In this second part of our three-part series on camshafts, I'm unraveling the inner workings of camshafts. If you haven't read the first part, I highly suggest you get a copy or at least have some basic knowledge about camshafts. In this month's issue I'll talk about the basic power concepts of camshafts and the detailed terminology used to describe them. You'll see that even though two camshafts might both be advertised at 290 duration, there could be huge differences between them.
Most of you are familiar with the terms duration and lift on camshafts. The more duration and the larger the lift the more power, right? A lot more goes into cam design than you think and sometimes a well-designed cam can make power with smaller advertised measurements. Here's a quick engine primer to get you thinking about how an engine makes power. On a four stroke engine four events happen to make one power event: intake, compression, power, and exhaust.
The crank makes two revolutions in this process. For each revolution, the piston starts at the top, travels down, and then goes back to the top. The very top position is called top dead center (TDC), and the very bottom position is called bottom dead center (BDC). The intake stroke consists of the piston starting at TDC and the intake valves opening, allowing fresh air and fuel to enter the engine while the piston is traveling down toward BDC. Next is the compression cycle where the intake valves are closing and the piston travels from BDC back up toward TDC. With the intake charge being compressed, the ignition sets off the charge near TDC and begins the power stroke. The combustion pressure pushes the piston down from TDC back to BDC. The final cycle is the exhaust cycle where the exhaust valves are open and the piston is pushing the exhaust gases out as it travels up to TDC. The cycle repeats itself with the intake valves opening and the piston traveling back down, drawing in fresh air.
Camshafts control when the intake and exhaust valves open and close. Camshafts also control how far the valves open. Seems pretty simple: Open the intake valve when the piston is at the top of the intake stroke and close it when the piston reaches the bottom. Same for the exhaust valve: Open after the combustion stroke and close it back at the top. However, if we did this we'd have an engine that wouldn't make good power and it would all be at a very low rpm. Realistically the intake valve opens early (before TDC) and close later, well after the piston reaches BDC and it's already on the compression cycle. The same with the exhaust, it opens early before the power stroke is fully completed (before BDC) and closes after the piston reaches TDC of the exhaust stroke. The precise timing of these events determines how the engine will run through the rpm range. Certain timing events will benefit high rpm running and make more peak horsepower, while others will make more power down low at the cost of upper end horsepower. If your brain hurts already, take a rest, re-read, and get ready for some more cranial punishment. Ready? Most of you know what your compression ratio is, but what if I told you that your compression ratio changes when you change your camshaft? All automotive manufacturers and aftermarket companies rate engines and piston specifications as a static compression ratio. Static compression ratio is the volume of the cylinder when the piston is all the way down at BDC, compared to the volume when it's all the way up at TDC.