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Author Topic: Compression test results...  (Read 2946 times)
Camaroon
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« on: November 10, 2010, 04:54:57 PM »

Hey...what is the rule of thumb for compression test results? I've heard that cylinders should not be more than 15% variation between the highest compression cylinder and the lowest.
So, if the highest cylinder reads 140 pounds, the lowest should not be lower than 119 pounds.
Does this sound resonable?
Also, if an engine delivers compression test results in the 180 pound range, does this reading seem like a high compression engine that would require premium fuel?
Is there a guide for proper compression readings, such as what should a healthy good old 210 horse 327 show on the tester, or how about a 300 horse 350?   
Thanks!
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JohnZ
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« Reply #1 on: November 11, 2010, 11:21:49 AM »

Compression test readings for all engines are shown in the Tune-Up Chart in the "Specifications" section at the rear of the Chassis Service Manual; 327/210 and 350/300 show as 160 psi; high-performance engines are generally lower (150 psi) due to cam/valve timing differences. Compression tests should always be done on a warm engine, with the throttle blocked wide open.
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'69 Z/28
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Camaroon
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« Reply #2 on: November 11, 2010, 03:59:26 PM »

OK, John...thanks for the reply. It sure would be nice to know what is considered OK for amount of variation i.e., as a percentage, between cylinders.

Also, your statement about high performance engines being generally lower (150 psi), seems conter-intuitive, as one would assume high performance naturally aspirated engines have generally higher compression ratios, hence a higher compression test result, rather than a lower result.

As always, thank you for your contributions!
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JohnZ
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« Reply #3 on: November 12, 2010, 12:53:35 PM »

OK, John...thanks for the reply. It sure would be nice to know what is considered OK for amount of variation i.e., as a percentage, between cylinders.

Also, your statement about high performance engines being generally lower (150 psi), seems conter-intuitive, as one would assume high performance naturally aspirated engines have generally higher compression ratios, hence a higher compression test result, rather than a lower result.

As always, thank you for your contributions!

The notes to the chart indicate that 20 psi is the maximum allowable difference between cylinders. The static compression RATIO is a simple mathematical calculation that compares the total volume of the cylinder and combustion chamber at bottom dead center vs. top dead center - nothing else affects that calculation. What you're seeing with a compression check is DYNAMIC compression, which is affected by valve timing events; the cams in the high-performance engines have more duration and overlap, so they produce slightly less pressure during a compression test than the same engine with a standard cam.
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'69 Z/28
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Camaroon
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« Reply #4 on: November 13, 2010, 01:36:14 PM »

OK, John...that makes sense, and is supported  by the specifications chart in the chassis book.
Thanks for bringing the spec chart to my attention, as now I know that my engine(s) are within the acceptable range.

 
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myty
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« Reply #5 on: November 15, 2011, 08:05:57 PM »

 Ok guys, has anyone done a compression test on a dz302?  Mine average 125 to 130 after 3 turns of the motor and 150 to 155 after maxing out at 7 turns.  Is this in the ball park or a little low?  All plugs removed, throttle open,11 to 1 pistons, comp 346 cam, rebuilt w new rings and broken in. Thank you, Gary
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myty
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« Reply #6 on: February 03, 2012, 11:14:09 PM »

 Any body?  Thanks, Gary.
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JohnZ
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« Reply #7 on: February 04, 2012, 10:52:46 AM »

Any body?  Thanks, Gary.

150-155 is fine.
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'69 Z/28
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myty
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« Reply #8 on: February 05, 2012, 03:34:04 PM »

thanks john
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