compression check?

batmanfc · 08-15-2009, 07:38 PM

Of I were to do a compression check on a normal compression tester what numbers would I look for?

sen2two · 08-15-2009, 07:55 PM

three even bumps is what your really looking for. so three bumps like 30/30/30 = 90. thats ok. but not great.

110 is a nice fresh motor.

batmanfc · 08-15-2009, 08:25 PM

Ok thanks. I knew it was something like that but couldn't remember for sure.

batmanfc · 08-15-2009, 08:44 PM

How do I do it exactly?

sen2two · 08-16-2009, 02:56 PM

get a compression tester from any autoparts store pull the spark plug out and thread in the compression tester. (I always remove the fuel pump relay so i no fuel goes in the motor while doing this) then have someone crank the engine while you look at th gauge for the right numbers. do this on the front and rear on the leading plugs.

RotaryDiagnostics · 08-16-2009, 03:23 PM

Quote:

Originally Posted by sen2two

three even bumps is what your really looking for. so three bumps like 30/30/30 = 90. thats ok. but not great. 110 is a nice fresh motor.

Not sure where you got that information, but the compression readings per compression face are not additive. They are what they are for each face. Letting the pressure build over time is meaningless. You should be looking for 3 even bounces of like 80 psi or more. Make sure you remove the schraeder valve or hold in the button on the side of the gauge while cranking the engine. Make sure the gas pedal is held to the floor when cranking the engine.

However, you cannot really evaluate the compression readings derived from such a procedure since you do not know the engine cranking speed during the test. Get a free copy of the factory service manual and read up on compression testing. Better yet, for accurate information, have a compression test done by someone with a proper tester who know what they are doing.

batmanfc · 08-16-2009, 03:25 PM

I thought it was supposed to be 80 or more for each face. Thanks I wasn't really sure. The help was greatly needed.

NoDOHC · 08-17-2009, 10:39 PM

If you take both leading plugs out and can get a friend to hold the gas down and crank the car. As long as you have a 550 CCA + Battery, you should get about 375 engine rpm driven on the starter. With the valve removed from the bottom of the compression gauge, watch how high the gauge bounces when cranking. It should bounce to 80-120 psi (110 is good for turbo, 120 is good for NA). With 5,000 miles on a rather meticulous rebuild my turbo engine made 120 psi on all faces, front and rear.

You want to make sure that the gauge bounces rather quickly (goes up, down, up down) any pauses in the needle at zero indicate a bad face. A long pause could indicate two bad faces (bad or stuck apex seal) while a short pause indicates a single bad face (bad or stuck side seal).

I hope this helps you a little.

Two notes:
Never check compression after flooding the engine - it will always read low (60 psi or less).
If the engine starts every time, idles well and has good low-end torque, your compression is probably acceptable.

Another interesting note on compression:
If you have good compression, you can flood the engine (like putting a 3 bar tune in the ECU while using a 1 bar sensor) and after you get the appropriate tune in the ECU the engine will start fine without going through a deflooding procedure. (Needing to deflood all the time is usually a sign of bad compression.)

batmanfc · 08-18-2009, 05:55 PM

Awesome thanks a lot guys. What is the hybrid turbo everyone talks about all the time? I have heard of it but don't know exactly what it is.

08-15-2009, 07:38 PM	#1
batmanfc Rotarys Ruin Lives Join Date: Aug 2009 Location: Wise, VA iTrader: (0) Posts: 66 Rep Power: 16	compression check? Of I were to do a compression check on a normal compression tester what numbers would I look for? __________________ '88 Turbo II '89 GXL '07 Cobalt: DD

08-15-2009, 07:55 PM	#2
sen2two KTEC Join Date: Jan 2009 iTrader: (2) Posts: 642 Rep Power: 0	three even bumps is what your really looking for. so three bumps like 30/30/30 = 90. thats ok. but not great. 110 is a nice fresh motor. __________________ what I have: 1985 Gs:12a All motor Drag car - 1973 Rx2: Play car What I had: 93 Touring: TRADED - 91 Coupe: TRADED - 90 GTU: RHD - 88 10AE: SOLD - 87 Base: SOLD - 86 Base: SOLD - 1985 GSLSE - 85 Gsl: SOLD - 80 Gs: TRADED - 1972 Rx2

