I've been riding my 400ex to death since I bought it 2 years ago. Now I want a little more power. I didn't like the feel of the Raptor, although it had good power, so I've decided to do some motor work on my 400ex. I've heard pros and cons about the big bore kits. All I want is more top end since I ride big dunes mostly. I have all the bottom end I need with my Core IV pipe. I would appreciate some good quality feed back from anyone who has experience. Should I go with a 440 kit or just put a 11:1 piston in? If it is just the piston should I bore it out and what is recommended? I know we all have our prefrences, but like I said I'm looking for top end. Thanks for any help you can provide.

 

I myself am looking for the exact same as you. I think I'm goin to go with a 10:1 compression. Maybe higher if I can go higher without running race fuel. Anyone know about anything about that? But I believe I'll bore it our to be a 416. I hear that the 416 gives more top end. I read that in a dirtwheels issue about the 416 H-Bomb EX. After that i believe i'm goin to get me a xr400 cam and cdi high rev box.

 

I've been told if you go to a higher compression piston say 11:1, you will need to run at least 50/50 race fuel and premium. That's all I know about the race fuel issue, however if it would give me more top end and not cause a problem with the "Honda reliability" I'll pay the extra for race fuel.

 

If you get the XR400 or any bigger cam, you can run premium pump gas with 11:1 compression. I'm running 11.25:1 on mine on pump gas with no problems. You might want to run some octane booster in each tankful, just to be safe. If you don't mind paying the extra for race gas, I would get a 12:1 or 12.5:1 piston and run straight race gas. In my opinion, high compression is where you're getting the power, not boring it out a couple of millimeters. I bought the standard size piston(11:1) and put in a XR400 gasket, which is a little thinner. When the power hits now, the thing pulls like a raped ape. When you get the piston, you might as well get a rev- box too. you will be constantly bouncing off the rev-limiter without one.

 

WHAT OCTANE DO YOU HAVE TO RUN IN THIS TYPE OF SET UP?

 

GT Thunder recommends alcohol with 14.5:1 compression, Cam 2( must be like 117 octane) with 12.5:1 compression, race gas with 11.5:1 compression, and high octane pump gas for 11:1 compression.