By the staff of Dirt Wheels
No rider who has ever thrown a leg over KTM’s 525XC has ever called it slow—and if they have, they might need to take some time off for a mental rebuild. As stock, it will outrun just about any other sport quad on the market today, save for KTM’s own 505SX. The single-overhead-cam, four-valve RFS (Racing Four-Stroke) engine spent a good amount of time in KTM’s off-road motorcycles, achieving a level of refinement and performance we came to value when testing the biggest Katoom. In stock form, the 525XC is an open-class fighter—tons of power, comfortable Ohlins suspension, a lightweight chromoly frame and a whole lot of attitude make it one serious performer. In the dunes, it’s a blast; the huge torque curve makes wheelies and huge roosts just a blip of the thumb throttle away. But what happens if you get bored? Surely some power-hungry ATV riders out there will grow tired of having only 45–50 horsepower. Then what? If you have a KTM, you call Thumper Racing.
The Thumper 600 kit looks stock from the outside, but when you crack the thumb throttle, it will warp your world!
Based out of Santa Clarita, California, Thumper Racing is a family owned and operated business, consisting of Dad (Travis), Mom (Francis) and sons (Jake and Garrett). Thumper has all their own milling equipment and plenty of testing ground to put in the R&D time to make fast motors, and it shows. We have ridden plenty of Thumper-built machines before (and not one of them has malfunctioned), but we have always wanted to try the famed 600cc KTM. When we got the chance, we couldn’t pass it up. Thumper offers plenty of affordable power options for KTM-powered ATVs and motorcycles, as well as J-models, Can-Ams, Polaris ATVs and more. Give them a call at (661) 424-1800 for more information, or visit www.thumperracing.net
FINDING THE EXTRA 90cc
This KTM has enough power to light up the Skat Trak Haulers and sling it sideways at a blip of the throttle.
Thumper’s KTM kits are extensive, and their enlargement options start off with a 540cc conversion kit for the 450 or 525 (actually a 510) engine that will set you back $525. We have tested this conversion kit in a Polaris Outlaw 450 frame with one of Thumper’s custom cam grinds and found it to be a very potent engine; it will blow the doors off many built 450s with ease. From there, you can go even bigger with a 570cc conversion ($725) kit that Travis says is like adding nitrous to a 426 Hemi V8. Huge power gains over stock are achieved, then comes the stroker crank. The stroker crank can be optioned with the 540 or 570 kit, producing 576cc and 600cc, respectively. The 600 we tested uses a custom-machined, ceramic-coated CP 12.5:1, 99mm piston that has an offset wrist pin that works only with Thumper’s stroker kit. The reason for the offset wrist pin is to allow the use of a stock-length connecting rod with the stroker kit. This reduces piston acceleration speeds and cylinder side loads on the upstroke, increasing reliability and durability of the engine quite a bit. Travis also machines and balances the crank 1400 grams lighter than stock, and the whole rotating assembly is balanced to the new parts to achieve a quick-revving smooth engine, producing more torque than you can wrap your head around. To allow the engine to breathe, spec head porting, 1mm-plus oversized valves and a 43mm Keihin FCR carburetor are hooked up to a Fuel Customs intake and FMF Factory 4.1 exhaust.
The Thumper 600 kit looks stock from the outside, but when you crack the thumb throttle, it will warp your world!
The KTM we tested was running black ITP wheels with ITP Sand Star front tires and Skat Trak Hauler rear paddles. The owner of the KTM also custom built a titanium-lined oil cooler using a billet adapter that replaces the second oil filter on the KTM RFS engine and routes hot oil past the engine to a cooler position behind the steering stem. This ups the reliability even further and plays a major part in keeping that Thumper motor fresh. The stock KTM XC arms were replaced with KTM’s OEM wider arms, and the shocks were revalved by Rocket Ron for a plusher ride. Fasst Company Flexx bars were installed, as well as a Precision steering stabilizer and Pro Armor bumper/nerf package. Overall, it’s a great-looking quad that looks relatively stock—but doesn’t sound like it.
TAMING THE BEAST
It’s much faster than stock but just as light, so tossing the 600 around in the air is no problem.
From the first time we fired up the 600cc earthmover, we were in love. The stock battery and starter spin the motor to life just as effortlessly as stock, and it starts and purrs at idle with just a slighty more aggressive cam lope than stock. When you blip the throttle, the “bop bop bop” of the big single engine quickly snaps to life like a clap of thunder in an intense storm, cracking off revs quicker than you could imagine for such a large piston. Clutch engagement is just like stock, so pulling away from a stop is about as simple as cracking a rev and dropping the throttle, letting the torque do the work for you. The 600 will lope around near idle all day, and from there on out, things start to blur. Stuffing the throttle into the stops from any low speed is a fruitless effort, as it either ends in too much wheelspin or a 90-degree wheelstand. It could really use an extended swingarm to keep the front end down, but doing so would slow its handing slightly. Roll into it and keep your weight forward in first and second, and it moves out quick enough to make you think twice about shifting into third under power. However, the power is so linear and smooth that full-throttle upshifts from third to fifth don’t produce anything more than a power-induced wheelstand and a ton of roost. It hits light speed in a hurry and will easily stand up without any weight shift in any gear. It’s like holding all of the world’s power in your hands—use it wisely or it will bite you.
