IO-520 Compressions

Interesting article on borescoping by Mike Busch.

https://www.aopa.org/news-and-media/all-news/2017/may/pilot/savvy-maintenance-borescope

Was changed in the latest revision of AC 43.13-1B CHG 1 Par 8-14 b:

(1) For an engine cylinder having lessthan a 5.00-inch bore; 0.040-inch orifice diameter;.250 inch long; and a 60-degree approachangle.(2) For an engine cylinder with 5.00inch bore and over: 0.060 inch orifice diameter,.250 inch long, 60 degree approach angle.

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Eddie Foy said:

One thing puzzles me. Why is this test called a compression test in the aircraft world.

It is actually a differential pressure test or a leakdown test in the automotive world.

It is not a true direct compression test.

Does anyone do a direct compression test?

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Eddie,

Its actually called a “differential compression test”. Why is it used in aircraft engines? Only legitimate answer is because it is the “recognized” test, and the one we’ve used forever in aviation.

the good news is that, properly administered, this test will pick up a lot of engine problems. That said, the specific numbers themselves (60 vs 70) are relative, and so not necessarily meaningful. Trends are more indicative of a problem, or, as others noted earlier, if you can hear where the Air is going, if the cylinder isn’t holding Air.

I once had an O-360 Lyc that ran hot, didn’t make much power (a REALLY relative function) etc, but it could pass a differential compression test. Just wiggle that prop and fiddle with it to get it to the right spot and it would hold pressure.

Finally, I called Lycoming Tech help, told them what I had and what we’d done, and first thing he said was go to NAPA and buy an automotive compression tester. Did that, and two cylinders would not pass. They could be made to pass a differential test, but that’s not how an engine works.

But, differential pressure is the aviation standard, and when properly applied, will reveal many cylinder problems.

MTV

I think the FAA and manufacturers call it a differential compression test and us mechanics call it a compression test. Some of us have a hard time putting a string of long words together. 8)

Compression test (differential pressure) is only one test. You listen to where the air is going, borescope, read the spark plugs, look at the color of the oil, oil usage, screens, filter, engine temps etc. The engine will normaly tell you when something is amiss if you pay attention to it.

Steve,
I understand all of that. Do you ever do a direct compression test on an engine with a subpar “differential compression” test??

Tom3holer said:

For a correction the log books were at the airport when I posted the numbers which were a bit off so here are the actual numbers.
Last year/This year

1: 70/60
2: 72/64
3: 71/70
4: 68/68
5: 72/70
6: 70/63

As I read the description for the two gauges one had a .040 restrictor orifice and the larger on has a .060 so it stands to reason it would give a higher reading but where does TCM make mention of this?

In cruise at 24/24 or WOT. In cruise all egt's are mid to upper 1400's and all head temps are mid 300's except #5 which runs the hottest at 360 ish. I do run LOP in cruise and see right around 160mph in cruise.

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I understand they are all different (and this is my first 520) but mine really didn't like LOP operations (and the wife really didn't like leaning it through the rough cough and sputter part) as I had oil consumption issues (1 in10 with new cylinders following break in) and had to steak a valve twice but all that went away with the "run it like you stole it" operations but I also think my flight profile really helps... I typically blast off and climb hard WOT with RPM at the top of the green and begin leaning at 5K, 7K and as I go above that then level off and heat it up for at least and hour, usually two these days and plus I fly it 220-250 hrs/year.

I think its about finding the pressure/heat combination that makes you and your engine happy and then sticking with it. You might give ROP a try for a few months and see, mine methodology is not per book but is based on 40-50 ROP baseline temps also I run cruise temps in decent.

I've also tried squeezing the fuel flow. It doesn't work in the long run. Maintenance hours (expense) are higher and more frequent. I had a turbo inter-cooled engine for a while and was very careful to lean it correctly. That engine was continually causing something to happen. When I increased the fuel flow one gallon per hour the issues disappeared.

Tom, Airline pilots are notorious tightwads. I've known a few. Remember I used to be one. :lol: Spend just a little bit more money on gas and the maintenance issues will diminish.

