffering a big low rpm torque penalty.

If you shoot for 9.2 corrected compression, you will get a bike that's easy to tune, and will run on pump gas without detonation problems. Use Intake valve close to select the cam that gets you closest to that, and you'll get a bike that works well right off idle. With the 103 in my bike, doing the calculations for adjusted compression suggested a cam with an intake valve close of between 34 and 38 degrees would keep the corrected compression between 9.1 and 9.2. The SE204 cams, with an intake close of 34 and moderate lift and duration looked like a good fit. It's also why the SE255 cams are a better fit for the 96 motor than for the 103.



Early closing cams will generally have shorter duration, later closing cams will generally have longer duration. More duration is important as the motor spins faster, because the time the valve is open becomes shorter. The sweep of the cam is the same, but it's rotating faster in the motor, shortening the length of time the valve is open, At higher rpms, a short duration cam won't have enough time to effectively fill the combustion chamber, which is why the classic torque cams like the SE255 (211* dur) are said to "fall off" at higher rpms. Racing cams have long duration, because they spend so much of their time at high rpms.





Centerline is the line through the point of maximum lift on the intake lobe, and passing through the center of the cam. It's then compared against TDC, and defined as the number of degrees ATDC. It's basically half the duration minus the point where the intake valve opens. It's also closely related to Lobe seperation angle, which is the difference between intake and exhaust centerline. Compare the intake centerline against the LSA to see how much advance the cam has ground in. For example, the SE204 cams in my bike have a intake lobe center of 96* against a LSA of 104, for 8* of ground in advance.

Overlap is the amount of time in degrees that both the intake and exhaust valves are open at the beginning of the compression stroke. As the exiting exhaust gasses leave the cylinder, if you also open the intake valve, the suction will pull in some of the fresh charge. The scavenging effect is why some performance cams are so much more effective than stock cams. Unfortunately, the EPA hates overlap, because some of the incoming, unburned fuel will escape with the outgoing spent gasses.






It's magic. And playing around with different cams in different types of engines. Harley twinkie cams have a lot in common with small block Chevy cams, which I spend a lot of time with in my early hot rodding days. Cams are a balancing act, and everything has to be in harmony. Exhaust designs also play an important part, especially with longer duration cams. The scavenging effect during overlap can be enhanced with effective exhaust, and keeping the exhaust gasses flowing quickly plays dividends for the incoming fuel charge.