How is the rpm range of a cam calculated? Or is it calculated? Is there a way to calculate what the rpm range would be given X info, or is it found by more concrete means(dyno testing, 1/4 runs, physical measurements, etc..) I ask b/c I want to better understand the relationships b/t cams, stall, tire diameter, and rear end gearing, among others.

 

Hey man. I got your call, just been so busy lately. I know different cam manufacturers and designers all have different ways of finding a cam's usable RPM range. Comp has a spintron which simulates how the cam will act inside a motor with the full valvetrain. A lot of people say that the Comp ranges are way off, though, so I think it will depend on the motor it's in more than anything else. My comp cam was right on in terms of predicted RPM range, which is relatively low for its duration.

I think it comes down to the valve timing, overlap, heads and those minute details that I know less about than others here on the board, so maybe they will chime in.

 

I think what you really want to know is the engine's rpm range, which is determined from the entire engine combination, with the valve timing (cam) only being one of the players.

If you study the torque curve of an engine you can pretty much tell where the engine does it's best job.

Example: If you have a 2000 rpm stall converter and the engine has a big torque hole below 3000 you are going to have a problem launching. If the engine was an LT1, one of the reasons for the "torque hole" could easily be too much cam or more properly, the wrong cam.

The same cam in different displacement LT1s with different heads will act differently and have a different "rpm range" where it works. There is no "one fits all sizes", except maybe in condoms.

At the risk of getting bashed by the brotherhood, let's take the LE series of cams. An "LE3" cam to be used in a 355 may not be exactly the same as an "LE3" meant for a 383 or a 406. Each engine combination has it's own valve timing needs. If you assume that the larger engines need "more" cam the "rpm range" for an LE3 meant for a 355 might be higher than a "larger" LE3 meant for a 396 with a lower rpm range.

Confused? It's all about the combination of displacement, head/intake flow characteristics, etc. I'm a biological kind of guy so I think of the cam as the "brain" of the organism (engine). It's not just the brain that determines whether a small, mildly muscled athlete or a large, fully trained athlete has more speed or endurance, it's the combination of hard parts and a brain that tells them how to work. More confused now?

To answer your initial question, YES there is a way, but it's not necessarily simple. Lots of things in life are like that.

Jon