A differential compression test or leakdown test will show problems with a cylinder long before a dynamic compression test. With a whirling propeller out on the front, the differential test is safer and easier to do as well. A dynamic compression test has so many variables like speed of the starter, throttle position, battery condition, etc, that comparing one year vs the next usually isn't a very valid comparison. As Steve said, with a differential compression test, where it's leaking is often times much more important than the numbers you read on the gauge. If one does a dynamic compression test, you get a final compression number, but if there is a problem, that number doesn't tell you whether you have a valve or ring problem. On a differential compression test, that's what the mechanic is looking and listening for.

As a A&P, when I'm inspecting an engine, I am doing the inspection to detect potential problems before they become flight safety problems, which is why a compression differential test is used. People are number driven, and the numbers are all you see in the logs when looking at history, but the actual inspection process is a bit more involved.

-Cub Builder

The two most disagreed upon topics in aviation with big Continentals? Low compressions and LOP operations. Funny, the big proponents of low compressions not being a problem are also the big proponents of LOP operations. Tom's owner reports of excessive oil discharge from the breather support sagging compression findings and blow-by. It doesn't sound like his rings are seating well with combustion pressures or compression test pressures. If I was to try a Hail Mary I'd put some MMO in the oil and run it hard for a couple of hours and test it again, just in case, but it's a low percentage play.

MMO's probably the third most disagreed on thing in aviation.

stewartb said:

The two most disagreed upon topics in aviation with big Continentals? Low compressions and LOP operations. Funny, the big proponents of low compressions not being a problem are also the big proponents of LOP operations. Tom's owner reports of excessive oil discharge from the breather support sagging compression findings and blow-by. It doesn't sound like his rings are seating well with combustion pressures or compression test pressures. If I was to try a Hail Mary I'd put some MMO in the oil and run it hard for a couple of hours and test it again, just in case, but it's a low percentage play.

MMO's probably the third most disagreed on thing in aviation.

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Would you use the MMO in a downwind turn?

Since operation has been introduced into this thread....

When I got my 185 (550D equipped) I heard many things on operation and leaning and LOP operation. I began my own research because I was getting confused with all the different recommendations from friends and acquaintances and came across these webinars presented by the Continental's Director of Customer Service.
Part 1. https://www.youtube.com/watch?v=4hmG5qwCpbw
Part 2. https://www.youtube.com/watch?v=3MPGP_GzXlI

Continental seems to reference GAMI's research as definitive and above their own in many regards, so I took the Advanced Pilot Seminar (GAMI's engine operation and leaning course, online version). There is a free sample available here. https://www.advancedpilot.com/onlinecourse.html In their demystification they explain the wives tales, rumors, and how so many people, including A/Ps and I/As, have misinterpreted the data over the years. The full online course is $400, steep but the additional education was worth it to me, maybe others will also.

Those youtube videos are by Bill Ross, who now is with Superior Air Parts. Don't forget you can get his book download for free.

http://www.superiorairparts.com

I'm looking forward to exploring LOP with my API injection. The exp alternative to GAMIs!

My engine via the previous owner had significant cylinder issues (two sets in 600 hours with oil consumption, valve issues and cylinder cracking problems) which I knew going in. I put up with it for awhile without any problem other than oil consumption growing to 1qt/6 and then put all new on and I still had a few valve problems running it easy per the previous owners notes both LOP and ROP so I did what is happening here I asked lots of questions...hence my operational change. I would like to study it more and when I hang a fresh engine (now scheduled in 2-2.5 years unless the oil dribble somewhere on the rear of the case wins) I will likely explore LOP operations at that time but right now I wouldn't change at gun point from what I'm seeing in a fairly high time engine (that earns a living) that is on track to keep it up well through TBO.

stewartb said:

The two most disagreed upon topics in aviation with big Continentals? Low compressions and LOP operations...... MMO's probably the third most disagreed on thing in aviation.

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Don't forget mogas.

