How much comp before race gas?

[speedy400]

Hey,
I have a 82gs850, and I'm trying to bump up the compression a tad, (for street use), I'll be having a oil cooler on, high perf. cams as for heat wise on my bike. But I am wondering how much can I get milled off the head to get to 10.5 to 1 comp? I thought I heard someone say about .009 inches? Thanks-darren

[RacingJake]

Before I'd do anything take a compression check. If it's below oem limits I wouldn't do anything until it's fix, worn rings or valve's??

Depending on what cams you use some require more compression due to valve timing, maybe 11-1, 12.5-1??

If you do mill the head be sure to check your valve to piston clearance, cause it's going to change, get smaller. Adjustable cam sprockets work great for this.

[wiredgeorge]

Speedy, I don't know how much to mill to get 10.5:1 compression but I personally wouldn't mill the head to achieve higher compression. I would use some Wiseco pistons which will bump your compression and also freshen your performance because of better ring seal (new rings are included). At 10.5:1 you won't need race gas. You MAY need to slightly retard your ignition timing which can be done by changing the position of your ignition plate and the 2/3 pickup but you could also pick up a Dyna 2000 or newer Dyna ignition (not a Dyna S) where they build in several advance curves that retard igniton. At 10.5:1 you may need to retard just a bit but you will likely be fine on 93 octane.

[speedy400]

I would love to get wiscos, but A. I cant find a set anywhere for stock bore for my 850, and B. I can get the milling done for free compareds to ~$200 for a set of high comp wiscos. Also my stock rings have been sealing real well, its only got 13,000 miles on her. Thanks though, sounds like 10.5 to 1 is a good number to shoot for. Thanks again-darren

[motorcyclemodeller]

I am running 10.75:1 on premium pump fuel with no problems. I decked the head and block a total of 12 thou. to bump my 10.5:1 piston kit compression up.

[speedy400]

motorcyclemodeller, Thank you! Exactly what I was looking for!-darren