Thumper Racing's 600cc KTM

"After that first test ride, I hooked up with Travis Smith at Thumper Racing. He was looking for someone willing to give up his or her quad for a while, as he was offering to do a custom 600cc build with an RFS-based engine. We talked about some of the stuff he wanted to do, which included lightening, balancing and stroking the crankshaft. I told him it sounded like something I wanted to do and the rest is history."

Travis Smith At Thumper Racing Speaks On His 600cc Motor

"We set out to build a motor that would run on pump gas and make awesome power. We first looked at the limits of the stock KTM RFS motor. For this build we didn't want to get into boring cases, so we increased displacement via stroke, bore and compression. We took our tried-and-true 99mm (+4mm) 12:1-compression big-bore kit and then added a 6mm increase in stroke giving the motor a 600cc displacement. Because of the stroke increase the 12.5:1-compression piston was now at 12.9:1, which is the limit of compression on premium pump gas. The piston was ceramic coated to keep the motor running cooler by insulating the piston from the combustion chamber.

"Normally any increase in stroke slows the motor and affects handling. In order to offset this and keep the motor vibration-free, we reshaped and lightened the crank by 1,400 grams and balanced the crank to the larger piston. Lightening and balancing keep the motor vibrations to a minimum, and revving out as well as stock, plus the increase in gyroscopic forces to a minimum. We also chose to use the stock length connection rod and offset the piston wrist pin in the piston instead of shortening the rod as most stroker cranks do. By keeping the stock rod length we keep the rod-to-stroke ratios better, which ultimately improves torque, decreases piston acceleration speed and piston side load, hence adding to motor reliability and longevity.

KTM 600XC

+	Ridiculous amounts of torque and power.
-	Swingarm should be longer with this power.
=	A motorhead's dream quad.