to read Pages 4-6

KTM 600_pg62-70:Layout 1
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12:33 PM
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This KTM has enough power to light up
the Skat Trak Haulers and sling it sideways at a blip of the throttle.
The Thumper 600 kit looks stock from
the outside, but when you crack the
thumb throttle, it will warp your world!
ing 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.
OTHER GOODIES
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.
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TAMING THE BEAST
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 real-
ly 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
KTM 600_pg62-70:Layout 1
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Page 6
The companies who helped build this
beast are cleanly displayed in a beautiful DeCal Works graphics kit, complete
with engine-size labels!
It’s much faster than stock but
just as light, so tossing the 600
around in the air is no problem.
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-sosmooth. 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
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-andtrue 99mm (4mm-plus), 12:1 compres-
KTM 525XC WITH THUMPER 600 KIT
Thumper Racing, $4729
www.thumperracing.com
• 600XC built by Travis Smith at
Thumper Racing.
• Stroked, lightened and balanced
crankshaft: $1650
• Thumper Racing stroker 99mm bigbore 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
68 www.dirtwheelsmag.com
• 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,
(800) 845-8378
• Tires
ITP
www.itptires.com, (909) 390-1905
• Wheels
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
www.rocketronracing.com,
(317) 319-4381
• 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
heat shield
• Carbon fiber exhaust shield
• ASV pro clutch lever
Precision Racing Products
www.precision-rp.com,
(209) 365-1850
• Steering stabilizer
SRC Schuler
Racing Components
www.schulermotorsports.com,
(619) 813-4873
• 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
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Rocket Ron revalved the stock Ohlins
shocks, and KTM’s extended A-arm kit
widens the front for more stability and
a stock look.
sion, 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 stocklength 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 overbored from the stock 39mm bore to
43mm bore and rejetted to match the
600cc displacement.” ❑
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