The idea is to gently lift the valve off the seat, open it quickly, hold it open, then close it quickly, and set it gently back down. At very low lifts there's very little airflow going on so most manufacturers rate their cam's duration at a certain lift. For example, the same cam can be listed as having 300 degrees of crankshaft duration at 0.004 inch of lift or it can have 240 degrees of duration at 0.050 inch of lift. Most of the cams that I'll be testing have advertised duration ranging from 270 to 280 degrees of duration. This advertised duration really must be taken with caution as it depends on what lift the manufacturer is specifying, some specify 0.004-inch lift while others use zero lift. Let's say we have two different cams rated at 240 degrees of duration at 0.050 inch, from the graph you can see how the same duration cams can have very different lift profiles.
If one cam is more aggressive on opening and closing the valve, it can have the valve open further for a longer period, theoretically supporting more horsepower. This type of aggressive profile will be hard on the valvetrain, requiring stiff valvesprings and lightweight valves that might only last 5,000 miles before wearing something out.
To show you how duration is calculated, let's look at this example. Taking an advertised duration of 0.004 inch lift, the intake valve would open roughly 20 degrees before TDC, stay open throughout the intake cycle, and then close 60 degrees after BDC during the compression stroke. The advertised duration of this cam would be 20 degrees, plus 180 (the intake stroke), and 60 degrees, for a total of 280 degrees.
Cam Lobe Data Collection
As I mentioned in the previous article I'm using the Performance Trends Cam Analyzer to record all of the cam data. The system consists of a test fixture that holds the camshaft and allows it to spin concentrically. A rotary encoder attaches to the end of the cam to measure the rotation and a linear encoder rides on the lobe to record the lobe displacement. All this data is fed into the Cam Analyzer software through a data acquisition box.
Not to complicate things further, but the valvetrain in the Evo head is complex. It's referred to as Cam On Rocker Arm (CORA). The cam lobe doesn't act directly on the valve, it transfers the motion through a rocker arm that pivots on one end and pushes on the valve stem on the other. On the opposite and easy end of the spectrum, the new Evo X has a cam-on-bucket design in which the lobe rides on a bucket lifter that acts directly on the valve stem. In the case of the Evo VIII, the trick is getting the cam lobe motion to translate correctly to valve motion. This procedure requires modeling the dimensions of the valvetrain in the Cam Analyzer software so it can compute one valve motion to the other. The tricky part is getting these exact dimensions; luckily, I had access to a coordinate measuring machine and was able to reverse engineer the head and valvetrain to get exact dimensions. Now I'm able to compare valve motion from one camshaft to another.