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"How To" Intstructions for Engine Compression Test

UnPossible

Well Known Member
Hey- I'm in the middle of my first condition inspection and I need to do an engine compression test. I have the differential pressure tool, but I bought it 2nd hand and can't seem to find the instructions.

I think I get the idea of how to preform the test, with the exception of how to find TDC for each piston.

I did a quick Google search, but came up blank. Does anyone have a link to the instructions, or a link to a step by step walk through on how to perform this test? The tool I have is the one linked below.

Thanks,
Jason


http://www.aircraftspruce.com/catalog/topages/atsdifferentialtesters_orifice.php
 
You might call ATS and see if they have the instructions in PDF and mail them to you... 1-800-248-0638
 
Here is an interesting SB on Lycoming engine compression checks:

http://www.google.com/url?sa=t&rct=...n-rrTgM8Gt9kr_Q&bvm=bv.46226182,d.eWU&cad=rja

Also, the way that we find the TDC is to put a finger over the upper plug hole and rotate the prop until we feel pressure against the finger. When the pressure peaks then that is TDC.

Of course, make sure all plug wires are removed and all upper plugs are removed. It is also easier to do a compression check with two people....
 
Another clue if you are running mags is that the impulse will click right at TDC.
 
I wouldn't do a leakdown test (compression test) without 2 people, especially if it were my first. Find someone who has done it before and take turns on who is working the plumbing and who is working the propeller. It is astonishing how much force the 80psi generates at different prop positions, closer and further away from TDC. At one position you think "easy peasy, what's the big deal?" and just an inch or two away it's "yikes, I can't hold it!"
 
not hard sts

1. Warm up engine
2. Remove cowling
3. Remove all four top plugs
4. Install adapter in a cylinder. You can start in one and follow the firing order or not. *You will just turn the prop more if you don't
5. Turn the prop while holding your finger on the adapter. You while feel when the cylinder is on the compression stroke. Be careful the air maybe hot.(It is best to do this with two people but you can do it by yourself)
6. If you have an impulse mag you will hear a click at TDC. *If not use your finger on the adapter to determine TDC. *The prop should be in the 8 o'clock position (you can check you index prop position by by setting your timing mark to TDC)
7. Back out the regulator on your tester and attach it to the adapter.
8. While firmly holding the prop with as much of your body outside the prop plane, attach the air supply line and slowly turn the regulator to add air pressure into the cylinder
9. If you are not exactly at TDC you will feel the prop pull. Slowly move it against the pull until the pull stops. *This will be TDC. *Keep a firm hold of the prop because it can spin at anytime if you let go.
10. Increase the pressure until the left gauge reads 80 PSI and read the right gauge. *If the reading is low, you can wiggle the prop but be careful it doesn't get away from you.
11. If you have a low reading, listen to the intake, exhaust and crankcase breather to determine the problem.
12. *Back off the regulator and remove the tester for the adapter. Don't let go of the prop until all pressure is off of the cylinder.
13. Remove adapter and repeat for the other cylinders.
 
Wow, I type slow

Six responses while I typed my response. Do you guys sit around and hawk this site all day.
 
I wouldn't do a leakdown test (compression test) without 2 people, especially if it were my first. Find someone who has done it before and take turns on who is working the plumbing and who is working the propeller. It is astonishing how much force the 80psi generates at different prop positions, closer and further away from TDC. At one position you think "easy peasy, what's the big deal?" and just an inch or two away it's "yikes, I can't hold it!"

Good points, all. I have seen a prop slip out of someone's hands during a leak-down test, and it can really gain speed quickly. I know one individual that removes only one plug at a time to have some compression from the other cylinders if the prop slips, which makes some sense. I guess that we remove all top plugs just because that is how we have always done it...

It appears that we are actually doing leak-down tests:

"In very basic terms, a compression test tells you about your engine's ability to generate cylinder pressure, and a leak down test tells you about it's ability to hold pressure. Another way to think of it is that with a compression test you are seeing how much pressure the engine creates, and with a leak-down test you are seeing how much pressure it loses."

I wonder why we all refer to the test as a compression test...
 
I wouldn't do a leakdown test (compression test) without 2 people, especially if it were my first. ... It is astonishing how much force the 80psi generates ...
Just don't do it with one person.

One time ... in a airplane fix things up shop ...
An A&P tried to do that by himself ...
Someone found him laying in a pool of blood :eek:
 
I"ve got a cross brace and plug wires and baffling etc all congesting access to the top #4 plug. When I was doing my second condition inspection and struggling with removing that plug, a friend wandered in and looked at me with bemused pity and said "why don't you just remove the bottom plug intead?"

