Hello!

Have anyone tried a 12:1 piston on the XRR?

How much loss in reliability? Big performance difference comparing a 11:1 piston?


I saw a guy here in sweden who bought a 12:1 Ross-piston, unfortunately he has not installed it yet, that's why I'm asking here.

thats alot of comp!! i think alot would depend on other engine mods along with this piston, and the fact that that comp would need prolly 50% race fuel also. would this be in a stock engine or what other mods???

That might be race gas with a motor like this, seeing as a 11:1 piston can be considered pushing it.

Reliability of the kickstart would be severely compromised not to mention the rest of the motor internals. It is all how you use it though. You would need a lot of porting and cam to make 12-1 worthwhile. And you would loose useful powerband width.

Denn10 said:

A new cam will be installed. Also a tm42 carb and a full arrow exhaust system.

We drive on 95 or 98 octane here in sweden, should be enough?

valvesrule said:

You say powerband width? Do you mean that I loose bottom or top?

Edit: I guess that depends on other stuff to, but in general? The 12:1 piston is almost 50 grams lighter than a 11:1 je-piston too.

loy said:

Is that research octane(RON), motor octane(MON), or an average of the two(R+M/2)? Here in the U.S., we use R+M/2. I know many other parts of the world instead go only by RON. That makes the numbers seem artificially high when compared to R+M/2 octane ratings.

I would say that 98 R+M/2 method fuel is likely adequate for 12:1 compression, but probably not 98 RON fuel.

you would definitely need to upgrade the rod, the stocker on a big build prolly isnt the best idea. just some info.

loy said:

For 12-1 you will need a bunch of cam duration (270+ degrees) or cranking pressures can be high enough to cause overheating even with liquid cooling, hard starting, detonation and other possible problems. Power will be shifted to higher rpms from lower rpms. It will start later and likely end around the same rpm. Generally what power is added on top is subtracted from the bottom. It can become hard to kickstart motors modified to this extent because they have to be turned over faster to get enough flow to start. Idle speed usually has to be set higher to reduce stalling. Bikes like this are a thrill to ride and a pain in the azz to keep running. I've had a few, user friendliness goes bye bye. New Gen bikes get away with these C.R.s because they have carbs with TPS sensors and ignitions much more complex than older bikes. They still routinely self destruct.

12 to 1 is nothing there's a heap of XR650R's here in OZ running high comp on super retards with no probs.
Also high comp benefits the bottom end and may sacrifice some top on an engine running high revs.The big XR doesn't run high revs so it's no issue.

That's great. High comp without a cam change will increase all parts of the power band but very top rpm where it will restrict some. Heat is increased a bunch however. Holed pistons, cracked heads and burned valves can result.If the internals are holding up to it fine! I'm curious how the CRF450/500 in your sig is holding up? Stroker, bore or both?

valvesrule said:

That sounds good. I don't use to rev it that much so.. :ride:


But you guys think that i rod-change is needed if putting in a 12:1, if I got it right?

I wouldn't change the rod if you're just going the piston but if you go full house do it.
It was a 490cc Athena kit on the CRF and it hauls the mail.I can do triples in 2nd,3rd,or 4th it's got that much grunt.
It had 1 thou gap when i put it in so i was carefull running it in,stayed a bit rich till she was bedded in then it was a hoot.
It has a heavy flywheel as well so it's the pefect old fat guy bike.
I recently sold it and the guy is way happy with it.