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� No rider who has ever thrown a legover KTM’s 525XC can call it slow—and if they have, they might need totake some time off for amental rebuild.Stock, it will outrun just about anyother 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-roadmotorcycles, achieving a level ofrefinement and performance we cameto value when testing the biggestKatoom. In stock form, the 525XC is anopen-class fighter—tons of power,comfortable Ohlins suspension, alightweight chromoly frame and awhole lot of attitude make it one seri-

ous performer. In the dunes, it’s a blast;the huge torque curvemakes wheeliesand huge roosts just a blip of thethumb throttle away. But what hap-pens if you get bored? Surely somepower-hungryATV riders out therewillgrow tired of having only 45–50 horse-power. Then what? If you have a KTM,you call Thumper Racing.

More power than you can hold on to By the staff of Dirt Wheels

MONSTER MACHINE TEST

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THUMPER POWERBased out of Santa Clarita,

California, Thumper Racing is a fam-ily owned and operated business,consisting of Dad (Travis), Mom(Francis) and sons (Jake and Garrett).Thumper has all their own millingequipment and plenty of testingground to put in the R&D time tomake fast motors, and it shows. Wehave ridden plenty of Thumper-builtmachines before (and not one of themhas malfunctioned), but we havealways wanted to try the famed 600ccKTM. When we got the chance, wecouldn’t pass it up. Thumper offersplenty of affordable power options forKTM-powered ATVs and motorcycles,

as well as J-models, Can-Ams, PolarisATVs and more. Give them a call at(661) 424-1800 for more information, orvisit www.thumperracing.net.

FINDING THE EXTRA 90ccThumper’s KTM kits are extensive,

and their enlargement options start offwith a 540cc conversion kit for the 450or 525 (actually a 510) engine that willset you back $525. We have tested thisconversion kit in a Polaris Outlaw 450frame with one of Thumper’s customcam grinds and found it to be a verypotent engine; it will blow the doors offmany built 450s with ease. From there,you can go even bigger with a 570ccconversion ($725) kit that Travis says islike adding nitrous to a 426 Hemi V8.

Huge power gains over stock areachieved, then comes the strokercrank. The stroker crank can beoptionedwith the 540 or 570 kit, produc-ing 576cc and 600cc, respectively. The600we tested uses a custom-machined,ceramic-coated CP 12.5:1, 99mm pistonthat has an offset wrist pin that worksonly with Thumper’s stroker kit. Thereason for the offset wrist pin is to allowthe use of a stock-length connectingrod with the stroker kit. This reducespiston acceleration speeds and cylin-der side loads on the upstroke, increas-ing reliability and durability of theenginequite a bit. Travis alsomachinesand balances the crank 1400 gramslighter than stock, and the whole rotat-

The FMF MegaBomb head pipe has an O2 sensor bung in itto fine-tune the jetting with a wide-band sensor. It makesgreat power without being overly loud.

Pro Armor’s nerf bar and PowerGrip peg combo offer astrong foothold, and you’ll need it to hold on.

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