DOOOOOOHHHHH:D

I now know to remove the bottoms...:D
 
I know some that always remove the bottom plugs. They say that removing the top plug might dislodge some carbon that could then prevent a valve from closing completely, therefore causing a leak.

Many ways to accomplish the same thing, I guess...
 
Wow - that was fast.......I'll make sure to have another helper there to help hold the prop while I take the measurements - thanks!
 
When the numbers are good, they're good...but when they are low it's helpful to listen to determine where the leak(s) are:

• exhaust > exhaust valve
• carb/FI servo throat > intake valve
• oil filler tube/crankcase breather > rings

This kind of detective work **does** require two people; however once you've done a few differential compression tests the basic test can easily & safely done alone. I like to start about 40 psig and once I'm sure I'm at TDC bring the regulator up to 80 psig. It's best to ensure your compressor can maintain >80 psig even with a leaky cylinder. If the readings are low there is another whole process to go through before becoming alarmed and wrenches start flying :D
 
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Procedures I’ve read say to remove a plug from each cylinder when doing differential pressure tests - did this and got unusually low differential pressure readings. Put plugs back in (except for cylinder being tested) and got expected readings. When doing a differential compression check do you remove a plug from each cylinder or only the cylinder your checking?
 
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Every cylinder. Safer, and easier (no compression to work against on the other three). I am not sure if this will change the readings, but unusually low but acceptable is ok in my book.
 
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Differential Compression Test

Important Points in this thread.
1. If you are inexperienced; remove only the lower plug on the cylinder you are testing. If the prop gets away from you; it won?t rotate as far. It still can kill you..!!
2. Disconnect all the spark plug wires from every spark plug. **Just a safety precaution**
3. Removing the lower Plugs decreases the chances of carbon or debri getting into the cylinder area to cause other problems.
4. If you have never performed this task; have someone with experience teach you how.
5. About a year ago: A pilot I knew at the airport I fly out of was performing a compression test on his aircraft with the assistance of another pilot. The pilot/owner holding the prop let it get away from him & it stuck him in the head causing a serious injury. He lived for about 7 months with a brain injury before he went West.
**The Prop is like a loaded gun?Treat it with great respect?Or it can & will kill or seriously injure you or someone you care about***
 
And one more point not mentioned yet, don't use a compression tester from a Bonanza mechanic.

Our Lycoming compression test numbers from the manual are based on a 0.040 orifice, not a 0.060 orifice - even if the bigger orifice tester does make your numbers look better... :)
 
Just realized that I had the orifice test port open during first round of tests. Closed this on the second round with all plugs in and got good readings. Point taken re plugs in would slow down the prop if it got away. I thought that if at exactly TDC the prop would lock in place due to crank position - don?t seem to get this as prop gets pretty hard to hold.
 
@ 80 psi there is around 1287 pounds of force on the piston, you don't want that kind of energy getting away form you. Treat that prop like and alligator that will eat you if you let go of it!
 
JI thought that if at exactly TDC the prop would lock in place due to crank position - don?t seem to get this as prop gets pretty hard to hold.

It should. You shouldn't have to hold it at TDC (and it's dangerous).

If it's moving when you up the pressure, it's not at TDC.

ETA: From an earlier post
8. While firmly holding the prop with as much of your body outside the prop plane, attach the air supply line and slowly turn the regulator to add air pressure into the cylinder
9. If you are not exactly at TDC you will feel the prop pull. Slowly move it against the pull until the pull stops. *This will be TDC. *Keep a firm hold of the prop because it can spin at anytime if you let go.
10. Increase the pressure until the left gauge reads 80 PSI and read the right gauge. *If the reading is low, you can wiggle the prop but be careful it doesn't get away from you.

The problem with this is that it's a bit unclear on adding pressure, etc. Here's what I do, YMMV, but this is how I was taught:
8. While firmly holding the prop with as much of your body outside the prop plane, attach the air supply line and slowly turn the regulator to add air pressure into the cylinder to about 15-20 PSI
9. If you are not exactly at TDC you will feel the prop pull. Slowly move it against the pull until the pull stops. *This will be TDC. *Keep a firm hold of the prop while moving it to find TDC because it can spin at anytime if you let go.
9A. You may have to reduce/remove pressure entirely and start again. Don't go past 20 PSI, and keep your arms and body and HEAD away from the prop!
9B. Once you've found TDC, STAND ENTIRELY CLEAR OF THE PROP and then:

10. Increase the pressure until the left gauge reads 80 PSI and read the right gauge. *If the reading is low, you can start over and try rotating the prop through a couple of revolutions before taking a reading.

