Variable Valve Timing

In an internal combustion engine, optimizing the airflow in and out of each combustion chamber is a key to just about every aspect of performance. Power, emissions, fuel economy and other critical parameters are all dependent upon how good the engine flows air in and out.

Clearing the Air
Four-stroke automotive piston engines have poppet intake valves in their cylinder heads to let air in, and separate poppet exhaust valves to let exhaust gases out. These valves are opened at the appropriate time by one or more rotating camshafts. The cams are turned in precise relation to the crankshaft by chains, belts or gears—or a combination thereof. Lobes machined on the camshafts push the valves open at just the right time in the engine's operating cycle, then keep the valves open for just the right duration. There are other ways to operate the valves, including various pneumatic and electromagnetic schemes, but for production engines, the inexpensive and long wearing camshaft is king.


For decades, engines had the relative position (or timing) of the cams and crankshaft fixed. That meant, for example, that in a given cylinder, the intake valve(s) always opened at exactly the same point in the piston stoke, regardless of how fast or slow the engine was running. This system was simple and worked cheap, but ignored hidden potential in the engine. It turns out that fixed cam timing is only really correct at one engine rpm and one throttle setting. At every other rpm and throttle setting, the valves are opening and closing a little (or even a lot) too early or too late.

Change is Good
The reason that valve timing needs to change to match the engine's operating condition from moment to moment is due to the physics of air. Though it feels pretty weightless to us, air actually has mass. That means that it takes time to be put into motion or to be stopped. At idle, a typical cylinder has to fill with air and empty itself of exhaust gases about five times every second. The airflow in and out of the engine likewise has to start and stop just as often.


At redline, it gets far more intense. The same cylinder might have to fill and empty 50 times per second—maybe more. Obviously, when working more than 10-times faster than it does at idle, there's very little time for the air that's stopped in the intake manifold to accelerate into the combustion chamber when the intake valve opens. What we need is a way to give the air a head start. Variable valve timing can do this.