Wednesday, November 22, 2006

AV - VW Cam Details

>When it comes to cams all I've ever seen listed is lift and duration<


That's because all the after-market cams advertised in VW-specific magazines are based on the assumption you're only interested in Peak Horsepower.

Torque is largely a function of displacement whereas 'horsepower' is a city slicker method of computing a certain amount of work done over a certain period of time. To produce a hundred horsepower from a VW engine all you have to do is spin it faster during that interval of time. Of course, the thing won't last very long but when you're selling junk to the Kiddie Trade you don't bother to mention such things :-)

What most people tend to overlook is the efficiency factor of their prop. In simplistic terms, your prop is a wing and as such responds to the same rules of lift, drag and aspect ratio, meaning a longer prop tends to be more efficient than a shorter prop. If you start with one of those gee-whiz 80hp VW's turning 4000 rpm the odds are your prop is going to be painfully inefficient, converting as little as half your torque into thrust. That means your expensive '80hp' engine with its dune buggy cam is burning 80hp-worth of fuel and wearing out at a 4000rpm rate yet delivering only 40 horsepower's-worth of thrust.

But that also sez the airplane is able to fly with 40hp. So why not simply turn the equation around? Start with the longest prop you can swing without mowing the lawn then use a high-torque cam to build an engine that will swing your longer prop at its point of maximum efficiency. Odds are, you'll end up with a slow-turning, long-wearing engine. Of course, it will only be producing fifty or sixty horsepower... obviously not as 'good' as 80hp, right? Yet in the air each engine produces the same 40hp-worth of thrust. Vast mystery, eh?

The advantage here is that a high-torque cam results in a slower-turning engine that will last longer and produces less 'waste' horsepower to satisfy the prop -- meaning you'll fly farther on the same fuel and enjoy a longer TBO than when you use a hot-rod cam. All of which is common sense when the task is to produce thrust with a directly-driven propeller, a fact easily verified by by simply examining the spec of cams used in small aircraft engines.


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