High Comp Piston and premium gas

Hi Fokks

Are any of you with Xr650L's using a 10.5:1 compression piston? if so, do you run premium? Have you used regular in it and have you had any issues by using regular. I'm in the process of doing the mods, Mikuni Pumper, 10.5 piston, aftermarket exhaust, and I got curious about fuel availability as some of the areas I ride in only have regular gas. I intend to bring octane booster with me when I ride in those areas but I wonder if regular will be a problem.

Thanks for your responses.


Now get out and :thumbsup:

10.5-1, cam,rod kit,carb,exhaust,airbox. I only use premium gas. I haven't been stuck yet. The only time there was only regular grade, I just waited for the next station as I have a larger Clarke installed. Read up on high comp engines. Heat, knock, pre-det, and a host of other things come onto play when running built motors. Most race motors use a blended fuel to their motors liking. For our app, we can get away with higher octane pump fuels without getting to nuts. If you were to go higher than 11-1, I would be careful as many more factors come into play.

10.5-1 piston, cam, Mikuni flat slide carb. No problems running premium gas. As absynth01 says be careful going with any higher than 10.5-1 compression pistons.

I have the same piston in my XRR and i have to run premium. i tried running regular but it started to ping so i switched back.

Thanks Guys, thats about what I thought. I'm going with the piston fron XR's its a 10.5.


Is there a reason why you haven't considered the Wiseco 10.25:1 piston?



I wasn't aware of the company or their products. can you send me a link, I'll check 'em out.




This is a link to their main site. Here is a link to the PDF of their catalog that has the XR650L pistons. http://www.wiseco.com/PDFs/Catalogs/CB07/HondaCycleDirt.pdf

This is the lowest higher compression piston I've found. Personally I'd really like to find a piston in the 9.5:1 range. I don't know if you are planning on changing the cam at the same time but a cam change will lower the dynamic compression ratio which may may affect detonation. My belief is it should reduce the chance of detonation but I'm still researching it. I notice absynth01 and Norge both say they have changed the cam.


Hi Tobin

I hadn't thought about changing the cam because of budget limitations. I looked around for a lower compression piston as well. When premium isn't avaliable I'll use an octane booster. Thanks for the links I'll look into wiseco.


i have only used premium since stock. now i have exh, cam, fcr and would never even think of it. unless thats all there is!

one other thing with the new pump fuels the detergents are washing away your valve lubricants so see if you can get bronze valve guide

I have a J.E. 10.25 to 1. Getting the motor hot and straining it, causes some pinging. For riding on the road, pump Premium works OK. When I race it, I cut it with some race gas. I have since removed the piston, and machined the top to 10 to 1, now I don't need to add the race gas.

did that affect performance noticeably?

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