Compression Gauge
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Stellar HDF Member
Joined: Mar 2012
Posts: 2,008
Likes: 596
From: Columbus, OH
Compression Gauge
What are you guys using? What do you trust for accuracy?
Here's the situation: When I calculate my CCP with Big Boyz I get 10.5:1 Corrected 9.63:1 and CCP 202.0 @sea level (Columbus OH is 902' according to Google)
107" (3.938x4.375)
0.040 head gasket
deck height 0.002 front
deck height 0.003 rear
chamber volume 101.5cc
piston dome 18cc
When my tuner checked compression he came up with 220/220 with his brand new Snap-on gauge, and 210/210 with an older auto parts store brand gauge. I just checked with my old beat up Snap-on with broken/missing glass and needle resting on 25psi all the time that I've had for 25+ years. I got 220/225. I had my Snap-on man replace my gauge and I was wanting to do an old gauge/new gauge comparison just for s#@ts and giggles, but turns out new gauge has a different thread and wont fit my hose. I'll resolve this tomorrow since he will be in. I find it interesting that my old beat up gauge (that I don't exactly trust) came up with similar results to a new one, but what gauge is accurate. I could probably check it with a pressure regulator, set regulator then see if there is a differential. More questions than answers right now?????
Here's the situation: When I calculate my CCP with Big Boyz I get 10.5:1 Corrected 9.63:1 and CCP 202.0 @sea level (Columbus OH is 902' according to Google)
107" (3.938x4.375)
0.040 head gasket
deck height 0.002 front
deck height 0.003 rear
chamber volume 101.5cc
piston dome 18cc
When my tuner checked compression he came up with 220/220 with his brand new Snap-on gauge, and 210/210 with an older auto parts store brand gauge. I just checked with my old beat up Snap-on with broken/missing glass and needle resting on 25psi all the time that I've had for 25+ years. I got 220/225. I had my Snap-on man replace my gauge and I was wanting to do an old gauge/new gauge comparison just for s#@ts and giggles, but turns out new gauge has a different thread and wont fit my hose. I'll resolve this tomorrow since he will be in. I find it interesting that my old beat up gauge (that I don't exactly trust) came up with similar results to a new one, but what gauge is accurate. I could probably check it with a pressure regulator, set regulator then see if there is a differential. More questions than answers right now?????
Seasoned HDF Member
Joined: Aug 2008
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hummmm??? is your pressure regulator correct??? they have quite a dead band. the best way to test a gauge is with hydraulics.
there are also many ways a gauge is built, the latest and greatest are the crystal or piezo types. with mechanical parts removed, less error. compression is just a ball park test anyway, many things can change the outcome.
there are also many ways a gauge is built, the latest and greatest are the crystal or piezo types. with mechanical parts removed, less error. compression is just a ball park test anyway, many things can change the outcome.
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Joined: Jul 2015
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From: SF Bay area
Sometimes things don't work out like the calculator says. My build with 57 cams was supposed to be 205 ccp according to the calc.... it was actually 225-230. I changed to T-Man 590 cams, according to the calc it should have dropped ccp 7 psi.... actually made zero difference.
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Seasoned HDF Member
Joined: Mar 2010
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From: Poolville
With the RB racing calculator I come up with 10.48 to 1 compression ratio and with that 41 degree cam everything comes out to right around 200 CCP at Sea level...I would think that your heads were not cc'd right... 101.5 is a huge chamber....
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Stellar HDF Member
Joined: Mar 2012
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From: Columbus, OH
Heads are CVO 110 (95cc average chamber size to start with from what I understand), so with a little chamber work, if the valves were sunk a little then it's possible. Heads were sent back and clearanced, when first assembled the piston domes contacted chamber right by the compression release. Sheet on the heads says 82cc and CP piston box said 18cc dome. I asked my machinist about this and he said heads were 100cc pistons were 18cc gave us an 82cc chamber, when heads were clearanced he said we gained about 1.5cc. 10.5:1 was our target for static and if all of those numbers are right then we hit our mark. This is where the accuracy of the compression tester comes in, 220 CCP doesn't agree with all of the other math. Bike runs great no problems there, but the .625 is a little soft down low, there is another cam that was recomended to me by my tuner but it wants a little more compression than 10.5:1. If the compression testers are right then I may try it, if the math is right then I don't see gaining anything on the bottom end.
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HDF Community Team
Joined: Jun 2006
Posts: 12,763
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From: san antonio
Rare when actual compression is the same as calculated; just doesn't happen very often. I suspect there is a variance is between the actual and measured chamber and dome volume.
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Thread Starter
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Stellar HDF Member
Joined: Mar 2012
Posts: 2,008
Likes: 596
From: Columbus, OH
But....... Back to my original question, what compression tester are you guys using? Or what compression gauge do you trust for accuracy?
Not trying to diagnose or pick apart my build, just wondering what you folks in the motorcycle industry use
Not trying to diagnose or pick apart my build, just wondering what you folks in the motorcycle industry use
Last edited by Fat11Lo; Nov 30, 2017 at 08:56 AM.
HDF Community Team
Joined: Jun 2006
Posts: 12,763
Likes: 2,597
From: san antonio
Not in the "industry" but a simple minded DIY guy that has built a few of these motors. I have used an OTC Model 5606 for years. Actual results have never matched calculated but have never had a variance of 10% between the two.