We took the KTM to the dirt and the dunes, spending the majority of our testing time in the soft sand at Pismo Beach, California. Jetting at sea level was spot-on, and the KTM ran without any hiccups. Riding through the dunes is still just as fun as it is on the stock KTM XC, albeit with much more thump behind your thumb. Crack the throttle and it practically skips across the surface of the sand at speed, but it’s tame, controllable and oh-so-smooth. The torque curve is near electric in its tractability, with a linear build that stretches your arms but won’t tear them from your sockets. Basically, it feels like the stock 525 power curve but amplified by nearly double. And with power numbers coming in a little shy of 68 wheel horsepower, it’s no joke. We eked a little over 70 mph out of the KTM on the top end, and the power could pull gearing tall enough to get it near 100 mph, we’re guessing. Out at the sand drags, there wasn’t a thing we came up against that would give it a run for its money—other big-bore 450s, modded dirt bikes, built Banshees. The torque pulled it far ahead of the competition on the uphill drag strip. Obviously it’s not going to take out a full-drag, 140-horsepower Banshee, but you can ride it to and from Comp Hill, and that we like.
WORDS FROM THE ENGINE BUILDER
The companies who helped build this beast are cleanly displayed in a beautiful DeCal Works graphics kit, complete with engine-size labels!
Travis Smith, owner of Thumper Racing:
“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 and bore and compression. We took our tried-and-true 99mm (4mm-plus), 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 to keep the motor vibration free, we reshaped and lightened the crank by 1400 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 makes better torque, decreases piston acceleration speed and piston side load, adding to motor reliability and longevity.
“In order to keep airflow into the big motor, the head received our Stage 3 porting, oversized 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 1 to 3 degrees. The carburetor was over-bored from the stock 39mm bore to 43mm bore and rejetted to match the 600cc displacement.”
Thumper Racing, $4729 www.thumperracing.com
• 600XC built by Travis Smith at Thumper Racing.
• Stroked, lightened and balanced crankshaft: $1650
• Thumper Racing stroker 99mm big-bore kit, 12.5 to 1. Includes cylinder sleeve and replate, CP 12.5:1 piston with ceramic coating, gaskets and jetting: $775
• Head porting by Thumper Racing: $350
• +1mm stainless steal kibble white valves: $164; valve springs with titanium keepers: $199; Hot Cam: $189
• 43mm carburetor jetted for 600cc motor: $350
• Engine gasket set: $180
• Carillo rod: $275
• Stage 5 cam
• High-flow water pump: $145
• Engine assembly labor: $640
KTM OEM Parts
• Extended KTM “SX” A-arms
• KTM billet ¼-turn gas cap
• KTM billet front sprocket cover
• KTM billet oil filter cap
• KTM billet brake reservoir cover
• KTM billet hydraulic clutch reservoir cover
FMF Racing www.fmfracing.com, (310) 631-4363 • Megabomb full-titanium exhaust system 4.1 exhaust
CP Pistons www.cppistons.com, (949) 567-9000
• High-compression big-bore piston supplied to Thumper
Cometic Gaskets www.cometic.com, (800) 752-9850
• Big-bore head gasket supplied to Thumper
Millennium Tech www.mt-llc.com, (920) 893-5595
• Over-bore and replating supplied to Thumper
Hot Cams www.hotcamsinc.com, (515) 402-8005
• Stage 1 Hot Cam supplied to Thumper
Dunlop Tire www.dunlopmotorcycle.com,
ITP www.itptires.com, (909) 390-1905
|CV4 www.cv4.net, (800) 874-1223
• Formed silicone coolant hose, orange
Pro Armor www.proarmor.com, (888) 312-7667
• Front bumper
• Pro Peg sport nerf bars
Rocket Ron Racing
• Rebuilt/revalved and upgraded triple-rate Ohlins shocks
Four Werx Carbon www.fwcarbon.com, (262) 501-9696
• Carbon fiber carburetor heat shield
• Carbon fiber rear shock
• Carbon fiber exhaust shield
• ASV pro clutch lever
Precision Racing Products www.precision-rp.com,
• Steering stabilizer
Racing Components www.schulermotorsports.com,
• Oil cooler mounting bracket by SRC
• Titanium plumbing for the oil cooler by SRC
• Jagg 10 row oil cooler
• Billet clamps for oil cooler
• Minot billet water pump
• Polaris bob oil filter adapter
• Extended brake pedal
• Crankcase pressure-evacuation valve
• AMSOIL motor oil