VALVE TRAIN HARMONY
How Timing Belts Eliminate The Discord
Not that long ago, you had two choices when it came to driving a camshaft – a timing chain or some sort of gear drive. They managed to get the job done, which, of course, was driving the cam by way of the crankshaft. In fact, timing chains and gear drives are still in use today, but in most racing applications, they’re only used if the rules mandate them. Gear drives and timing chains have been eclipsed by belt drives. And they see use in everything from Super Stockers to Pro Stockers (and the lot in between -- including endurance applications such as Winston Cup). One of the big benefits of a belt drive is the ease of camshaft tuning (phasing), but we'll look at that in a minute. Why have belts eclipsed timing chains and gear drives in both Sportsman and Pro motorsports? The real key is harmonics.
Turning Back The Pages….
Decades ago, a number of very savvy drag racers (Dan Jesel, Bill Jenkins and several other well respected “thinkers”) noticed that there was a tremendous amount of harmonics, vibration and resonance transmitted from the crankshaft to the camshaft. It was no big step to discover this harmonics dilemma eventually had a profound effect upon valve and spark timing -- to the point that racers had (and in some cases, still have) a difficult time holding the timing stable in their engines. To make matters worse, gear drives have a tendency to physically tie the crankshaft to the camshaft even more fervently than a timing chain. This means that every pop and bang seen by the crank goes directly to the cam. You can imagine what this does to the dynamics of the valve train. What about the harmonic damper? Doesn't it stop this internal monkey motion? Not really. Remember it's outboard of the timing chain or gear drive on the engine. It dampens the harmonics after it passes through the lower cam gear.