08-15-2009, 08:25 PM	#3
batmanfc Rotarys Ruin Lives Join Date: Aug 2009 Location: Wise, VA iTrader: (0) Posts: 66 Rep Power: 16	Ok thanks. I knew it was something like that but couldn't remember for sure. __________________ '88 Turbo II '89 GXL '07 Cobalt: DD

08-15-2009, 08:44 PM	#4
batmanfc Rotarys Ruin Lives Join Date: Aug 2009 Location: Wise, VA iTrader: (0) Posts: 66 Rep Power: 16	How do I do it exactly? __________________ '88 Turbo II '89 GXL '07 Cobalt: DD

08-16-2009, 02:56 PM	#5
sen2two KTEC Join Date: Jan 2009 iTrader: (2) Posts: 642 Rep Power: 0	get a compression tester from any autoparts store pull the spark plug out and thread in the compression tester. (I always remove the fuel pump relay so i no fuel goes in the motor while doing this) then have someone crank the engine while you look at th gauge for the right numbers. do this on the front and rear on the leading plugs. __________________ what I have: 1985 Gs:12a All motor Drag car - 1973 Rx2: Play car What I had: 93 Touring: TRADED - 91 Coupe: TRADED - 90 GTU: RHD - 88 10AE: SOLD - 87 Base: SOLD - 86 Base: SOLD - 1985 GSLSE - 85 Gsl: SOLD - 80 Gs: TRADED - 1972 Rx2

08-16-2009, 03:25 PM	#7
batmanfc Rotarys Ruin Lives Join Date: Aug 2009 Location: Wise, VA iTrader: (0) Posts: 66 Rep Power: 16	I thought it was supposed to be 80 or more for each face. Thanks I wasn't really sure. The help was greatly needed. __________________ '88 Turbo II '89 GXL '07 Cobalt: DD

08-17-2009, 10:39 PM	#8
NoDOHC The quest for more torque Join Date: Sep 2008 Location: Sheboygan, Wisconsin iTrader: (1) Posts: 855 Rep Power: 17	If you take both leading plugs out and can get a friend to hold the gas down and crank the car. As long as you have a 550 CCA + Battery, you should get about 375 engine rpm driven on the starter. With the valve removed from the bottom of the compression gauge, watch how high the gauge bounces when cranking. It should bounce to 80-120 psi (110 is good for turbo, 120 is good for NA). With 5,000 miles on a rather meticulous rebuild my turbo engine made 120 psi on all faces, front and rear. You want to make sure that the gauge bounces rather quickly (goes up, down, up down) any pauses in the needle at zero indicate a bad face. A long pause could indicate two bad faces (bad or stuck apex seal) while a short pause indicates a single bad face (bad or stuck side seal). I hope this helps you a little. Two notes: Never check compression after flooding the engine - it will always read low (60 psi or less). If the engine starts every time, idles well and has good low-end torque, your compression is probably acceptable. Another interesting note on compression: If you have good compression, you can flood the engine (like putting a 3 bar tune in the ECU while using a 1 bar sensor) and after you get the appropriate tune in the ECU the engine will start fine without going through a deflooding procedure. (Needing to deflood all the time is usually a sign of bad compression.) __________________ 1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers) 1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic)

08-18-2009, 05:55 PM	#9
batmanfc Rotarys Ruin Lives Join Date: Aug 2009 Location: Wise, VA iTrader: (0) Posts: 66 Rep Power: 16	Awesome thanks a lot guys. What is the hybrid turbo everyone talks about all the time? I have heard of it but don't know exactly what it is. __________________ '88 Turbo II '89 GXL '07 Cobalt: DD