It's human nature to want more, but when is more too much? Honestly I don't hold the answer to that question, but when it comes to power and speed on ATVs, it seems we always want a little more. With that said, wouldn't it be sweet to get 600cc out of a 370-plus-pound sport ATV? That thought prompted my introduction to a guy named Brian Schuler through a friend, Jay Clark. Brian was having his KTM 525 transformed from an already extremely fast sport quad into a 600cc monster. Thumper Racing had the task of carefully crafting this motor into a beast while maintaining its reliability. It was such an interesting process that I decided to let both Brian and Thumper's Travis Smith tell the story from their perspective.
"The KTM 525XC is an awesome machine in stock form, is a great race platform and has been winning races in all different classes across the globe. It really shines in the dunes and on the trails alike, and has proven reliable. The first performance-enhancing endeavor on this machine was to get the suspension upgrade offered by Ron Elmore at Rocket Ron Racing out of Indianapolis, Indiana. The setup consists of new valve components and different coil springs that were set up for my weight and style of riding. Once the suspension was completed, I could feel a huge difference in the improved overall handling of the KTM. This particular 525 spends most of its time in the dunes of SoCal where power is the name of the game. Anyone who's ridden one of these knows the power it possesses in stock form and understand that it's quite adequate yet not so for most die-hard power extremists.
"So the building of horsepower began in the form of the factory KTM 540 kit and the KTM 8/06 cam. I had the head initially ported by Moto Heads Racing and he installed plus-1mm Kibble White stainless steel intake valves and exhaust valves in stock sizing. Then to help keep the 540 cool I added the Minot oversize water pump. I was later contacted by Elmore at Rocket Ron to be the first one to receive a triple-rate shock upgrade on the stock Öhlins shocks for the KTM setup. I told him I was getting the SX-style A-arms for the quad, and sent the entire suspension package to him so he could mock it up on his quad and get the proper geometry mapped out for this particular upgrade. From the time I sent the package of new A-arms and shocks out, only a week went by before I received them ready to mount up. That's what I call a fast turnaround on a complete new shock setup.
"After I installed the new suspension I hauled it out to Gordons Well Sand Dunes (in California near the border with Mexico) for testing and immediately felt how good of a setup I now had compared to stock and even my original revalve job. The power of the KTM 540 was amazing everywhere in the rpm range, and the suspension upgrade was unbelievable to say the least. Everyone who'd ridden the quad in this form raved about its new handling, often saying it handled like a magic carpet. The power the 540 made with the ability of the upgraded suspension to soak any of the harsh terrains promoted confidence in the ability of the KTM. I then purchased a Precision steering stabilizer to finish out my powerhouse. Now, it will handle the logs, washed-out crevices and the rocks in the trails we ride while we are in the off-season for the dunes. As a 540 I could outrun about anything out there except the heaviest-modded quads I came up against. It had more than enough power to loft the front end easily over any obstacle that may get in your way. The power was very predictable and yet forgiving when you found yourself in a predicament where you needed to use the extra power right now. I loved the quad in this form.
"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.
FourWerx's carbon accessories...
FourWerx's carbon accessories give the orange machine a very unique look.
"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.
"In order to keep airflow into the big motor, the head received our Stage 3 porting, oversize high-flow valves, Stage 4 racing cam and high-lift valve springs. To get the full benefit of the cam, it must be timed to the crank as factory indexing can vary one to three degrees. The carburetor was overbored from the stock 39mm bore to 43mm and rejetted to match the 600cc displacement."
||Ridiculous amounts of torque and power.
||Swingarm should be longer with this power.
||A motorhead's dream quad.
To maximize rider comfort...
To maximize rider comfort for long rides on trails or in the dunes, ASV levers were installed on a set of Flexx bars.
