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Low compression on 4 mil Rebuild


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4mil stock cyl. 66.50 pistons  with custom domes by builder has 135 psi compression how am I able to bump this up to higher compression. He told me if I changed domes they would not fit. He said it is already at .015 and very high compression, and race fuel is needed.  I am running +4 timing and 33 pwk keihn. What are some ways I could bump the compression up to 150-170 or so to get more power. I’m new at this so I’m having a hard time understand, but I am willing and wanting to learn more.

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Gusto is correct, 135psi is perfect for pump gas and running race fuel on a play bike is expensive for no real reason. Who is your builder? If your builder told you to run race fuel at 135psi ask again to make sure you didn't misunderstand. That would be a very odd setup.

Save your money and build a bigger engine if you want more power. 0.015" squish is stupid tight. You should be about three times that.  Squish band and dome size are two different things. You can get smaller cc domes cut with the same squish but again its a waste of money in my opinion.

Edited by 97Screamer
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  • 2 weeks later...
On 5/8/2020 at 12:22 PM, 97Screamer said:
On 5/8/2020 at 12:22 PM, 97Screamer said:

Gusto is correct, 135psi is perfect for pump gas and running race fuel on a play bike is expensive for no real reason. Who is your builder? If your builder told you to run race fuel at 135psi ask again to make sure you didn't misunderstand. That would be a very odd setup.

Save your money and build a bigger engine if you want more power. 0.015" squish is stupid tight. You should be about three times that.  Squish band and dome size are two different things. You can get smaller cc domes cut with the same squish but again its a waste of money in my opinion.

Yes this is what he said that it was really vital I run race fuel. 
I’ve just built the motor for a 4mil. Crank. I think he kinda messed it up because I have cool heads so he should have to cut the domes or anything right? 

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On 5/8/2020 at 9:53 AM, gusto said:

Smaller cc domes or a thinner head gasket.

135 psi sounds perfect to me though, paying for race gas is so expensive and sometimes even finding it is a real pain.

Its not fast enough?

Just want some to real kick the newer bikes butt lol. I’ve had the builder do a 4 mil but I think he messed up the Cool heads so it’s not Getting it’s full potential

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