Bringing the pressure up slowly to about 20 PSI will let you find TDC without overly "fighting" the prop, and without the serious danger of having your hands/arms/whatever messing with the prop when it's at 80 PSI.

Just my 0.02.
 
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Procedures I?ve read say to remove a plug from each cylinder when doing differential pressure tests - did this and got unusually low differential pressure readings. Put plugs back in (except for cylinder being tested) and got expected readings. When doing a differential compression check do you remove a plug from each cylinder or only the cylinder your checking?

To me this sounds like your engine sat for a while before the first test, then you got some oil back onto the rings while repeating with plugs back in? There should be no effect from plugs on other cylinders. You can confirm this by loosening a plug on the adjacent cylinder(s) while pressurized.

Also, my humble opinion is that holding the prop (or being anywhere near it, let alone moving it as shown in the EAA video wtf) with pressure applied is terrible advice regardless of how many persons are there. If it is at true TDC the crank won't move when you apply pressure. Jiggling it back and forth to get a better reading defeats the purpose of the test and invites serious injury. Any time you apply pressure you and any helpers should be standing three feet behind the prop. Helpers get distracted and can distract you, so you're always safer doing this alone.

Hunting for TDC is easy: pulling the prop through by hand (with plugs in) will tell you roughly around what prop angle it is--where it starts to escape your fingers. Leave the prop near this angle, remove plug, apply pressure slowly and see which way it moves. If it goes forward you overshot it; if it goes backward you didn't quite get to it. (If no movement and no pressurization, you might be a full revolution away for that cylinder.) Remove pressure, pull through two full revolutions and repeat. Once you locate the TDC mark it with a sharpie or pieces of tape on the engine and prop flange so you won't have to hunt for the adjacent cylinder.

All this is probably much more complicated on the Rotax.

FWIW, I just had a 20/80 episode on one of my jugs (broken rings) and didn't really notice any performance loss in flight. New rings did reclaim some 20 RPM static, maybe. I could easily feel the compression drop on pull-through, and oil would get nasty after a couple of hours from blow-by. Ergo: static compression test is a useful diagnostic tool, but by no means the last word re airworthiness. Here's some good advice from Mike Busch, and more.
 
Thanks all. My main problem was having the orifice test valve open on the first run and not finding exact TDC on the second run and having to really HOLD the prop. I?ll do it again at the completion of my Condition Inspection when I can fly it again and warm things up.
 
When the piston is exactly TDC it won't move when air is applied. You can use a wooden dowel in the spark plug hole and move the prop to exactly TDC. I do this every annual on Robinson piston helicopters. There are no props to "hold" or rock back and forth. If you are careful about hitting TDC it wont move.
 
Gil,

The TCM SB03-3 (for TCM engines used in Bonanzas) calls for the .040 orifice. The .060 orifice is not the one to be used on Bonanzas (or Lycomings).
 
Gil,

The TCM SB03-3 (for TCM engines used in Bonanzas) calls for the .040 orifice. The .060 orifice is not the one to be used on Bonanzas (or Lycomings).

Thanks for the info - I'm not familiar with the Continental engines.

If they need 0.040 testers then all I can say is that there seem to be quite a few incorrect 0.060 testers floating around our airpark. :)
 
If they need 0.040 testers then all I can say is that there seem to be quite a few incorrect 0.060 testers floating around our airpark. :)

Either tester will reveal the same problems. Doing the test with the wrong tester is still preferable to no test, or to misinterpreting the results obtained with the correct tester. :D
 
why not the automotive way?

Anyone know why we don't do it like we do on cars? Seem like it would be much safer and faster.
 
Anyone know why we don't do it like we do on cars? Seem like it would be much safer and faster.

Whilst it's possible to spin the motor over as they do on cars it's generally not the accepted practice for a few reasons. The danger aspect is one, spining over a prop in a workshop not a clever idea. You can't listen to the escaping air (if any) as effective as the 80/80 method.
The Std method is what the major engine manufacturers expect and other mechanics.
 
spinning the prop

Whilst it's possible to spin the motor over as they do on cars it's generally not the accepted practice for a few reasons. The danger aspect is one, spining over a prop in a workshop not a clever idea. You can't listen to the escaping air (if any) as effective as the 80/80 method.
The Std method is what the major engine manufacturers expect and other mechanics.
True, I wouldn't recommend spinning a prop inside a crowded hangar. Seems with the car method you could buy 4 ebay compression testers, put them all in, spin the prop a few times (outside the hangar) and be done in a few minutes. If there are some cylinders that need a closer look, you could do it the aviation way. I've got zero experience doing this, and will of course be giving it a try "soon", but with so many warnings about the danger of a manufacturer-recommended procedure, thought it might be good to see if there is a safer way.
 
