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Old 04-02-2004, 08:56 AM
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Compression reading on a new re-built motor?

I have a question, I helped a buddy rebuild a big block chevy. He used 9:1 pistons in this re-build. Now that the engine is in the car and running we took a compression check just to check on things. Now this is my problem, we are only getting reading about 130# across the board ...I was anticipating reading around 150#!!? ...SO is there something wrong some where? OR with 9:1 pistons, is 130# a normal reading? THX! ...Mark
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Old 04-02-2004, 09:06 AM
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Cranking psi depends on alot of other things other than just flattop pistons...Cam selection, head cc's and a ton of other things figure into the reading you will get...Plus, if the motor is REAL new, the rings may not have seated completely yet, just another thing to think about...If the reading is even across the board, I wouldn't be concerned...

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Old 04-02-2004, 11:34 AM
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Re: Compression reading on a new re-built motor?

Quote:
Originally posted by 11echo
I have a question, I helped a buddy rebuild a big block chevy. He used 9:1 pistons in this re-build. Now that the engine is in the car and running we took a compression check just to check on things. Now this is my problem, we are only getting reading about 130# across the board ...I was anticipating reading around 150#!!? ...SO is there something wrong some where? OR with 9:1 pistons, is 130# a normal reading? THX! ...Mark

My big block reads around 150 on all cylinders.

Now as said ealier, the rings probably aren't seated yet, that would cause a low reading. Check it after a few hundred easy miles and let us know the result.
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Old 04-02-2004, 12:16 PM
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It's probably that low because of the overlap on the cam. A cam with little or no overlap should read 160 psi on that motor. It's not a big deal but you will probably want a loose converter to get it moving off the line. My 8.5:1 SBC reads 150 psi with 11 deg. of overlap on the cam (Crower 267 HDP). A slightly larger Isky 270H (IDK the overlap) gave 135 psi and poor low end response, but ran great after 3000 rpm.
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Old 04-02-2004, 01:27 PM
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When you did the reading, did you disconnect the distributor and block the throttle to wide open? if not, the reading will be somewhat lower, the carb has to be open so the cylinders can get air, kind of hard to with them closed. But on the flip side I say the same thing, wait til it`s broken in, then do another test in the way I suggested.
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