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Highest piston compression on a crf450 w/ pump gas
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Posted by: Glamis400ex

How high of compression can I go with and still be run on 91 pump gas? This will be in the sand dunes also so...we all know sand is very hard on a motor. 13:1?

thanks

Posted by: deathman53

stock is at the very limit of pump gas, which I believe is 12:1.

Posted by: stocktires

quote:
Originally posted by deathman53
stock is at the very limit of pump gas, which I believe is 12:1.

Not at all, but compression is surrounded by opinions and myths.
Who's right? Who knows...
Lots of people are running 13.5:1 on pump in CRF motors on MX tracks. BUT that's with aftermarket cams also.
It all depends on your skill, riding stlye, cam, etc etc. Lots of variables.
For a stock cam in the dunes, I'd say a 12.5:1 would be very safe.
Aftermarket cam I would run 13:1
250f's are running 12.5:1 from the factory all day long.
Those are being way on the safe side too, you could easily bump it up a 1/2 of a pound if you'd like.

Posted by: 2muchquad

I have been running 13.5 to 1 in my 426 with stock cams with pump fuel for the past 2 yrs now.For the dunes make sure you go up a size on your main jet as the motor is under more load at the dunes.