Accuracy

Also, the aircraft engines used to have large blowby as compared to cars. This will probably affect the accuracy of the results.

But whatever you do, dont stand in the arc of the prop... with your legs open.
 
True, I wouldn't recommend spinning a prop inside a crowded hangar. Seems with the car method you could buy 4 ebay compression testers, put them all in, spin the prop a few times (outside the hangar) and be done in a few minutes. If there are some cylinders that need a closer look, you could do it the aviation way. I've got zero experience doing this, and will of course be giving it a try "soon", but with so many warnings about the danger of a manufacturer-recommended procedure, thought it might be good to see if there is a safer way.

Since you don't have experience doing engine compression tests of any kind, I recommend you stick with the book method for your plane. With over 56 yrs of car work, hundreds of rebuilds, and decades in engine design, analysis, testing, and development, I can say either method can work using the right standard. However, I have managed many of my personal cars/trucks to over wear on-out on the engines, and tested both methods on my cars for years. The leak down is much more trouble free in providing consistent, definitive, results. You can easily get false information on the spinning method if you don't count the number of compressions. That means a lot of spinning which aviation batteries are not well suited.

You were on the right track about one thing, though. In 1971 VW began installation of the "diagnostic plug" and we installed a special test bay for that at my dealer. The two tests it did well was a high accurate milliamp load for the battery test, and an measuring the starter amp draw during a spin test of the engine to compare compressions. It was very good a picking a week cylinder, and only took a few seconds. Modern electronics and a flywheel tooth pickup could easily do the same for our planes too. In a controlled and repeatable way. I hope modern OBD will come to pass for our planes. Surely in experimental first. Electronics guys could do this with an oscilloscope, a hall effect sensor on the starter cable, and hall effect sensor on the flywheel teeth. Easy for an EE, hard for me as an ME :(
 
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True, I wouldn't recommend spinning a prop inside a crowded hangar. Seems with the car method you could buy 4 ebay compression testers, put them all in, spin the prop a few times (outside the hangar) and be done in a few minutes. If there are some cylinders that need a closer look, you could do it the aviation way. I've got zero experience doing this, and will of course be giving it a try "soon", but with so many warnings about the danger of a manufacturer-recommended procedure, thought it might be good to see if there is a safer way.
FWIW, I tried this method this year out of curiosity. $120 experiment, for four compression gauges that have one-way valves to retain the pressure. Pull top four plugs, screw in four gauges, route them away from the prop, hop in, and spin the prop for 5-6 seconds on the starter. All switches off, mixture rich, throttle wide open. And yes, I did it in my hangar, after ensuring nothing was in the vicinity of the prop, and the door was closed to keep looky-loos out. I intended to do it outside, but the rain came faster than I could get it set up so I elected to accept the risk and do it indoors, with multiple double-checks to reduce risk.

I don't have all the numbers right with me now but I had pressure readings anywhere from 120psi to 146psi on the four gauges. I did a few runs to see how the numbers varied, and they did, by up to 5psi per cylinder. Every run showed the same pattern the differential tester showed, that my #2 cylinder is a little low (the 120psi number) and one of the rear cylinders is a little high (the 146psi). On a differential tester, the cylinders usually read 78-79/80 except #2 which reads about 76/80.

What I haven't been able to figure out is how i'm getting 146psi on a 7:1 compression cylinder (O-320-E2D). 14.7psi being standard pressure at sea level, shouldn't I at best be able to get 7x that, or 113.9psi? All I can surmise is that either the gauges aren't that accurate, or the momentum of the needle due to the rate of pressure rise is enough to carry it beyond the actual number when the gauge is measuring.

I did try rotating gauges between cylinders, but the numbers didn't change significantly, so I think all four gauges are reading about the same (at least, the difference was within the 5psi variation that I saw between runs).

It was an interesting experiment, and i'll do it again from time to time to see if I can spot a trend. It's an easy test to do. And i'll do it outdoors from now on... :)
 
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Math

Using siimple math, the volume of the hose from the cylinder to the gauge will affect the compress ratio. So the hose lengths should make a different in the psig reading
 
Nice article

How-to instructions, principles of operation, etc. The how-to is a method of doing the task solo. Personal opinion, but I don't want anyone holding the prop, or even being in the vicinity.

https://www.danhorton.net/Articles/0913_MaintMatters_Compression.pdf
Thanks Dan - very helpful article.

