I would say the biggest difference is 400 vs 250 cc of displacement. Typically more displacement means more torque. Torque is what you immediately feel, not horsepower!

Also the quad is geared lower, which only makes the torque feel like there is more.

 

According to HMF our 250's are rated at 18 horse wherease I believe the 400 atv is 28 and the 400 thats in the bike is like 35, if I remember my back issues of Dirt Wheels correctly.

 

iowaguy,

Man, I hope you're not too disappointed in the 2 to 3 hp difference you're gonna see!

Considering that the bike is making 18.2 hp in stock configuration, and with the mods listed on the HMF Engineering site, they get 2.6 with the pipe and a mod package equivilent to what has been done here, another 1.5 hp if you remove the air box - not recommended - that 4 hp up. Considering that is more than a 25% increase, I'm gonna say that's quite significant!

To get more power you're gonna have to go to the 300 kit!

 

you might even try to find a used 300 cylinder and bore it out with one of the big bore kits for the 300.that would be alot of torque.just remember the more umph you give it the more maintenance and reliability probs will occur,there's alaways a trade off.the motors in quads are usually more beefy(tranny gears and stuff)to compensate for the extra weight.they are geared much lower and you also have 2 wheels pulling you.i love quads and have one that i mostly single track with,the klx is like my pit bike at the track.here's my yfz450,i don't think it would be possible to make the klx scream like it,it's pushing about 45hp now without any internal engine mods.but they both are a blast to ride in their respective arenas.
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it can wheelie. just not easily!! but i do agree that i wish it would pull a 2nd gear power wheelie. i can blip the clutch in 2nd and ride through 4th on pavement. but i know its hard on the clutch. but thats the price for fun!![sm=icon_rock.gif]

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DAMN 1iviper, that's a bad *** quad

 

actually in this pic. its all stock, still is. i put the pipe on without having the jets just to hear it then took it back off. but my jets and needle should be here tomorrow. i just got my sprocket yesterday so maybe next week ill tear everything down and get to work.