BIG-BORE KTM 365SXF
THE MASTER PLAN
After riding the 2012 version of KTM’s 350SXF, most
test pilots enjoyed the engine characteristics, but wanted
more power starting earlier in the power curve. There are
many different ways to achieve this, such as exhaust systems, porting, cams, high-compression pistons, Acme
rocket boosters and so on. We chose displacement for
simplicity and cost-effectiveness.
Initially, we weren’t sure if a big-bore kit was even
possible for the 350SXF. KTM engineers had already
taken the motor to its limits with the goal of making it
compact and 250-like. But the guys at Cylinder Works
took their time, did their homework and eventually
determined that there was room for a 90mm bore, which
would provide 365cc. The big-bore kit includes a brand-new 90mm cylinder, a 13.5:1 compression Vertex piston
kit, and a complete top-end gasket kit. With a retail cost
of $649.95, the Cylinder Works kit is easy on the pocket
book compared to other paths to increase performance.
We remapped the stock ECU, giving it more fuel all
around, along with a more aggressive ignition curve. At
the other end, we figured we could do better than the
stock exhaust, so we installed an FMF 4.1 full-titanium
system with a Megabomb header.
With the engine pumped up and ready for action, we
OUR REPORT CARD
turned our attention to the chassis. Applied Racing
supplied us with one of its brand-new R/S top triple-
clamp kits, featuring urethane elastomer bar-mount sys-
tems for reduced vibration. Race Tech took on the
duties of giving our KTM a plush ride without compro-
mising its resistance to bottoming on big hits. The
wheels on our test bike had seen better days, so RAD
Manufacturing laced us up a complete wheelset. Eagle
lightweight aluminum hubs teamed with DID Dirtstar LXT
rims went with the build quite nicely, giving us style with a
lot of function.
With any project, you don’t know what you’ve done
until you’ve done it. In this case, the finished product was
even more of a mystery, because this was uncharted territory. No one has raced a 370 since DeCoster’s GP days.
But we had good reason to be confident. Virtually all big-bore kits have two results: more torque and less rpm. The
KTM 350 needed more torque, and it had rpm to spare. In
theory, it would be perfect.
So how would we grade ourselves? On the Dirt Bike
self-made curve, we’ll give ourselves 90 percent. Right
away we noticed that the power came on sooner off the
bottom and pulled stronger in the mid-range, all without
killing the top-end revving power the stock bike is known
for. Make no mistake; the KTM is still a screamer. When
you get to the top of the powerband, it still has that high-rpm overdrive that allows it to hang with an angry 450.
The peak output is virtually unchanged, but it does occur
earlier. After that, you can still rev the 365 just as far as
you could when it was a 350, but there’s no real advantage way up there. The real difference is what happens
down low. You can get away with a mistake on the 365. If
you only downshifted once when you should have downshifted two gears, the bike doesn’t fall on its face. You
can use the clutch and get back into the happy zone.
Don’t expect miracles, though. It’s only 15cc of extra displacement, so you only get 15cc of extra torque. The 365
still can’t be ridden a gear too tall and power its way
through the soft stuff.
With more bottom
end, you have a safety
net for a misjudged
PROJECT MIDDLE GROUND