Cam Profile Rate

Sometimes fast rate, slow rate, or aggressive might be the term used. Like most camshaft terms, they are used in marketing to sell camshafts. What makes a cam profile fast rate or slow rate? What makes a cam profile aggressive? Sometimes the term only applies to the profile ramps, such as the ramps are fast or the ramps are slow, the ramps are aggressive.

If you look at enough cam profiles, some definitely move the valve faster than others. Basically the movement of the valve (or tappet) per degree of lobe rotation is considered the cam profile rate. Velocity is the actual term used when designing cam profiles. How quickly that velocity is reached is the acceleration which is also part of the rate. In flat tappet profiles the maximum velocity is determined by the tappet face diameter. Roller profiles will usually have a faster maximum velocity and most are about the same for a specific profile. The acceleration will need to be controlled on a roller profile to prevent excessive negative radius in the flanks.

Some ramps will open the valve faster, some ramps will close the valve slower. It's just all part of the design. If someone is trying to sell you a camshaft that has fast ramps than slow ramps are bad. If the maximum velocity is higher than it is better. Sound familiar. If you give-in to this type of marketing, read my last post. Maybe read it a few times, it is very short and to the point.

The best way to compare cam profile rates is by looking at the duration at the higher lifts. There is a reason most camshaft manufacturers do not publish duration figures past 0.050 tappet lift. Measure the duration at 0.100 intervals. Compare the duration numbers. If the camshafts are the same at 0.050 but one is larger at 0.100, 0.200, 0.300, and so on, the larger one has a faster rate. More importantly, it has a faster rate where is really matters. The faster rate at the higher lifts gives you more area under the lift curve. What goes on above 0.050 on a cam profile is a whole lot more important than what goes on below 0.050 as far as making power. Below 0.050 has more to do with the affect the cam profile has on the valve train (valve springs, push rods, valve seats, valve lash, etc.)