Brian Wraps Up And Tests The 600
"When I got the quad back from Travis I wanted to install an oil cooler of some sort to help keep the oil cool. I found a guy on www.ktmatvhq.com
forums by the name of Polaris Bob. Bob had designed an adapter that fit in the place of the rear oil filter on the RFS engine. At first I was a little skeptical, to say the least. After a lot of homework and talking to people in the KTM racing industry, I found that none of the KTM RFS engine experts have ever found any debris in the rear filter after the hundreds of the RFS engines they had torn down. I then felt comfortable enough to eliminate the rear filter for the sake of keeping the engine temps down. I bought the adapter from Polaris Bob for $140 and then purchased the 10-row JAGG oil cooler from Thumper Racing. The adapter came equipped with high-flow fittings, so I had to come up with the plumbing and the oil cooler mounting bracket on my own. I had some spare 0.019x0.375-inch lightweight titanium tubing and preformed .0375 rubber hose on hand. I had the clamps made by Joe Krafft with a little of my own special touch. Then I visited Western Metals of El Cajon, California, and got some 1/8x3/4-inch chrome-moly strap to make the mounting bracket. Some time measuring, bending, drilling and welding and my bracket fit perfectly. The JAGG oil cooler helps to keep the oil about an average of 35 degrees Fahrenheit cooler.
"In 600cc form, this quad is extremely fast. Lifting the front end sky high is just a slight throttle bump away, while making sure to hang on for the ride. It has so much usable torque in the bottom and midrange the only reason you need to go in the upper rpm range is for racing. Riding the dunes is such a blast in the lower and midrange. Even with the big cam that Travis installed it is very predictable yet comes on extremely strong in the higher rpm range. Blasting up Oldsmobile Hill in fourth gear, I often found the front wheels wanting to loft. You have to breathe the throttle a little bit to let the front end back down, or it can and will bring itself over backward. About the only thing this KTM 600XC needs is a longer swingarm to tame the wheel standing when blasting off from starts. For all you motor heads out there, look up Travis Smith at Thumper Racing. You won't be disappointed."
The MegaBomb 4.1 titanium...
The MegaBomb 4.1 titanium exhaust system from FMF Racing not only helped to increase the power of our machine, but also saved a ton of weight compared to the sto
Build Sheet: KTM 600XC
Thumper Racing: www.thumperracing.net
Stroked, lightened and balanced crankshaft:
$1,550; stroker 99mm big-bore kit 12.5:1, includes cylinder sleeve and replate, CP 12.5:1 piston with ceramic coating, gaskets and jetting: $775; piston coatings: $50; head porting: $350; +1mm stainless steel Kibble White valves: $164; valve springs with titanium keepers: $199; Hot Cam Stage One cam: $189; carburetor jetted for 600cc motor: $350; engine gasket set: $180; Hot Rods connecting rod: $137; high-flow water pump: $145; engine assembly labor: $640 [total engine work: $4,729]
10-row oil cooler, custom with SRC: $259.95
FMF Racing: www.fmfracing.com
This triple-rate shock upgrade...
This triple-rate shock upgrade is a cost-effective way to increase suspension performance.
MegaBomb 4.1 full-titanium exhaust system: $599.99
CP Piston: www.cp-carrillo.com; 949/567-9000
High-compression big-bore piston supplied to Thumper: listed above
Cometic Gasket: www.cometic.com; 800/752-9850
Big-bore head gasket supplied to Thumper: listed above
Millennium Technologies: www.mt-llc.com
The Pro Armor front bumper...
The Pro Armor front bumper offers plenty of protection as well as a racy look for our trail-terrorizing KTM.
Over-bore and replating supplied to Thumper: listed above
Hot Cams: www.hotcamsinc.com; 515/402-8005
Stage One cam supplied to Thumper: listed above
Pro Armor: www.proarmor.com
Front bumper: $124.95; Peg Sport nerf bars: $379.95
FourWerx Carbon: www.fwcarbon.com
Podium carbon fiber hood: $155; carbon fiber carburetor heat shield: $60; carbon fiber exhaust shield: $55