Cams, or camshaft, the reasons and effects of tappet or valve clearances
All four stroke engines have valves! The thing that opens them is called a cam or camshaft, (shown opposite). The term valve clearance or tappet clearance is referring to the "gap" between the cam and the cam follower, or between the top of the valve and the tappet or rocker arm that pushes the valve open. When an engine sounds noisy, rattly or "tappety" this is what they are referring to. Getting valves done, etc is all part of routine servicing. There HAS to be some free play here as the valve will expand when hot and take up the gap, with disastrous consequences. The valve will not seal correctly and will overheat and burn, while causing pre-ignition, detonation, valve and seat damage and less power.
We are interested in power though. In an ideal world a valve would be instantly opened and closed. This way they could flow as much air or exhaust gas as possible for a any given opening time.
In practice this is impossible because valves cannot be instantly opened or closed. Race cams try to get closer to this ideal, but in turn need heavier valve springs to close the valves faster and follow the cam. One thing we can do to help power with a stock cam is to INCREASE the valves running clearance slightly.
This works because camshafts are designed to run quietly in your road car, and have what are termed quietening ramps on both the opening point and closing point of the camshaft. The problem is that for a good few degrees of engine rotation, the valve is S L O W L Y lifting off the seat, and the same in reverse as it returns. This means that its not sealing so the effective compression, power, induction stroke for example is shortened, while at the same time the valve is not open enough to allow any meaningful gasflow to take place. Setting the valve clearances to the larger end of the allowable range therefore gives more power rather than the expected less power at the possible expense of some valve noise.
All content and layout (c) John C Williamson, and respective owners, all rights reserved Aug 2003