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Two stroke over four stroke?. Ever since four stroke dirt bikes have become competitive with the two
strokes people have been arguing which one is faster and just all around
better so lets compare them.

what is the horsepower difference?
The first difference between the Yamaha four stroke dirt bike and the two stroke
is that the 4 stroke 250 produces 8500 rpm and 36 horsepower. The 2
stroke 250 makes about 46 horsepower at 11,300rpms. That’s a 10
horsepower advantage. The difference between the KTM 250sx two stroke produces around 49.77 horsepower. The KTM 250 four strokes make about 35.5 horsepower. So the 2 stroke has 13.22 more horsepower. So no matter what brand of bike you get the 2 stroke is going to have more horsepower.

Horsepower doesn't matter if you don't have good torque so the torque comparison is:
The KTM 250 four stroke has 19.42 foot pounds of torque at 8600 rpm. While that is good torque the KTM 250 two strokes crushes it with 31.93 foot pounds of
torque at 8000 rpm. The KTM two strokes has way more than the four stroke. But its not the only brand were the 2 stroke has more horsepower the Yamaha 2 stroke has 30.6 foot pounds of  torque. The YZ 250 four strokes has about 20.1 foot pounds of torque.

There is also a price difference Both the Yamaha 2 stroke and four stroke have the same retail of $7150 The 2012 KTM 250 two stroke is $800 cheaper than the 2012 KTM 250 four stroke. It retails for around $6899 (MSRP). The 2012 KTM 250 four stroke’s price is $7699


The author of dirt bike magazine said "Privateers could compete at a higher level for a lower cost. Even at the National level, engine mods on a two-stroke top out at around $1000—you can't buy the top-end parts for a four-stroke for that amount. "

Conclusion 

My conclusion based on my research is that the 2 stroke is a faster bike. It is also
cheaper bike. So why would you choose something that is more expensive and slower?