Compression test results, normal numbers?
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Pro
Joined: Apr 2011
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Compression test results, normal numbers?
Just curious as I have no clue what's normal and the service adviser didn't either unfortunately. He told me the tech said they were all withing gm specs. I wanted to get a compression test done because of a noise that doesn't go away on the drivers side(slight metallic rattle/*****, very faint have to lean in to hear it). Can someone give me feedback on these numbers? Thanks
cylinder 1=190 cylinder 2= 190
cylinder 3= 185 cylinder 4= 185
cylinder 5= 185 cylinder 6= 180
cylinder 7= 185 cylinder 8= 185
cylinder 1=190 cylinder 2= 190
cylinder 3= 185 cylinder 4= 185
cylinder 5= 185 cylinder 6= 180
cylinder 7= 185 cylinder 8= 185
Burning Brakes
Joined: Nov 2009
Posts: 1,114
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From: California
I don't remember exactly what our road race bike engines would test at...but those numbers seem really good. We hardly ever tested cranking pressure. We would just re-build every 1000K 
What type of oil are you running, and what's your oil pressure at idle?
How often do you change your oil, and how many miles are on the engine?
What type of oil are you running, and what's your oil pressure at idle?
How often do you change your oil, and how many miles are on the engine?
Thread Starter
Pro
Joined: Apr 2011
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I don't remember exactly what our road race bike engines would test at...but those numbers seem really good. We hardly ever tested cranking pressure. We would just re-build every 1000K
What type of oil are you running, and what's your oil pressure at idle?
How often do you change your oil, and how many miles are on the engine?
What type of oil are you running, and what's your oil pressure at idle?
How often do you change your oil, and how many miles are on the engine?
today, 82 degrees out, oil temp 230, oil pressure was 25 at idle.
miles: 46,178, oil changed every 3k
Burning Brakes
Joined: Nov 2009
Posts: 1,114
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From: California
Interesting...seems a little low on oil pressure, but not bad.
For sure stay with Mobil 1; but you might consider going to 0-40. I used Mobil 1 in all my race engines, and it always kept the bearing wear to a minimum. And it also produced the least amount of top end noise when compared with our sponsored oils...and some others I paid out of pocket for.
The funny thing about the Mobil 1 was, the oil pressure was a little higher than a few of the others I tested, but the power was a little higher. I saw about 1hp increase across the board compared to the next best. This is on a 600cc engine producing around 125hp at the tire. Every hp counts when .01's of a sec. count
Hard to say what the sound is without hearing it. Have you had a cam put in it or anything? There's no info in your profile.
For sure stay with Mobil 1; but you might consider going to 0-40. I used Mobil 1 in all my race engines, and it always kept the bearing wear to a minimum. And it also produced the least amount of top end noise when compared with our sponsored oils...and some others I paid out of pocket for.
The funny thing about the Mobil 1 was, the oil pressure was a little higher than a few of the others I tested, but the power was a little higher. I saw about 1hp increase across the board compared to the next best. This is on a 600cc engine producing around 125hp at the tire. Every hp counts when .01's of a sec. count
Hard to say what the sound is without hearing it. Have you had a cam put in it or anything? There's no info in your profile.
Thread Starter
Pro
Joined: Apr 2011
Posts: 689
Likes: 47
Interesting...seems a little low on oil pressure, but not bad.
For sure stay with Mobil 1; but you might consider going to 0-40. I used Mobil 1 in all my race engines, and it always kept the bearing wear to a minimum. And it also produced the least amount of top end noise when compared with our sponsored oils...and some others I paid out of pocket for.
The funny thing about the Mobil 1 was, the oil pressure was a little higher than a few of the others I tested, but the power was a little higher. I saw about 1hp increase across the board compared to the next best. This is on a 600cc engine producing around 125hp at the tire. Every hp counts when .01's of a sec. count
Hard to say what the sound is without hearing it. Have you had a cam put in it or anything? There's no info in your profile.
For sure stay with Mobil 1; but you might consider going to 0-40. I used Mobil 1 in all my race engines, and it always kept the bearing wear to a minimum. And it also produced the least amount of top end noise when compared with our sponsored oils...and some others I paid out of pocket for.
The funny thing about the Mobil 1 was, the oil pressure was a little higher than a few of the others I tested, but the power was a little higher. I saw about 1hp increase across the board compared to the next best. This is on a 600cc engine producing around 125hp at the tire. Every hp counts when .01's of a sec. count
Hard to say what the sound is without hearing it. Have you had a cam put in it or anything? There's no info in your profile.
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St. Jude Donor '09
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From: Des Moines, Iowa
185-190 is pretty good, that is what my engine made just after break-in. I'd say 160 is pretty low (for a newer motor), I would expect that from really low compression pistons maybe, like under 9:1, or a really high overlap cam that wont build as much compression during the test. Heck even my Talon with 8.5:1 pistons made 170psi.
