Dry,wet and running compression tests

[wrench459]

I'm thinking the engine is a little low on power. So some basic tests where in order for my street 2.0L Opel.
I started with a compression test.

.Dry .Wet R@idle R@WOT
1 120 152 (115) (150)
2 120 150
3 115 145
4 120 150

The motor was at operating temp for all tests.
The dry readings was with all plugs out and the throttle wide open as also the wet readings.
Running was after clearing out all the oil in the cylinders.

My thoughts are to run though this whole process again after adjusting the valve lash. Unless ya'll see a problem.

[blancojp]

Those are the figures I am getting out of my 1.9L and the engine runs quite well.

What comp ratio you have on the 2.0L?

[wrench459]

rhzdg

[traytodd]

You using a screw in gauge or a press in rubber tip? 120/150 D/W is what I got on my low compression 1.9L engine in 2000 with a press in guage. I don't have the numbers but I did it again about 2-3 years ago and got 135/155 if I remember right. The screw is the better gauge. My engine is unrebuilt, so the numbers sound normal for an original low compression engine to me.

[hrcollinsjr]

Originally Posted by traytodd 120/150 D/W is what I got on my low compression 1.9L engine in 2000 with a press in guage. <snipped> My engine is unrebuilt, so the numbers sound normal for an original low compression engine to me.

I only recall the dry compression numbers for my old engine, a low CR - '73 engine, having between 120 and 125 PSI and that was with the top compression ring broken on every cylinder. I expect a minimum of 140 PSI on a 9:1 engine. Dan, I'm assuming we're speaking of your personal engine. If so, I'm not sure what to expect since IIRC your cam isn't exactly stock. How about a leak down test?

Harold

[RallyBob]

FWIW, I've had a perfect-running, 11.02:1 compression ratio 2.0 litre engine only give 145 psi (hot) in all 4 cylinders. Blame the overlap of the camshaft for that one, with a stock cam it would probably have 220 psi+.....

[blancojp]

Originally Posted by wrench459 I'm thinking the engine is a little low on power. So some basic tests where in order for my street 2.0L Opel. I started with a compression test. .Dry .Wet R@idle R@WOT 1 120 152 (115) (150) 2 120 150 3 115 145 4 120 150 The motor was at operating temp for all tests. The dry readings was with all plugs out and the throttle wide open as also the wet readings. Running was after clearing out all the oil in the cylinders. My thoughts are to run though this whole process again after adjusting the valve lash. Unless ya'll see a problem.

I believe I get your drift here....
I think you are trying to determine if you have a leakage problem which is causing the engine to lose power. Do the dry/wet test again with the rockers off and see how it compares to the previous figures. If they are the same then you know you have a problem.

[Jerry C]

It is very good that your compression numbers from cylinder to cylinder are so close. My stock 1.9 was about the same, 120 PSI or so. If you can get a leak down test done to the engine it will tell you a lot more. 5% or less is very good. A nice built fresh engine will show 1% or 2% loss I would consider anything less than 10% acceptable on a stock street engine.

[wrench459]

dzhzeth

[RallyBob]

Originally Posted by wrench459 Yes the motor has a mild cam(low lift) nothin fancy with some.... overlap. Could it be because of all the advance of the cam I put in?

I retarded the cam on my friend's circle-track Opel by 4 degrees, the cranking compression went from 215 psi to 185 psi! It ran like a muther on the top end, and that was all that mattered, she never saw less than 5700 rpms on the track anyway.

[wrench459]

Got the leak down test done the other night.
And the numbers are even better then when the motor was first built and sitting on the engine stand.

I'm sold on Total Seal rings even in the second ring land!

This is after close to 5k miles on the speedo and 1.5 years later.

[wrench459]

I literally stumbled upon this while working in another area.
lost jet in the 32-36 resized.jpg

Humm is this the problem and where did it come from? Here's a pic showing proper location.
jet in place resized.jpg

I'm thinking there calibration jets for the different sized motors.
So could I resize the 38 orifices to run on the 2.0 with the standard main and air correction jets?
38 resized.jpg

[strassed]

All

I pulled up an old post of mine (2002) from the Classic Opel groups and updated and added a bit.

Wrench 459 - I think your compression numbers look great. If you are having performance problems, its probably something else.

Long post but might help.

Doug

++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++

IMHO, lifter noise doesn't really have much to do with compression or a lack of compression. Just so long as the lifters are getting your valves off their seats. Noisy lifters = feeler gauge work on a hot engine for solids and replacement (?) for hydraulics.