I realize that many folks have great results running LOP, and we all have had good and bad with engines, that being said I have never had an instructor, pilot, owner or Chief pilot on any commercial plane tell me to run LOP.

Operations that changed 15 engines a year (TBO requirement for 135) ran them rich!

Why? Fuel is cooling, as said above 75% it is a big deal- why you should always run FULL THROTTLE on take off- there is an extra jet that opens and sends cooling fuel into the cylinders further adding to cooling.

How critical is this? We had a fuel controller set 1 to 1.5 gallons leaner than what was called for in the manual. We went through 2 cylinders in 500 hours, and almost lost a third. Adjusted the controller up and the engine is run flawlessly. Fuel costs at over $5/gallon is still far less than cylinder changes.

Having enough fuel on take-off to keep the cylinders cool is very important, as is cooling in flight. LOP might be just fine, but be sure to keep eyes on all temps. to ensure you are not melting the 3&4 jugs.

aktango58 said:

I realize that many folks have great results running LOP, and we all have had good and bad with engines, that being said I have never had an instructor, pilot, owner or Chief pilot on any commercial plane tell me to run LOP.Operations that changed 15 engines a year (TBO requirement for 135) ran them rich!Why? Fuel is cooling, as said above 75% it is a big deal- why you should always run FULL THROTTLE on take off- there is an extra jet that opens and sends cooling fuel into the cylinders further adding to cooling.How critical is this? We had a fuel controller set 1 to 1.5 gallons leaner than what was called for in the manual. We went through 2 cylinders in 500 hours, and almost lost a third. Adjusted the controller up and the engine is run flawlessly. Fuel costs at over $5/gallon is still far less than cylinder changes.Having enough fuel on take-off to keep the cylinders cool is very important, as is cooling in flight. LOP might be just fine, but be sure to keep eyes on all temps. to ensure you are not melting the 3&4 jugs.

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we set our high pressure to be needle strait down, beyond redline for the reasons you mention on takeoff on 185

An old-timer once told me that fuel is the cheapest coolant you can buy.

RK

Eddie Foy said:

Does anyone do a direct compression test?

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Yes. There was an old guy that worked as a roving mechanic in Ohio. He had a set of gauges that screwed, one into each top plug hole. He would rotate the prop two revolutions or maybe it was three. Then he would read the numbers off the gauges. Pretty slick-no air compressor needed-no electric needed. Just like doing a compression check on a car engine while rotating it with the starter. Nobody I knew ever double checked his numbers with a leakdown gauge but his numbers were mostly in the low sixties to high seventies and appeared normal for the engines. I saw one set of those gauges get sold on ebay and I was disappointed that they sold before I saw them. I have no idea who manufactured them but I would still like a set if I could afford them. Of course I also have a Seyboth fabric tester for the guys who want to see the low green or high green looks like.

This is a great discussion that I am enjoying very much and learning a lot.
I have read almost everything I can find on LOP and ran it as far back as the mid 90's in my IO-470 powered Debonair long gone to Dr. Zarum who lives nearby and takes very good care of her. The J3 I had in the intervening years did not want any part of LOP so I ran FM or fixed mixture all the time. I did keep abreast of LOP operations and the updates on the learning curve. It is nothing new as they ran that way back in the Super Connie days with the 4360's or whatever they were and it indeed extended their longevity. There are many benefits to LOP operation other than fuel consumption. Head temps are cooler than at ROP for one thing. Just because an operator has had success running one way does not mean a new method may not be better. If I were a commercial operator and had what I considered success running one way I would be reluctant to change too until the day came when my competitor with the same equipment was operating differently and being just as reliable at a considerable cost savings. It is more involved and requires a learning curve I agree and is not for everyone.

