ASK THE MXPERTS: WHY DO KTM’S HAVE TWO-STROKE PIPES ON THEM?

The exhaust system on the 2019 KTM 450SXF and 2018-1/2 KTM 450SXF Factory Edition (shown) are reminiscent of two-stroke expansion chambers, but their only connection to the exhaust pipe inside the cone is via a small 1/2-inch hole that bleeds exhaust into the empty resonance chamber.

WHY DO KTM’S HAVE TWO-STROKE PIPES ON THEM?

Dear MXA,

I was shocked to see the photos of the KTM 450SXF. I thought it was strange a couple years ago when their four-stroke pipes began to resemble two-stroke pipes, but now they are two-stroke pipes. I don’t understand how KTM can put a two-stroke pipe on their four-strokes and get it to work. What’s the deal?

We’ve been over this a million times, but we said the same thing about 2007 Honda CRF250 jetting specs (and we still get letters about how to jet one). First and foremost, it is not a two-stroke pipe, nor would a two-stroke pipe work on a four-stroke. The actual 2019 KTM four-stroke pipe is a normal-looking, constant-diameter tube—you just can’t see under the part that reminds you of a two-stroke expansion chamber. The two-stroke looking section is actually a resonance chamber. It is stamped just like a two-stroke expansion chamber and then welded over the four-stroke exhaust pipe (after a hole is drilled in the four-stroke pipe to allow excess exhaust to be sucked into the empty space of the resonance chamber). We find it hard to believe that you never noticed this before; although, on the 2019 KTM 450SXF, the resonance chamber is much larger than it was in the past.

 

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