Safe 91 octane compression on 650R???

K, ive had my bike for over 2 months, and i havent even ripped the engine apart yet. Just wondering, how much compression can you run without detonation? The yamaha WR and YZ are running like 12.5:1 while the ole BRP is at a measly 10:1, even crf is at 11.5:1. I am asuming stock cam, (bigger cam has more overlap usually, reduces cranking pressure and effective compression ratio so you can run more). More power is always good. Anyone out there taken one to the edge of detonation? I figured that the aircooled limit of like 11:1 is B.S. since the 650R is liquid, runs cooler and it has a plated cylinder instead of steel sleeve right? Thanks in advance, JR

ive had my bike for over 2 months, and i havent even ripped the engine apart yet.

Wow, a whole 2 months, how have you been able to hold back for so long? j/k.


I don't think I would play around too much with timing, it doesn't take too much detonation, for too long to do real damage...unless you are looking for an excuse to pull the eng. apart.

Hi JR i just put in a new hi comp piston this weekend and she rips around the neigbor hood with no noticable problems yet. Im road racing it and it does pretty well.

I got it from XRS only and it was supposed to be 11:1 but it was engraves 12:1 on the underside?? piston was made by Ross. I also went with the Hot Cams stage 2, have a XRs only muffler, fmf powerbomb header and mikuni flatslide as well as the hi rev CDI box. I plan on getting it on the dyno this week and post the results.

Keep us up to date how your motor turns out.



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You replaced the piston and Cam? Many times you can run higher compression with a new cam because the new cam lowers cylinder pressure at lower rpm,(it doesn't actually lower it on purpose, it just works out this way so the cam can build higher pressure at mid, and top rpm)so the piston raises the comression ratio to get back some of the low end. Detonation is still an issue, but it's a bigger problem at lower rpm.

This would most likely be a winter project (hard to ride in 2 feet of snow), just getting a feel for what i should order. Need something to keep me entertained in -30. Thinking of going to the hrc cam as well. i guess if i change piston i could go 102mm and be at like 675cc..... lol, just what the BRP needs, more cubes. Look forward to seeing those dyno results, the BRP is kinda detuned for a bike its size, nice to see what one in proper form will put out. Talk to ya later, JR

Any one out there have any comments on the HRC motor kit thats used it?

Its a lot of money (over a grand) for a piston cam cam chain and some other small stuff.

The after market piston and cam and gaskets is only 360 bucks from XR's and seamed like a much better deal at the time.


Just some random stuff on building the XR motor...

There are numerous motard dudes in Germany that run a XR 700cc bike using the J E piston. Check out ( for more info. They even have a 710cc bike making well over 60 horses...Theres a good link with pictures of the piston/engine etc. Unfortunalety its in German....I assume its a stroked motor with the 102/'680' piston but i cant read german!

Rumor has it Team Honda rebuilds their crank after every 3 or so races. I've know of 3 cranks going out on BRP's.

Never heard of a stock cam chain / tensioner going out. Anyone? The HRC tensioner seems unnecessary.

Seems like the crank is the reliability issue-not the 11.5:1 pistons either standard or 102 bore... although for us Baja riders the higher compression is a bit of an issue.

Valves: seems like valve 'tuliping' is becoming as issue with the BRP. Gotta check/adjust those valves at least every 1000 miles. I do mine every race or 500 miles now.

Sprecen zie duetch? Its amazing how many euro motard links there are with the BRP featured. Even found a picture of Valentino Rossi on his BRP motard racer...

Search google for a german-english translator. Some times it turns out peculiar translations but you will be abler to get the gist of the text. :)

The new Baja Designs catalog says a 715cc kit is coming soon. :D Available in different configurations to meet your high speed needs. :) That sounds like a good winter project. :D

A guy from Sweeden raced last years Dakar Rally with a turbo-charged XR650R!

Maybe the 715 kit with the turbo? :D Of course theres always the option of bolting a small bottle on the frame filled with Nitro :)

Nitro is too much of a bitch to light and u have to use alot to keep from blowing the motor up :) but the 715 kit..... turbo would add weight, just what i need, but a stroked BRP with a 102mm piston, 11.5 compression, stage 2 hot cam, nice port job on the head and a pumper carb could be some real fun :D, maybe a little nitrous for those real nasty hills and swamps, my boredom this winter is gonna get me killed, lol.

Just out of curiosity, anyone know the duration of the XRs only stage 2 hotcam? do i need new springs? What is the meanest cam that anyone has found that still has the auto decompression? i like starting my bike without the usual 4-stroke BS.

BTW for anyone i might have confused earlier, longer duration cams usually hold the exhaust valve open longer for better scavenging, the result is that some of the incoming intake charge is blown out (overlap means all the valves are open, only near the bottom of the stroke) reducing cylinder pressure. like loufish said though, cylinder pressure is not decreased nearly as much at higher rpm where detonation is not usually a problem.

Why go with a small bottle? Go Big and launch that Puppy! :)

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This lists the specs of the hot cams

and only the honda cam uses the decompressor as far as i have found.

Its still no problem to klick over if you use the manual decompressor to get past tdc

thanks for the info, u know if those specs are advertised or at .050 lobe lift? or do they even rate bike cams this way? not really important im just curious. I think we should ignore Rockatt, he has a cr500 which eliminates any possibility of sanity :)

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