Ok heres the dealy , i have an aging engine and alot of friends with them, so Just to have one id like to get a compression tester however i really know nothing about how to use one , or what the right one for the ecotec would be, could someone point me in the right direction



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

Go to autozone or napa or whatever and get a $35 one there, or rent one if you can.
It will have the proper adapters and instructions for that particular unit.
In general you just take a spark plug out, screw the tester lead in, connect the tester to it's lead, then crank the engine a few times (start the engine briefly).
It should show the pressure. The exact number will vary per engine, especially if you have mods, but should be around 130s.
The pressure on different cylinders should not vary more than 10 psi.
Usually if you have a problem cylinder it will be way low, like half what the others are.
If they're all below 100 you're doing something wrong, or you have serious problems.

Disable the ignition and pull the fuel pump relay out.
Pull all 4 plugs out.
Crank the engine the same amount between each cylinder. like 4-5 times.

130? Damn... my 2.4L was 210 across the board... until I blew one cylinder then it was only 120.

yea i know the pressures should be above 200 across the board just wasnt sure about the rest



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

No way 200 psi.

From the 2004 Factory Service Manual:

Perform the following steps in order to conduct a compression test for the L61.

1. Perform the following steps in order to check cylinder compression.
1.1 Engine should be at room temperature.
1.2 Disconnect wiring from the ignition module.
1.3 Remove the spark plugs.
1.4 Throttle body valve should be wide open.
1.5 Battery should be at or near full charge.

2. For each cylinder, crank engine through four compression strokes.

3. The lowest reading cylinder should not be less than 70% of the highest.

4. No cylinder reading should be less than 689 kPa (100 psi).

The results of the compression test will fall into the following categories:

Normal
Compression builds quickly and evenly to specified compression on each cylinder.

Piston Rings
Compression is low on the first stroke, tends to build up on the following strokes, but does not reach normal. Compression improves considerably with the addition of oil.

Valves
Compression is low on the first stroke, does not tend to build up on the following strokes, and does not improve much with the addition of oil. Use approximately three squirts from a plunger-type oiler.

If you have 200 psi you're runnig around 6 psi boost. I think a SUPERCHARGER may effect your readins.

The equation is simple, assuming 100% volumetric efficiency: PSI x CR = Reading. PSI = 14.7 (ideal) and CR= 9.5 (or there abouts depending on the different years and engines). So the reading should be 139 under ideal conditions.

The compression ratio for the L61 is 10:1. However, this is a static rating and the dynamic CR is always lower due to intake valve overlap. Dynamic CRs (the real CR) is always lower than the rated CR depending on the cam.




Edited 1 time(s). Last edited Friday, September 29, 2006 12:16 PM

Well it was only 25 bucks so i just bought the thing

#1 - 210
#2 - 209
#3 - 180
#4 - 209

so not so great I take it



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

hey where did you get it for 25 bucks, and is there a way you are cranking the engine other than just turning the key?

autozone and no



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

around 200 psi is correct.

Gm says no less than 100 psi, correct. But a little over the minimum is not ideal.

^^^No, 200 is not correct or even attainable in a n/a L61 engine. CTS is correct in his calculations and his assumption that if pressures of 200 psi are being attained, then it is the result of running a supercharger with approximately 6 psi boost. If I'm not mistaken, the RSM Stage I is rated at 6 psi. 14.7 + 6 = 20.7 x 10 = 207. At sea level, a n/a engine with a true 10:1 compression would not produce pressures higher than 147.

Then how did my built motor with 8.9:1 compression push 178 180 179 181???? and thats with no turbo on it!!!!

i dont make boost at idle, and in compression test the charger isnt even spinning so dont continue to post if you dont know what your talking about



1989 Turbo Trans Am #82, 2007 Cobalt SS G85

Wrench Monkey wrote:Then how did my built motor with 8.9:1 compression push 178 180 179 181????

I have no idea. If you compress air at atmospheric pressure by a factor of 8.9, you get approximately 131. How do you explain this discrepancy? I'm not trying to be a wiseass. If all these high numbers are legitimate, I really want to understand why the formulas that normally predict engine cranking pressures are invalid for this application.

I had #'s in the 180-190 range when I did my last compression test, NA 2.4, throttle body was closed as I hadn't heard that one at the time. The formula makes sense, but the expected results are not what I've seen in the real world.


15.3 @ 89.97mph, 14's on the way?

My cressida with stock compression of 9.2:1 had cranking pressures of 185psi...When I stroked it and ran up the compression to 11.5:1 I got cranking pressures of 225psi. My cav got 185 across the board this last time I tested it.

Different engines can have the same CR and have different psi ratings. Like Mitsubishi engines for example. The 2.4L NT Spyder engine has a c/r of 9.5:1 and compression pressure of 192. The Galant SOHC has 9.5:1 c/r also but only 185psi compression pressure.

And no, You're not doing the math right. Boyle's Law(P1xV1)/T1=(P2xV2)/T2. We are going to assume 10:1 for ease of calculation on a 2.0L engine. That will give us .05L of displacement at TDC. We are going to solve for P2. P is 1atm(14.7psi) T is the temerature of the gas in Kelvin(using 85degF as a base #). Temperature rises about 50degK durring this compression, and Volume is in L so we have .5L and .05L.

It'll be P2=(P1xV1xT2)/(T1xV2)

P2=(1atm*.5L*352K)/(302K*.05L)
P2=176atm*L*K/15.1K*L
P2=11.66atm
P2=171.4PSI

You aren't calulating the temperature increase that is involved in compression. That's why the ratio is different then just 14.7*10 being 147PSI.

Forgive me if my numbers are off I was running off what I can remember. And windows calc program.


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'96 Cavalier Good ol' Pushrod 2.2
-24X,000 miles on factory build
-Some oil loss between changes, me thinks it be rings.

I forgot to add that I didn't use air as my gas for compression and that it's just an example. Air has different compression properties so I beleive it gets hotter with that compression. It's safer to go with the factory compression numbers because the engines will all be different. some will absorb more heat formt he air making cranking pressure lower.


--------------------------------------------------
'96 Cavalier Good ol' Pushrod 2.2
-24X,000 miles on factory build
-Some oil loss between changes, me thinks it be rings.

Lees04Cav wrote:^^^No, 200 is not correct or even attainable in a n/a L61 engine. CTS is correct in his calculations and his assumption that if pressures of 200 psi are being attained, then it is the result of running a supercharger with approximately 6 psi boost. If I'm not mistaken, the RSM Stage I is rated at 6 psi. 14.7 + 6 = 20.7 x 10 = 207. At sea level, a n/a engine with a true 10:1 compression would not produce pressures higher than 147.

You aren't creating boost at 1K rpm or less. Another example: 04 saab 9-3 with the 2.0L ecotec. Lower static compression ratio than n/a ecotec 2.2. Cranked out 210 psi across the board.