Cylinder Compression Values

SupraSleuth · 08-16-2009, 03:20 AM

I have an '89 Supra (7MGE) with 133K miles. It runs great, but I decided it was time to change the spark plugs. I also wanted to do a compression check before installing the new plugs. The service manual indicates a desired compression value of 156 psi or more, a minimum desired value of 128 psi, and no more than 14 psi difference among cylinders. The average values I obtained ranged from 207 to 217 psi. I did not expect to see values this great and now I am wondering if my compression guage is not properly calibrated. Insight anyone ??

TONY! · 08-16-2009, 02:48 PM

Quote:

Originally Posted by SupraSleuth

..The average values I obtained ranged from 207 to 217 psi. I did not expect to see values this great and now I am wondering if my compression guage is not properly calibrated. Insight anyone ??

Possible factors for high readings are:
1) Faulty readings
2) Carbon build up on the pistons and/or head
3) Someone put in high compression pistons
4) The head/block was/were milled enough so that it would change the compression
5) A thinner head gasket was used than stock specs

SupraSleuth · 08-16-2009, 03:22 PM

Thanks for the insights Tony.
I can rule out reasons 3, 4, and 5: I have owned this car since the day I drove it off the showroom floor and have made no such modifications. Some time ago I read something about de-carbonizing an engine. Do you think this is something I should consider doing? Is there anything harmful about running an engine with these high compression values (assuming the readings are not faulty) ?

TONY! · 08-16-2009, 03:37 PM

I have only "de-carbonized" pistons once, but the engine was fully disassembled...and that is the extent of my experience with that.

As far as continuing to drive a car with that much compression (like you said IF true), pinging/detonation/pre-ignition would be your biggest concern.
Do you hear pining when going uphills? Do your spark plugs look like what illustrations show for pinging? Sometimes plugs can be a clue, but your ears should pick up on it.
Engineers want a car to be as high compression as possible without pinging problems.
Higher compression engines are more fuel efficient and more powerful (all else being equal).

SupraSleuth · 08-16-2009, 03:56 PM

She seems to run fine; no "pinging" even under engine strain such as pulling a hill in 5th gear. At this point I think I will recheck compression with another guage (if I can get one) then get her back on the road ...

scaggsj · 08-17-2009, 03:56 AM

Those are pretty high but the important thing is that they are all pretty close to each other. How many cranks did you do? I do three revolutions then check. Are you doing it wet or dry? The wet test numbers are higher than the dry test numbers.

SupraSleuth · 08-17-2009, 11:53 AM

I did my first test with the engine warm, and had my helper crank the engine until I obtained a maximum value at each cylinder. This typically occurred after 4 or 5 revolutions. I wanted to do both dry and wet tests. However, I had a lot of oil in the plug wells that found its way into each cylinder after I removed each plug. For practical purposes, I consider the first test a wet test. I repeated the test the next day to see if the numbers would change. The numbers from my second test were consistently less than those from my first test (typically by about 5 psi). I used a Craftsman compression gauge that I purchased from Sears several years ago. This gauge had been used 3 or 4 times and is in "like new" condition. I don't think this gauge gave me erroneous values, but I have not confirmed its accuracy. Because I had so much oil in my plug wells, I am planning to install new cam cover gaskets and a new "number 3 cylinder head cover".

08-16-2009, 03:20 AM	#1
SupraSleuth Stock Join Date: Aug 2008 Location: USA Posts: 9	Cylinder Compression Values I have an '89 Supra (7MGE) with 133K miles. It runs great, but I decided it was time to change the spark plugs. I also wanted to do a compression check before installing the new plugs. The service manual indicates a desired compression value of 156 psi or more, a minimum desired value of 128 psi, and no more than 14 psi difference among cylinders. The average values I obtained ranged from 207 to 217 psi. I did not expect to see values this great and now I am wondering if my compression guage is not properly calibrated. Insight anyone ??

08-16-2009, 03:37 PM	#4
TONY! 12psi boost Join Date: Sep 2005 Location: Troy, NY 12180 Posts: 353	I have only "de-carbonized" pistons once, but the engine was fully disassembled...and that is the extent of my experience with that. As far as continuing to drive a car with that much compression (like you said IF true), pinging/detonation/pre-ignition would be your biggest concern. Do you hear pining when going uphills? Do your spark plugs look like what illustrations show for pinging? Sometimes plugs can be a clue, but your ears should pick up on it. Engineers want a car to be as high compression as possible without pinging problems. Higher compression engines are more fuel efficient and more powerful (all else being equal). __________________ -------------> ITEMS THAT ARE STILL AVAILABLE FROM AN '88 PART OUT <------------- _____\|_____ /. . . \ . . . \ . .\ ( ]/ . . . . \ . . . \ . .\[ ) _________________ /OO]====(W)====[OO\ [___________________] [ [O] ///______\\\ [O]_] ____________________[\|\|]______________[\|\|]____________________

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08-16-2009, 03:22 PM	#3
SupraSleuth Stock Join Date: Aug 2008 Location: USA Posts: 9	Thanks for the insights Tony. I can rule out reasons 3, 4, and 5: I have owned this car since the day I drove it off the showroom floor and have made no such modifications. Some time ago I read something about de-carbonizing an engine. Do you think this is something I should consider doing? Is there anything harmful about running an engine with these high compression values (assuming the readings are not faulty) ?

08-16-2009, 03:56 PM	#5
SupraSleuth Stock Join Date: Aug 2008 Location: USA Posts: 9	She seems to run fine; no "pinging" even under engine strain such as pulling a hill in 5th gear. At this point I think I will recheck compression with another guage (if I can get one) then get her back on the road ...

08-17-2009, 03:56 AM	#6
scaggsj 3" Exhaust Join Date: Aug 2009 Location: Florida Posts: 87	Those are pretty high but the important thing is that they are all pretty close to each other. How many cranks did you do? I do three revolutions then check. Are you doing it wet or dry? The wet test numbers are higher than the dry test numbers.

08-17-2009, 11:53 AM	#7
SupraSleuth Stock Join Date: Aug 2008 Location: USA Posts: 9	I did my first test with the engine warm, and had my helper crank the engine until I obtained a maximum value at each cylinder. This typically occurred after 4 or 5 revolutions. I wanted to do both dry and wet tests. However, I had a lot of oil in the plug wells that found its way into each cylinder after I removed each plug. For practical purposes, I consider the first test a wet test. I repeated the test the next day to see if the numbers would change. The numbers from my second test were consistently less than those from my first test (typically by about 5 psi). I used a Craftsman compression gauge that I purchased from Sears several years ago. This gauge had been used 3 or 4 times and is in "like new" condition. I don't think this gauge gave me erroneous values, but I have not confirmed its accuracy. Because I had so much oil in my plug wells, I am planning to install new cam cover gaskets and a new "number 3 cylinder head cover".