Cam selection can be really complicated. It takes a while to learn this stuff. The amount of lobe shapes is , well, crazy. While I’m far from an expert on the subject, I read a lot of the experts words. From there, I can now say I can make a decision while being a little informed.
A lot depends on what the target is. Driver, hot rod, street/stripper etc.... I get the whole lobe thing pretty good. And the different lobes out there can bend your mind up pretty good. As a basic direction when people ask me about a cam, after the above direction is discussed and the running gear to be used and the cars weight.... A cam can be selected. Hence the many questions asked before such a answer is given.
For the sake of and just for the sake of simplicity, I can offer the following which is a bit of a over simplification of the matter, but it will work to a large degree for most people out there willing and wanting to take a stab at a cam selection. And if you don’t mind wrenching and experimenting, then by all means, DO IT!
It was The only way to learn before the internet. And I had a ton-O-fun doing it!
Duration;
Is the rpm band. The duration @ .050 lift is a good way (but not the end all) to find out where the cam will generally operate. The rpm band of which it operates in is effected by a few things. Those being:
Engine size in how the bore and stroke relationship is. A lot of stroke? Lower rpm peaks. Short stroke engine? Higher rpm peaks.
Cylinder heads stock? Lower rpm peaks. Ported (& how well ported is also a key!) Higher rpm peaks.
And there’s more kids!! We will leave that for another day.
A split duration cam can help extend the operating rpm range slightly. It was originally done to help poor flowing exhaust ports. It is possible to use a single pattern cam. (And this is where those dang head flow numbers come in handy.)
For guys with stock converters, keep the duration @.050 @ 218 or less. You won’t have to change your converter for street duty. That’ll save you some bucks that you should roll over into head work or better heads all together.
Center line.
Will produce the rhythm of the exhaust - or not.... LOL!
At a 112 or higher C line, the idle will be smooth. It is also good for fuel injection, more so on earlier systems.
At 110 or 108, THAT Muscle Car sound is now present. The idle can be muscular to a little choppy.
At some 108’s and the lower 106 C lines, the idle can become rough to radical. Lower than that and your probably racing and know this stuff already.
Lift:
I always suggest getting as much lift as your heads can handle. More the merry. If your stock or enhanced stock heads flow well to .500, then a .500 lift can will do well.
If your ported heads flow well to .600, then get a cam that goes to .600. Or as close as possible.
A little more lift than the head flows isn’t a bad thing. Just keep it limited for most street duty where the car is actually used a lot.
If your trying to extract more than the average bear on a hot rod, go another .030, street stripper, - .050 inch an lift. Generally I don’t recommend more lift than the head is capable of for street, hot rods and lower powered street strip machines. It’s really not needed. Just grab what you can. It’s a few extra HP over a low lift cam. Since they cost the same.......
How Quick the valve lifts is also something you should look into. I personally would like to lift the valve by the cams lobe rather than extra rocker ratio because that is extra stress you can avoid on the rocker on down.
BUT! A quick opening valve is, IS POWER!
There can be an issue with quick opening valves on low & medium level cars. Noisy valve trains is the biggest complaint. Failed lifters is next I think. I hear it a lot.
And generally it is not needed even though the engine will appreciate it.
But I just can’t recommend a super trick, magical atomic bomb cam lobe with a lift that is stupid fast for the average guy here.
The higher the performance the higher the maintenance!
Remember that on your current or next build. When it’s with big and radical cams, you will be changing out springs. The more radical the cam and it’s lift, the shorter the springs life span.
And that’s about it in a nutshell. An over simplification that will work for you on your street car, hot rod & mild to mid level street/stripper. (Small blocks under 500 hp. Which is quit a bit on the street.)
The MP engines book had a “Tip” section that gave combos with part numbers. And yes, it included cams.
That’s my take (& story) & I’m stickin to it!
