Compression test--calibration problems?

I'm using an Eastern Technology E2M compression tester, on a low time (90 hr) engine.
The readings across all cylinders are consistently 74/80, which seemed a bit low.

I blocked the output end of the spark plug adapter with some firm thumb pressure to simulate "zero leakage" and found the measurement to be 76/80. Am I correct in assuming that 80/80 would be the expected reading with this testing, and thus the gauges are out of calibration by 4 psi? If so, my "corrected" measurement would be 78/80, four psi higher than the 74 that I measured?

I contacted Eastern Technology with this question, but haven't received any reply.

Hi Rob. Perhaps your gauges have a offsetting error (they don't read the same for a given pressure input). I have seen this on test sets.

To check your tester put 80psi on the input gauge and with the output valve off, read the output gauge. They should both read 80.

If not, what I do is, set the regulator to show 80 on the output gauge and then do the test.

Others may have different approaches...including returning the test set for replacement or recalibration.

78/80 is better than 74/80, but I would never consider 74/80 a low compression. For me, I check compression cold. If they are all over 70 I leave it at that. You could go flying, come back and check again and they could all be different. If you get 74, call it good and move on.

Jesse said:

78/80 is better than 74/80, but I would never consider 74/80 a low compression. For me, I check compression cold. If they are all over 70 I leave it at that. You could go flying, come back and check again and they could all be different. If you get 74, call it good and move on.

Click to expand...

This is the RV-12 forum, and he is talking about a Rotax 912 engine.

The Rotax 912 is a very tight clearance engine so comp of 74 on one with only 90 hrs would be very unusual.

I agree that there is an error in the gauge set. With the hose disconnected from the spark plug adapter (zero airflow) the two gauges should read the same.

I had a similar problem and after 1/2 day of getting varying readings, I found a a brittle fracture in the air hose between the gauges and the plug adapter. You couldn't see it unless you flexed the hose. The compression test would vary based on how much the hose was flexed. Very frustrating.

If you block the residual flow gauge outlet it should read the same as the test pressure gauge. If they do not match then you may want a new gauge.

Depending on what test pressure you use, 80 or 87 psi the differential residual should be between 2-5 psi loss. If your gauges show an 80/80 test then I usually suspect the gauge set to be off. This may be more common than some think. If you have anything past a 10 psi drop I would want to know why. Even though the manual says a 25% drop that might be okay for an air cooled engine it would be way out of line and not acceptable for a Rotax. Your results can vary 1-2 psi from annual to annual and this is normal.
This is a leak down differential test and that means there should be some difference because the cylinders are not absolutely air tight. Some air must leak by or the oil would have no way to return to the oil tank. Oil is pushed back to the oil tank by crankcase pressure. You can and should hear a slight gurgle in the oil tank when you have pressure on a cylinder. That is normal. Usually when using 80 psi test pressure the normal result is 76-78 psi. When using an 87 psi test pressure 82-85 residual is normal and because the engine is tight the cold and warm test many times are the same. That said a warm engine should be used. If the cylinder is at perfect TDC then you don't need to hold the prop as it will remain in that position with pressure on it, but stand clear unless you don't have it at perfect TDC.

Sometimes you can get a low erroneous reading. If that happens just rotate the prop a couple rotations or run the engine for 1 minute and that will go away if it isn't a real internal issue.

gauges

I suspect your gauges are not so good. When I got my compression tester it would not repeat the same cylinder within 5 lbs. a minute later. Replaced both gauges with liquid filled "calibrated' gauges from McMaster-Carr and it works great. I think the original gauges had the needle dragging on the face. Blocked off, both gauges should read the same and repeat as long as your supply air pressure is adequate.

Gurgling during differential pressure test

While doing the differential pressure test my A & P and I experienced gurgling from the oil tank (like that when doing the dip stick test). Is this common?
Pressures across all cylinders were 78-79/80 across the board even with the gurgling.

When you do a compression test remove the oil dip stick, while each cylinder has pressure in it listen for air seeping from the dip stick hole, exhaust and intake (carb or servo). Rings leaking will be air out the dip stick hole and valves out the exhaust or intake.

ben barron said:

While doing the differential pressure test my A & P and I experienced gurgling from the oil tank (like that when doing the dip stick test). Is this common?
Pressures across all cylinders were 78-79/80 across the board even with the gurgling.

Click to expand...

Yes it is normal.
The cylinders do not have a 100% seal. They do leak a little bit of air just like they have a small amount of blow by when the engine is running. It is this leakage that slightly pressurizes the crank case and pushes oil back to the oil tank (the same thing you are doing when burping the engine to do an oil level check).
Is you A&P not trained and /or familiar with working on a Rotax 912?

FasGlas said:

When you do a compression test remove the oil dip stick, while each cylinder has pressure in it listen for air seeping from the dip stick hole, exhaust and intake (carb or servo). Rings leaking will be air out the dip stick hole and valves out the exhaust or intake.

Click to expand...

The question is posted in the RV-12 forum. The Rotax 912 is not the same as a Lycoming.