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.

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.