Choosing a camshaft
Seriously, this is the trickiest choice you have to make. At least that’s the impression you can get when looking in some books. 30-40 pages on the subject. And it is true that choosing a camshaft has great impact on the power and personality of the engine. But there are some ground rules, and if you keep them in mind, you’ll get what you need.
A cam is known by its duration, lift and profile (how fast the lift is). We’re not going into the profile, because while it’s important, there’s no real measurement or at least it’s not publicized by cam manufacturers. Buy a brand name cam and they’ll have the research and engineering covered to give you a good profile.
Lift is quite simple, because more is good. The higher lift the more power and it doesn’t change where in the rpm band the power comes. But there are limitations of course. One limitation is the profile, there’s simply a limit to how big a difference between the inner circle on the cam (where the lifter is at no lift) and the top of the cam (where the lifter is lifted maximum). To create high lift, the cam has to be steep, and at one point it becomes so steep that the cam and lifters get worn too fast. How steep is also part of the profile, so don’t worry about this, the cam manufacturers have this covered. Roller cams are much more forgiving on this, so any roller cam is normally producing more power than an equivalent non-roller cam because the profile can be made steeper. Another limitation is your valve springs and rockers, how much physically can they lift? And in extreme situations the valves could hit the piston. Bottom line is that you should go with as much lift as the cam manufacturer tells you your engine can physically take.
Duration is measured as the number of degrees where the lifters are lifted more than 0.05”. A standard cam would be around 200 degrees, a very wild cam around 250 degrees. So what’s a wild cam? Having more degrees open should let more air/fuel mixture in and out, so that’s more power. But there are side effects. The first is that power comes at a higher rpm, so just like a big carb or single plane manifold, you risk having the power in an rpm band where you rarely drive and where the engine might break. Second side effect is less vacuum and a rough idle. Truth told, the sound of a long duration camshaft engine at idle is one of the sweetest sounds. But drivability goes out the window. In conclusion, if your entire engine is built to be a 4000-7500 rpm screamer, get plenty of duration, if not, be conservative, and buy a lot less duration than what everybody tells you. Our advice for a non-roller camshaft would be max 235 degrees, and you probably be happiest around 225 degrees.
A note on engines with blower/turbo. Here the intake side is aided by pressure, exhaust is not, so you’d need a camshaft with more exhaust duration than intake duration.