High Comp Piston and premium gas

12 replies to this topic
  • GarrettXRL

Posted December 03, 2007 - 04:46 PM


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:

  • absynth01

Posted December 03, 2007 - 05:27 PM


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.

  • Norge

Posted December 03, 2007 - 06:04 PM


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.

  • Jon-D

Posted December 03, 2007 - 06:15 PM


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.

  • GarrettXRL

Posted December 03, 2007 - 07:17 PM


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


  • tobinbakner

Posted December 04, 2007 - 10:43 AM


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



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  • GarrettXRL

Posted December 04, 2007 - 12:58 PM


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



  • tobinbakner

Posted December 04, 2007 - 01:43 PM


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.co...daCycleDirt.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.


  • GarrettXRL

Posted December 04, 2007 - 10:01 PM


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.


  • martinfan30

Posted December 05, 2007 - 09:57 AM


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!

  • usarfire

Posted December 10, 2007 - 08:47 PM


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

  • Captain Midnight

Posted December 14, 2007 - 12:53 AM


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.

  • martinfan30

Posted December 14, 2007 - 08:37 AM


did that affect performance noticeably?

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