By way of discussion:

The way I was taught to do a compression test is to do everything the same for each cylinder. Steps and rationale below -

Test the engine at operating temperature. This gives you results under conditions your engine normally operates at with all metal parts expanded and sealing as usual.

Some folks like to disconnect the coil I think for fear of burning it out. I have never disconnected and haven't had a problem.

Pull all the spark plugs - eases the work your battery and starter have to do.

Throttle and choke wide open. I use a long screwdriver down the carb. Improve your volumetric efficiency! Be careful not to bend or ding anything in the carb.

The same amount of revolutions for each cylinder - 4, 5 or 6 should be enough. Don’t give weaker cylinders the advantage of more cranks to build up pressure.

Realizing that cranking your engine so much is going to drain your battery - connect a battery charger while you are doing your compression tests.

Use a threaded pressure gauge. I don't know about you folks, but I can hardly chew gum while I drive a standard tranny (automatics - no problem!). I'm supposed to hold a rubber tipped gauge in a well recessed spark plug hole of a hot engine (with greasy hands) and crank the engine at the same time???? Threaded will increase your chances of accuracy. It does for me.

Test all holes dry first then wet. If you do a dry and then a wet immediately after, that cylinder is lubed. When you test #2 cylinder dry, it won’t have to do as much work on the dry test as #1 cylinder did. Minute differences, but by the time you get to #4 cylinder, who knows??

Evaluate the results;

If you have low compression on a dry cylinder then great compression results after the wet test on the same cylinder - the cylinder rings are bad. The oil helps seal the piston rings.

If your compression is low on the dry and it stays low on the wet - your valves are the culprit.

If you arn't gettting any compression at all, make sure the spark plug hole isn't plugged some how. Could be your valve rockers are over tightened and arn't seating at all. Stranger things have happened.

If you are only getting one bump in pressure, your intake valves may not be opening because the rocker adjustment is too loose.

Also, look for a >10% difference between cylinder compression. On the results below, look at the difference between #1 and #3. 15lbs is greater than 10% of the cylinder pressures. # 1 is doing the work, #s2 & 4 are keeping up and #3 is dragging the other three down

If your dry is high and your wet result is low - Ask the Opel experts on the list :-)

Did I miss anything?

If you have an attached garage, I also highly recommend rolling your fine machine out of your garage before starting the motor after a wet compression test. Your spouse will let you know about it if you don’t. Makes a great mosquito fogger.


-----Original Message-----
Sent: Monday, September 30, 2002 7:01 PM
To: classicopels@yahoogroups.com
Subject: [Opel] compression

Hello to all:

I'm trying to get my manta properly tuned and on the road. While I
was replacing plugs, etc. yesterday I thought I would do a quick
compression check. The engine was dead cold when these numbers were
taken, but I don't know how much difference that makes. Engine is
a '74. Throttle held wide open, all plugs removed.

Cylinder PSI
1 ----------- 130
2 ----------- 120
3 ----------- 115
4 ----------- 120

My reason for doing the compression test was that one of the lifters
seemed a little noisy the last few times I've started it (though it
has gone for extended periods between starts). These numbers seemed a
little low to me, but since the engine is a lower compression motor
and it was cold when I ran the test, I don't know that they're
necessarily out of line. The car seems to have plenty of power, even
though it is obviously out of tune. I'd appreciate any comments...

[wrench459]

Originally Posted by RallyBob FWIW, I've had a perfect-running, 11.02:1 compression ratio 2.0 litre engine only give 145 psi (hot) in all 4 cylinders. Blame the overlap of the camshaft for that one, with a stock cam it would probably have 220 psi+.....

it only took me a week and another post to get your drift.
I've even talked with Dennis from the now defunct Reed cams about this issue. First question from him was "what's the compression readings?" So it seems he assumed I had a stock cam to start with. I'll need too clarify the point Monday morning.

Originally Posted by strassed All Did I miss anything? If you have an attached garage, I also highly recommend rolling your fine machine out of your garage before starting the motor after a wet compression test. Your spouse will let you know about it if you don’t. Makes a great mosquito fogger.

Great post Doug
Thanks for your input. Thats the nice thing about the running compression test no oil involved with the results very close to the wet readings.

BTW whats those jets called in the third picture from above?
For a lack of knowledge I'm calling then high speed fuel jets. And for what its worth DON'T remove them! The carb will be fat as you know what on the top end.

