fm 107 compression
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Novice
Joined: Mar 2013
Posts: 20
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From: knox county maine
fm 107 compression
finished assembling my 107 on a 13 fatbob. went with 777 cams and big boyz port and polished heads. ive run 3 heat cycles and it idles great(cant ride it yet do to maine weather) anyways im alittle concerned by the fact that big boyz calculator with my +3 pistons and 85cc heads says i should be 205ish ccp. im only at 170 with throttle wide open. fuel moto says dont worry about it util after brake in and none of there kits will push over 190ccp with average about 185. so... should i be conscerned? is bigboyz calculator that far off or do i even have an issue?
Elite HDF Member
Joined: Sep 2012
Posts: 3,791
Likes: 57
From: N Texas
Big Boyz should be pretty accurate if you put everything in perfect. Did you measure your deck height, even on a + 3cc piston, you may still be in the hole some, stock is usually .008" in the hole. Did you plug in the altitude part? There are a lot of builds that go that high, but usually on bigger builds. Probably OK in Maine, not so much in Texas, and you have ACRs so that will help. Run premium gas too.
Stellar HDF Member
Joined: Jan 2013
Posts: 2,464
Likes: 50
From: Rouses Point, N.Y.
your compression is a little low, my stock was 185. on stage 2 with 555 's set at 10.3 was 198 the FM 107 I built for the maddghost with there level b heads and the 777's was 200 that was after 3 heat cycles. if you are running a plus 3 cc piston it would be 211 ccp with 0.30 head gasket with 0.40 would be 206ccp I believe you have a -3.0 cc piston with 0.30 head gasket that would give u 195 ccp is your battery fully charged that will make a difference within about 4 revolutions it should be at peak compression. if its still low add four or 5 squirts of oil in cylinders and re check compression if it comes up your rings have not seat yet if no change good possibility valves are adjusted wrong.
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Novice
Joined: Mar 2013
Posts: 20
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From: knox county maine
well i tried another compression tester and read 195 on both. i tried oil and made no change. they are a +3 dome piston, .030 gaskets but i did not measure deck height. at 195 i think i will be good to go and my other tester was just causing me to over think and freak out a bit.
Extreme HDF Member
Joined: Sep 2006
Posts: 10,114
Likes: 29
From: Vancouver B.C. Canada
Exactly! Break it in then we talk. Most ppl target on numbers, I never do. If it runs good, all that matters. 
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