Welcome to a new department in Stock Car Racing. In each Racers' FAQ, we'll go to an industry professional to get answers to common questions they receive from Saturday night racers. This month, we speak with Chris Brown of COMP Cams.
Q: What information should entry-level, grassroots racers have when selecting the right camshafts?Brown: We must know as much as possible about the engine and car. The size, surface, and shape of the track are very important. Cam selection is based on all these and more factors. All racers have egos, and sometimes we have to pry out some of the answers.
Q: What do they ask for as far as cam size?Brown: Racers often ask about the biggest cam they can use. This is a mistake. They need to understand the whole package has to work together. When I started at COMP Cams I built an engine for myself and used a "big" cam. After a few races and not going too fast, one of the old hands in the shop told me to put in a smaller cam. I listened to him and did exactly that. My car was immediately faster. That night I learned something: The right cam was faster than a big cam. Racers always want to go faster, but sometimes smaller works better.
Q: What if a track has a lift rule?Brown: Often we have to work within local track rules that call for a certain level of engine vacuum or a prescribed lift rule. When grinding a cam to fit a vacuum rule there is not much we can do. However, the lift rule is a different story. Lobe center and ramp angles leave some room to play. The biggest thing we have with a lift rule is the dynamics of cam-to-lifter operation.
Q: What about launching the lifter off the cam?Brown: This leads to a big increase in lift, maybe 0.070 to 0.100 inch over the actual measurements of the cam. A lifter is somewhat like an airplane. It is not too difficult to get together enough power to take off, but it takes many things working together to land it without damage. We used to launch the lifters off the nose of the cam accidentally. This would lead to lifter and lobe damage. Realizing what was happening, we began working with landing-ramp designs to catch the lifter safely. Remember, this only works with low lift rule cams, but it can flow a lot more air when the lifters begin to launch at about 5,500 rpm.
Q: How important is maintenance?Brown: It has become very important. With launching the lifters, the valvespring tension becomes very important. If the springs weaken excessively, the lifters may float much higher. This can cause the valve to contact the piston. This is not a good thing. I recommend getting an on-the-head valvespring checker and checking the springs at least every four weeks. The valve lash should be checked every week.
Q: What about using hydraulic lifters with the cams designed to launch the lifters?Brown: I'm not sure I would recommend it, but it should work OK with everything right. Our racing hydraulic lifters have only 0.080-inch compression as opposed to OE types having 0.200 inch. Ours should be adjusted to a preload of 0.002 to 0.003 inch. (Adjust to zero with no compression of the lifter. Then tighten the adjusting nut half of a flat. The resulting preload will be approximately 0.003 inch. These numbers will be true if you have stock-size studs that are 31/48 inch x 24 threads per inch.) Solid lifters should be used, but where track rules leave few options and racing is the name of the game, you do what has to be done.
Q: How does the driver affect cam selection?Brown: When I get a call for a cam, I ask all the usual questions about the car and track, but then I try to find out about the driver. A new driver needs a cam with more torque so he can pull out of a corner if he is going a bit slow. It also will allow him to recover easier when he makes an error and scrubs off too much speed.