high comp pistons

[6858-400ex]

how do just keeping same bore size but higher comprssion


what comp would be better what power do they produce

[cals400ex]

most people tend to stick with the JE or Ross pistons. i run the ross because they are lighter. i am not sure what the stock bore pistons compression ratio is. i bet it is close to 10.4:1 or 10.6:1. either of these will run just fine on pump gas. as a matter of fact, i would run a thin head gasket with any of these to raise the compression a bit a get a better flame travel across the top of the piston. as a rule of thumb, the more compression the more power. however, higher compression the more strain on the motor and the higher octane fuel you need.

[400ex2072]

I had a wiseco 11:1 compression stock bore piston in mine before I got my 416 built and with the hotcam I used to have, it made a significant difference. Compression influences power alot more than the bore so if youre looking for a good boost without boring, that would be a good setup for you. When you order your head gasket, get one that is made for an XR400 though and not the 400ex since its thinner. This way youll be close to running a full 11:1 compression instead of the 10.4 or 10.6 like Cal mentioned already...

[Toadz400]

Isn't the stock compression closer to 9:1?

I've heard that many people have gone 11:1 and not had a problem running 93 octane, some have had to run mixed gas though. It probably all depends on your elevation, if you're higher up it's not going to be as bad.

[400exstud]

400ex stock comp is 10:1

[nosliw]

Originally posted by 400exstud
400ex stock comp is 10:1

im pretty sure it's 9:1 like toadz400 said

[cals400ex]

the stock compression is 9.1:1.

i don't know if you got that from my post or not. when i was talking about the 10.4 or 10.6:1, i was referring to the stock bore high compression pistons. i wasn't referring to the stock honda piston that comes in the bike.