[strassed]

Wrench459

I had to check out the "running compression test". Never heard of it before. Thought it might be an analysis type evaluation off a Sun machine.

I checked it out and found a pretty good run down -

Credit where Credit is due dept~ this came from Dynamic Compression Test


HOW TO PERFORM A RUNNING COMPRESSION TEST

1. Start with a normal ("static") compression test. To eliminate rings, valves, holes in pistons, that sort of things. A normal cylinder balance test is also helpful (so you know which, if any, cylinder is presenting a problem). Engine should be warm.

2. Put all spark plugs but one back in. Ground that plug wire to prevent module damage. Disconnect that injector on a port fuel system.

3. Put your compression tester into the empty hole. The test can be done without a Shrader valve, but most people recommended leaving the valve in the gauge and "burping" the gauge every 5-6 "puffs".

4. Start the engine and take a reading. Write it down

5. Now goose the throttle for a "snap acceleration" reading. Reading should rise. Write it down NOTE: Don't use the gas pedal for this snap acceleration. The idea is to manually open then close throttle as fast as possible while without speeding up the engine. This forces the engine to take a "gulp" of air.

6. Now write down your readings for at least the bad cylinder (if there is a single bad cylinder) and maybe 2-3 good ones. Make a chart like this: CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 125 Cyl 2 175 80 130 Cyl 3 160 75 120 Cyl 4 160 80 125

7. ANALYSIS: Running compression at idle should be 50-75 psi (about half cranking compression). Snap throttle compression should be about 80% of cranking compression.

EXAMPLE 1 - RESTRICTED INTAKE CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 80 If Snap reading is low (much less than 80% cranking compression), look for restricted intake air- severely carboned intake valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the runners. (Toyota, Vortec etc. with variable runner length) Comparing measurements between cylinders is important.

EXAMPLE 2 - RESTRICTED EXHAUST CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP Cyl 1 150 75 180 If snap measurements are significantly higher than 80% of cranking measurements, look for restricted exhaust on that cylinder-such as worn exhaust cam lobe, or collapsed lifter. Or, if they are all high, look for a clogged cat converter.

WHAT IS GOING ON?

When you do a normal compression test, you are checking cylinder sealing, not cylinder breathing. When you check engine vacuum at the manifold, you are looking at the breathing of the entire engine, by checking vacuum at a common (plenum) source. You aren't testing a specific cylinder. This test looks at the breathing of an individual cylinder.

Say the engine is running at 18 inches vacuum. Atmospheric pressure is about 30 inches, so the difference (30 inches - 18 inches = 12 inches) is what the engine is sucking in. 12 inches mercury is equivalent to about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you should get 6 x 8 = 48 psi pressure if all the air makes it into the cylinder and then gets pushed out. So your idle reading on running compression is about 50 psi.

When you snap the throttle, the manifold vacuum drops, so the absolute air pressure going into the cylinder increases.

In fact, you can do running compression tests at various constant manifold vacuum readings (by brake-torqueing the engine momentarily), and the running compression should roughly correspond to the manifold vacuum. For example, at 10 inches vacuum, engine should be breathing in about 10 psi air pressure, so you should see a running compression reading of about 80 psi (at 8 to 1 compression ratio).

If one cylinder reads low running compression compared to the rest it means that the air didn't make it in. If one cylinder reads high, the air didn't make it out (and the next pulse of air raised the pressure).

Not sure I will ever do one of these, but interesting. Thanks for teaching me something!

Doug

[wrench459]

Thats the one Doug
I've read that article before and employ the method with good results.
IMO there's one step missing...the WOT running, with the gauge installed
take the motor up to 3-4 grand and note the peek readings. If the motor is
breathing correctly the numbers will mirror the cranking give or take a pound.

[wrench459]

I brought home a colleagues compression gauge the other night.
His gauge showed 180lbs cranking on my 2.0L with an wot reading of 210.

Being a hard headed kind of guy. I grabbed the scope and a pressure transductor to triple check. After calibrating the sensor to the scope this is what I found.
idle.jpg

Idle speed
What do ya'll think about the 50lbs?
Do I need to subtract the reading from peak to get a true compression number?

[wrench459]

wot
wot.jpg

aint this interesting

[wrench459]

While I'm at it
I should post the decel
decel.jpg

[traytodd]

from your first chart, I get about 130 psi. It is 50 psi/major div. 2.3 div x 50 = 130. Not the 180 that you measured with the gauge, but the conditions may be different.