FWIW, I tried this method this year out of curiosity. $120 experiment, for four compression gauges that have one-way valves to retain the pressure. Pull top four plugs, screw in four gauges, route them away from the prop, hop in, and spin the prop for 5-6 seconds on the starter. All switches off, mixture rich, throttle wide open. And yes, I did it in my hangar, after ensuring nothing was in the vicinity of the prop, and the door was closed to keep looky-loos out. I intended to do it outside, but the rain came faster than I could get it set up so I elected to accept the risk and do it indoors, with multiple double-checks to reduce risk.

I don't have all the numbers right with me now but I had pressure readings anywhere from 120psi to 146psi on the four gauges. I did a few runs to see how the numbers varied, and they did, by up to 5psi per cylinder. Every run showed the same pattern the differential tester showed, that my #2 cylinder is a little low (the 120psi number) and one of the rear cylinders is a little high (the 146psi). On a differential tester, the cylinders usually read 78-79/80 except #2 which reads about 76/80.

...
Thanks Rob - pretty much exactly what I was thinking might be another way to do it. I think doing this to watch trends could add value. I have the standard tester which I will use as in Dan's article, and I also just ordered some ebay testers - they are now less than $20 each shipped globally.
 
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Using siimple math, the volume of the hose from the cylinder to the gauge will affect the compress ratio. So the hose lengths should make a different in the psig reading
True. However, that volume is added to the total volume of the cylinder, and the piston still moves the same amount. So the "error" should be a *reduction* from the theoretical maximum, not an increase. On one cylinder I read 146psi, which is almost 10x ambient.

They are, of course, cheap gauges, which is my leading explanation until I can figure out any other reason they'd read so high.

The gauges have a schraeder air valve (like on your tire nipple) that has a very weak spring on it... Probably intended as a check valve or a backup to the thumb-operated valve on the outside. I saw mention online of removing that schraeder valve because the person wasn't getting a reading due to lower compressions on their engine. I wonder if it could be affecting readings at higher compressions too? Something to try the next time the cowling is off.
 
Follow-up

How-to instructions, principles of operation, etc. The how-to is a method of doing the task solo. Personal opinion, but I don't want anyone holding the prop, or even being in the vicinity.

https://www.danhorton.net/Articles/0913_MaintMatters_Compression.pdf
Thanks again Dan for the excellent article. I finally got a chance to do my compression tests using the ATS 2EM https://aircraft-tool.com/shop/detail.aspx?id=2EM and it worked fine.

Just a couple of things I learned that helped me:

  • Use the TDC marker on the ring gear
  • American and Europe air hose connects are different
  • Get the 14mm adapter with your tester if you want to avoid pulling the 18mm-14mm adapters
  • Single person seems safer or at least keep that person away from the prop, not holding it
  • My engine dryer hose popped out of the breather during each test, which was at least some indication that the air was going past the rings, as expected

The TDC marker works directly for two cylinders, and for the other two, find the 180° point. My cylinders didn't budge once I had them at TDC.

The whole process was much easier than I had anticipated, and in fact, much easier than using the automotive type compression testers, since you can do it in a quiet hanger, don't need to be outside. Also, when I work on my FWF I like to remove the ground from the battery, just in case.

Thanks again gentlemen for all the hints and tips!
 
[*]Use the TDC marker on the ring gear

The TDC marker works directly for two cylinders, and for the other two, find the 180° point. My cylinders didn't budge once I had them at TDC.

Assuming a 4-cyl and a two blade prop, the propeller blade angle will be the same for all cylinders. Personally I just move from cylinder to cylinder, finding TDC for each with a finger over the spark plug hole while spinning the prop with my other hand. However, it may help the novice to see the angle once with the TDC mark. With visual picture in mind, the finger-on-the-hole thing is really easy.

Note it's desirable to take the pressure reading with the piston somewhere between TDC and just a few degrees before TDC, after coming up to it with a little air pressure on the gauge. You want the rings seated in their normal compression position.
 
Starting with #1. Find TDC and test, rotate prop ccw 360 degrees and test #3 next. (You should be holding the high prop for 1 and 3 if working alone) Go to #2 and rotate 360 again. (you should be holding the low prop for 2 and 4) Finish with #4 and again rotate 360 to get TDC......done.
 
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