Last edited by ashanson; Mar 24, 2012 at 05:05 AM.
Burning Brakes
Joined: Feb 2004
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From: Sanford NC
Are we talking Cranking Compression? If so, those are good for a stock motor. I actually figured they would be a tad higher. All of my motors that have 8.6-8.8 DCR (which is about max for pump gas) come in in the 200-215psi range. What you are looking for is difference between cylinders. 5psi is nothing to worry about. The only issue is all the variables of the test. Engine temp, Engine revolutions, TB blade restrictions, Revolution speed, Guage bleed down, Lifter bleed down. I have a method that I use every time for consistancy.
But oil pressure of 25psi at idle is a little concerning.
But oil pressure of 25psi at idle is a little concerning.
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Just curious as I have no clue what's normal and the service adviser didn't either unfortunately. He told me the tech said they were all withing gm specs. I wanted to get a compression test done because of a noise that doesn't go away on the drivers side(slight metallic rattle/*****, very faint have to lean in to hear it). Can someone give me feedback on these numbers? Thanks
cylinder 1=190 cylinder 2= 190
cylinder 3= 185 cylinder 4= 185
cylinder 5= 185 cylinder 6= 180
cylinder 7= 185 cylinder 8= 185
cylinder 1=190 cylinder 2= 190
cylinder 3= 185 cylinder 4= 185
cylinder 5= 185 cylinder 6= 180
cylinder 7= 185 cylinder 8= 185
http://www.ls2.com/boggs/torques/engcompress.htm
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Unfortunately, getting decent results from a compression test can be very misleading.
Case in point, when my LS7 let go back in 2009 I did a compression test before pulling the heads. Believe it or not, even with a cracked piston the numbers on this cylinder were fine.
Case in point, when my LS7 let go back in 2009 I did a compression test before pulling the heads. Believe it or not, even with a cracked piston the numbers on this cylinder were fine.
Drifting
Joined: Jun 2008
Posts: 1,746
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From: Jupiter Fl
A general rule of thumb is no more than a 10% variance between the highest and lowest cylinder. So in your case 10% of 190 = 19 psi, so anything lower than about 171psi, could be an issue. As others have stated, any adjacent cylinders that share low compression could be a head gasket problem etc.
Still the cranking compression test is just one diagnostic test. A Cylinder Leak down test is the best for determining if a cyl has a bad piston, valve, cracked block etc. Good luck with your issue.
Still the cranking compression test is just one diagnostic test. A Cylinder Leak down test is the best for determining if a cyl has a bad piston, valve, cracked block etc. Good luck with your issue.
Le Mans Master
Joined: Jul 2005
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185-190 is pretty good, that is what my engine made just after break-in. I'd say 160 is pretty low (for a newer motor), I would expect that from really low compression pistons maybe, like under 9:1, or a really high overlap cam that wont build as much compression during the test. Heck even my Talon with 8.5:1 pistons made 170psi.
Also, your engine would have made 185-190 at 10 miles...there is no break-in for the engine, the rings are seated by the time you pick your car up. That's the result of GM using moly rings with a plateau finish honing process...they've been doing it that way for many years. The break-in is mainly for the ring and pinion, brakes, and somewhat for the transmission.
Tech Contributor
Joined: Jul 2002
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From: St. Louis, MO
St. Jude Donor '08
I wouldn't worry about that engine and I'd continue to run recommended oil.
If you have a rattle, if you have headers, check where they run close to the pinch weld the floor board. Sometimes they hit a little there, I had to bend my pinch weld a little.
If you have a rattle, if you have headers, check where they run close to the pinch weld the floor board. Sometimes they hit a little there, I had to bend my pinch weld a little.
Le Mans Master
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Instructor
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From: Des Moines, Iowa
Overlap has zero impact on the cranking compression pressure test. If anything, the exhaust valve held open during the piston down stroke allows more air into the cylinder for compression on the piston up stroke. The exhaust valve closes at some point well before BDC. The intake valve closing point and static compression ratio are the main determinates of cranking compression pressure...or you can just use DCR which combines the two in one handy number.
Also, your engine would have made 185-190 at 10 miles...there is no break-in for the engine, the rings are seated by the time you pick your car up. That's the result of GM using moly rings with a plateau finish honing process...they've been doing it that way for many years. The break-in is mainly for the ring and pinion, brakes, and somewhat for the transmission.
Also, your engine would have made 185-190 at 10 miles...there is no break-in for the engine, the rings are seated by the time you pick your car up. That's the result of GM using moly rings with a plateau finish honing process...they've been doing it that way for many years. The break-in is mainly for the ring and pinion, brakes, and somewhat for the transmission.
For break-in, I was referring to my aftermarket build, I believe it takes more than a couple miles to get full compression, no?