68 L71 compression test results, high?
Thread Starter
Burning Brakes
Joined: Jul 2016
Posts: 767
Likes: 74
From: Blaine MN
68 L71 compression test results, high?
Finally got around to doing a compression test on my 68 tri-power L71,
Engine warmed up, throttle/choke wide open.
Wasn't super concerned with number of rotations (I know, we should have counted), I'd say all of them were 5-6 rotations I just had the helper crank til we hit max value.
205, 215, 210, 212, 203, 198, 205, 210
This is a CE replacement engine dated 69, Mech. Cam ( I set lash last july).
Do these values seem high (carbon build up maybe)?
I'm planning on pulling off the Heads to install a set of AFR Alum. Oval Ports rather than pull the Engine and rebuild.
Engine warmed up, throttle/choke wide open.
Wasn't super concerned with number of rotations (I know, we should have counted), I'd say all of them were 5-6 rotations I just had the helper crank til we hit max value.
205, 215, 210, 212, 203, 198, 205, 210
This is a CE replacement engine dated 69, Mech. Cam ( I set lash last july).
Do these values seem high (carbon build up maybe)?
I'm planning on pulling off the Heads to install a set of AFR Alum. Oval Ports rather than pull the Engine and rebuild.
Melting Slicks
Joined: Sep 2016
Posts: 2,215
Likes: 972
From: Wauconda IL
Quote from Super Chevy regarding compression testing:
What You're Looking For
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
What You're Looking For
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
Static compression together with the cam (intake valve closing point) determines your cranking compression. If you have a mild cam and a high compression motor 200+ PSI is normal.
For comparison: My 454 is a true 10.15:1 (everything measured). Cam has 231/239 dgr @050 and is a hydraulic roller. My cranking compression is 202-205 PSI.
I would be a little worried about the one cylinder at 198 PSI. That one gets close to the 10% variation that is typically considered max for a healthy engine.
For comparison: My 454 is a true 10.15:1 (everything measured). Cam has 231/239 dgr @050 and is a hydraulic roller. My cranking compression is 202-205 PSI.
I would be a little worried about the one cylinder at 198 PSI. That one gets close to the 10% variation that is typically considered max for a healthy engine.
Last edited by zwede; Apr 13, 2017 at 08:56 PM.
Burning Brakes
Joined: Mar 2006
Posts: 793
Likes: 1
From: Houston Texas
Finally got around to doing a compression test on my 68 tri-power L71,
Engine warmed up, throttle/choke wide open.
Wasn't super concerned with number of rotations (I know, we should have counted), I'd say all of them were 5-6 rotations I just had the helper crank til we hit max value.
205, 215, 210, 212, 203, 198, 205, 210
This is a CE replacement engine dated 69, Mech. Cam ( I set lash last july).
Do these values seem high (carbon build up maybe)?
I'm planning on pulling off the Heads to install a set of AFR Alum. Oval Ports rather than pull the Engine and rebuild.
Engine warmed up, throttle/choke wide open.
Wasn't super concerned with number of rotations (I know, we should have counted), I'd say all of them were 5-6 rotations I just had the helper crank til we hit max value.
205, 215, 210, 212, 203, 198, 205, 210
This is a CE replacement engine dated 69, Mech. Cam ( I set lash last july).
Do these values seem high (carbon build up maybe)?
I'm planning on pulling off the Heads to install a set of AFR Alum. Oval Ports rather than pull the Engine and rebuild.
