Candid Cameron

Candid Cameron

Q What is it about Harley-Davidson XR-750s that have allowed them to dominate dirt-track racing, while that same traditional H-D design has proven not to be competitive in other forms of racing? Posted on www.cycleworld.com

Peter Whitworth

A Some people claim that the XR's uneven firing order is very helpful to traction, and I won’t dispute that, but there is another factor: Their peak torque appears at the lowest rpm at which the engine can smoothly pull on the dyno-/'.e., at about 5500 rpm-and it falls steadily and slowly from there to peak power, just at or a bit above 9000. That smooth, almost featureless torque gives the engines excellent throttle controllability that matches the driving force to what the tire can manage. Years ago, XRriding Hank Scott used to be much admired as a “human traction control.”

It would be easy to make a lot more power with an engine designed not so long ago; but homebuilt race engines usually have peaks and valleys in their torque curves that cause them to break traction more easily and lose time. Maybe most important of all is that the XR-750 engine has been on the dirt for 40 years now, and a huge amount is known about how to make it work well there. It takes so long to learn the equivalent about any other engine that people have usually given up (Yamaha parallel-Twins, various British “jumprope” Twins, etc.). Besides, whenever some other engine type has managed to win a race or two (Yamaha TZ750 two-stroke Four and Erv Kanemoto’s three-cylinder Kawasaki “Banger”), the usual amount of muttering gets it banned for being too dangerous.

I guess that old Harley two-valve is just dangerous enough, but not too dangerous.

-Kevin Cameron