High Compression at High Elevation?

I live in Wyoming at about 5000ft. Elevation. I was thinking about purchasing a

high compression piston from C.P to help compensate for the air density loss at

this elevation. My question is, What would be the highest compression ratio I

could go at this elevation before I would have to run a higher octane fuel? And

how low could I go elevation wise before I would have to increase the octane of my fuel.

I can't answer your question specifically but I can tell you that I am running a 13.5:1 piston at between 5000 and 6000ft on 91 pump gas with no trouble. Hope that helps a little.


Anyone else have any info for me?

At high altitude, air is less dense, so at any given compression ratio, the specific compression pressure will be lower. Thus, either less octane is required to prevent detonation, or higher compression is tolerable at a given octane rating. If you can get legitimate 91 octane ( (R+M)/2 method, standard pump gas), you should be able to run 13.5:1 at your elevation with no problems at all. In fact, that compression ratio will in part compensate for your altitude.

Thanks Gray. Would it be possible to gain all of the air density loss back with a high comp piston? What would be the ratio of the piston that would be the closest.

Possibly, you could completely offset the altitude as far as compression goes, but I doubt that would make up for the lack of density in the fuel air charge, even so, still, if you get it jetted right, your bike won't be too far down on what the flat landers are running, and everyone you run against will be in the same air you are.

Eddie Sisneros posted a dyno run on a stone stock '06 YZ450 and got 49.75 at 5000 feet, so it ain't so bad.

That dyno run wasn't corrected for altitude was it? The HP seems kinda high for up here but I guess the 450 was supposed to be around 52 stock right?

That dyno run wasn't corrected for altitude was it?
Not sure. I'll dig it up and see. Either way, Eddie ran a stock '06 CRF the same day as a comparison, and the Honda put up .5 less.

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