My 3.4 that I swapped in last year has started smoking under high vacuum situations and suspected that maybe the rings are getting tired. I ran a compression test today and these are the #'s I got:

CYL 1 195 psi
CYL 2 198 psi
CYL 3 195 psi
CYL 4 179 psi (adding some oil to the cylinder 220 psi)
CYL 5 195 psi
CYL 6 150 psi (adding some oil to the cylinder 210 psi)

Based on this I have confirmed that 4 and 6 are definately not sealing so I know what I will be doing this winter.
What I am puzzled about is how high the numbers are. I had expected to see the good cylinders in the 150-160 range and the bad in the 100-110 range.
I am not sure what a 3.4 pushrod motor usually tests at, but based on the #'s I have seen on here from other people these seem high.

I did do a compression test on the engine when installed it and the #'s at that time were 195 psi on 4 of the cylinders and 190 on 2 of them. I am an Acura mechanic for a living and at the time the #'s made sense as they are about what I am used to seeing on most of the engines I work on and they were very consistent from cylinder to cylinder, so I went ahead and installed the motor as it came from the junkyard.
So can anybody tell me what the numbers should be on a norrmal 3.4?
I should note that I am running a GM crate motor cam (same specs as Crane H260)
I have heard that a cam designed for high torque values at low rpm can increase the readings on a compression test so maybe thisis wat I have experienced.

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85 GT 4-speed
3.4 pushrod, 390 Holley carb, Edlebrock intake, MSD 6A ignition. 04 Gran Prix exhaust tips, Ported manifolds and lots more to go.
Richey

[This message has been edited by STRATOHACKER (edited 11-13-2004).]

220 psi works out to 14.9:1 compression ratio. i'd be suspicious of your guage, unless the 3.4 is a hi-compression engine. what's the book compression ratio?

ok, here's how i figured this. nominal air pressure at sea level is 14.7 psi. at higher altitudes the pressure decreases. 220/14.7=14.965...

[This message has been edited by lurker (edited 11-14-2004).]

Depending on how much oil was added to the cylinder the reading would go wayyyyy up(can't compress a liquid). I would tend to suspect valve stem seal before rings.

quote Originally posted by lurker: 220 psi works out to 14.9:1 compression ratio. i'd be suspicious of your guage, unless the 3.4 is a hi-compression engine. what's the book compression ratio?

Stock the 3.4L is suppose to be 9.0:1

quote Originally posted by lurker: 220 psi works out to 14.9:1 compression ratio. i'd be suspicious of your guage, unless the 3.4 is a hi-compression engine. what's the book compression ratio?

Theres to many variables to say that XXX psi = XX:1 compression. 200+psi isnt suprizing on a higher-compression engine with a tight camshaft.

A better way to check the health of the motor is to do a differential air pressure/compression check. You will need an air compressior, air regulator, the gauge from your compression gauge, the hoses and fittings for the spark plug. You are pressurising the cylinders and seeing if the rings, valves and stem seals are sealing. If the pressure leaks down, listen at the exhaust, the intake, and the valve cover (oil fill ) for escapeing air. This will tell you where the damage/wear is and gives a better idea what needs to be repaired. Your basic compression check only tells you if the motor is sealing or not.

[This message has been edited by ron768 (edited 11-14-2004).]

Thanks for the replys. Now to clarify a few things.My guage is MAC tools top of the line guage and I also have their differential cylinder pressure tester. It may be that my guage reads high but I would be surprised if it did, as it has been really consistent alll along and I do both compression and leak down tests on an almost weekly basis. The valve stem seals are new and the motor does not smoke at start up which would be more indicative of seals. I did not run a leak down test due to the fact that adding a small amout of oil to the cylinders caused the compression to come back. If the compression stays about the same when adding oil, then it is more likely valves causing the lack of a good seal. My compression ratio should be 9.0-1 as Oreif stated.
It is a stock block 3.4 out of a Camaro. The only changes I made were the cam and I replaced the valve springs with the correct ones for the cam along with the retainers and locks to give me the correct install height on the springs.

Does anyone know what the psi range is for this motor? Has anyone ele gotten similar numbers when running a compression test on a 3.4 pushrod motor?

Richey

When I first bought my 3.4L I did a compression check before pulling it apart (to make sure I was starting with a decent block) and with my gauge I had between 185-190 between the cylinders. As a reference, The 2.8L I pulled out was reading 165-170 on an 86K mile engine. I never ran one on the 3.4L after I built it because everything was new and it ran great.

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Happiness isn't around the corner...
Happiness IS the corner.
ZZ4 Powered !!

[This message has been edited by Oreif (edited 11-14-2004).]

I was doing a tune on my 3.4 GM crate motor a few weeks ago. The motor has a little over 35K and I hadn't previously done a compression test. I pulled all the plugs and did a compression test. The motor was cold as as I really don't like pulling the firewall side plugs on a hot engine. I used a new tester (Mityvac) that I hadn't used before. I held the throttle body fully open and used ten revolutions since I was working alone and couldn't see when the compression stopped rising. All six cylinders came out to be 195+ pounds. Since all of the cylinders were within a couple of pounds, I thought the pressure was higher than normal and thought the new guage was probably reading high. I know there is no real carbon build up and the plugs are running light gray.

The HT3.4 runs the same 9.0 ratio as the Camero 3.4. I had planned to try another tester to verify the pressure but I haven't been yet. Your pressures are not out of line with what I found.

My engine runs smoothly and put out 139HP at 4500 and 190# of torque at 3000 on the dyno.

James

Hmmm, wonder what happens if you get a drop of oil in the compression gage?

Oreif and Ammotrup:
Thanks for posting your numbers It sounds like mine are in line for a 3.4 and I have the same cam as the HT motor so 190-195 is probably where it should be.

I had a problem with oil blow by in the intake when I first put the motor in the car that traced to a bad breather on the rear valve cover that was not breathing and I think I probably washed the 2 cylinders causing the ring wear. In any case i will most likely be rebuilding the motor when the snow flies anyway so I wil figure it out when tear it down.

Cadillac Jack:
I am not sure you could get oil in the tester itself though it my be possible to get it in the hoses that connect to the guage. When adding oil to the cylinders you are putting in a very small amount and only cranking the engine for a very short period of time so it is unlikely that it would get blown in.
However when you start the car after doing this it will blow a nice oil cloud

Richey