I ordered the 5" and up diff pressure gauge today and we will try it again in a couple of days when it comes in.
As luck would have it the borescope broke the other day so it will be a couple of days before my mech can get a good look at the jugs.
On the oil out the breather thing I have a theory that it might be due the angle of the sweepback of the breather as it exits beneath the firewall. ON mine it extends back parallel to the bottom of the fuselage. As a test I put about a 2 1/2 extension of vynl tubing on it to see if perhaps there was a venturi effect going on and if extending it further back might increase the effect. I went through about a quart of oil in a little over an hour and a half. Tomorrow we are changing the tube to exit an inch or so below the firewall and cut it parallel to the ground in level flight and see if that helps. That was the recommendation by several members here and a good friend in Greenville with a 0-520 powered 180.
Tom

Nope, WA, has a beautiful private strip nearby in Rockwood.

N86250 said:

Yes. There was an old guy that worked as a roving mechanic in Ohio. He had a set of gauges that screwed, one into each top plug hole. He would rotate the prop two revolutions or maybe it was three. Then he would read the numbers off the gauges. Pretty slick-no air compressor needed-no electric needed. Just like doing a compression check on a car engine while rotating it with the starter. Nobody I knew ever double checked his numbers with a leakdown gauge but his numbers were mostly in the low sixties to high seventies and appeared normal for the engines. I saw one set of those gauges get sold on ebay and I was disappointed that they sold before I saw them. I have no idea who manufactured them but I would still like a set if I could afford them. Of course I also have a Seyboth fabric tester for the guys who want to see the low green or high green looks like.

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He used to annual my cub, I thought those screw in gauges were pretty neat. I miss those days.

Tom3holer said:

Nope, WA, has a beautiful private strip nearby in Rockwood.

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Bill's a great guy

Glenn

Cubdriver2, Yes he is and his wife Maria cooks at Maynards and makes the best mushroom linguini I've ever had.

The new gauge should be here tomorrow and if things go well should be able to fly it and retest the compression.

behindpropellers said:

https://www.beechtalk.com/forums/viewtopic.php?f=37&t=145502

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Tried to go there but I couldn’t see the keyboard with a bag over my head.

Rudy said:

He used to annual my cub, I thought those screw in gauges were pretty neat. I miss those days.

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His name is in many, many logbooks. Everybody liked his work. Never heard a single bitch.

Tom3holer said:

This is a great discussion that I am enjoying very much and learning a lot.
I have read almost everything I can find on LOP and ran it as far back as the mid 90's in my IO-470 powered Debonair long gone to Dr. Zarum who lives nearby and takes very good care of her. The J3 I had in the intervening years did not want any part of LOP so I ran FM or fixed mixture all the time. I did keep abreast of LOP operations and the updates on the learning curve. It is nothing new as they ran that way back in the Super Connie days with the 4360's or whatever they were and it indeed extended their longevity. There are many benefits to LOP operation other than fuel consumption. Head temps are cooler than at ROP for one thing. Just because an operator has had success running one way does not mean a new method may not be better. If I were a commercial operator and had what I considered success running one way I would be reluctant to change too until the day came when my competitor with the same equipment was operating differently and being just as reliable at a considerable cost savings. It is more involved and requires a learning curve I agree and is not for everyone.

Tom

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While I have no reason to refute your research and historical knowledge, your assertion on LOP being great to operate would carry more influence if it was not preceded by the question: Why is my engine having problems?

Again, if all cylinders were 10 lbs lower than before I would be more apt to think gauge, the one is only 2 lbs lower indicating something else is amiss.

Ok, I just did a differential compression check on a Lycoming O-320 150 hp which has a bore of 5.125". I normally bring it up on compression to where I hear the impulse coupling snap and stop, install gauges and adjust air pressure. Using the E-2M1000 with the .060" orifice I got 78/80 without having to mess with the prop. Immediately installed the E-2A with .040" orifice and it showed 64/80 at the same prop position. Moved the prop around and best I got was 74/80. Went to the other three cylinders and they were all 78-79/80 with little prop movement using the bigger bore gauges and they were all 4-6 psi lower with a lot of prop back and forth with the .040" gauges. Both gauges are fresh out of calibration from Eastern Technology.


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Thank you for going to the extra work to determine this Steve! Great Information.