Minimum Compression

Jaybird180

Jaybird180

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Jaybird180
At what point does low compression manifest with a noticable lack of performance?
 
Pretty low, especially if it's on one cylinder.

I had a cylinder at 30/80 (really low) on the last Aztec annual, and the engine performed beautifully.
 
I'd guess you'd be fouling plugs in cylinders that bad and consuming oil / blowing it out at a pretty noticeable rate. If it is the rings anyway
 
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dell30rb said:
I'd guess you'd be fouling plugs in cylinders that bad and consuming oil / blowing it out at a pretty noticeable rate. If it is the rings anyway
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80 psi for the test is a mere fraction of the pressure in the cylinder of an operating engine. If the leakage during the test is through the rings, there's a good probability that the rings still seal adequately during operation.
 
Witmo said:
80 psi for the test is a mere fraction of the pressure in the cylinder of an operating engine. If the leakage during the test is through the rings, there's a good probability that the rings still seal adequately during operation.
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If its adequate for performance and oil retention then who cares if the compression is 30/80?
 
dell30rb said:
If its adequate for performance and oil retention then who cares if the compression is 30/80?
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I do. That sort of leakage usually means a leaking valve, and if it's an exhaust valve it's going to burn, and if it burns it might get hot enough to lose its head right into the cylinder, and things suddenly get serious.

Quite often low compression can be fixed by lapping the valves, if the rings are otherwise ok and the valve guides aren't shot. Letting them continue to leak might result in enough wear to require much more expensive work.

Dan
 
Understand Dynamic compression.

when we see low compression it tells us that the cylinder is not as the manufacturer made it. and keeping running it won't lower the cost of the repair.

the compression check is a simple tool we use to determine the health of the engine, but 1 compression check is not what we need to make a good decision, we need to know what the trend looks like.
 
Looking in the Lycoming Troubleshooting Guide there is a reference to Service Instruction 1191, but I can't locate that.
 
Jaybird180 said:
Looking in the Lycoming Troubleshooting Guide there is a reference to Service Instruction 1191, but I can't locate that.
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The FAA advises that 60/80 is as low as they consider airworthy. unless you have other guidance by the manufacturer, like the master oriface continental used in their service bulletin.
 
60 is really the bare mins. I get p1ssed if mine is any where below 75. Of course if you dont run your engine for a while it will be low and come up after a little run time.
 
Jaybird180 said:
Looking in the Lycoming Troubleshooting Guide there is a reference to Service Instruction 1191, but I can't locate that.
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Lycoming SI 1191A is just a basics of compression testing - what size orifice to use, rocking the prop to set the rings, listening for source of leakage etc.
It is similar, though not as detailed as Continental SB03-3

I think both were written to address the fact that people were taking compression tests to literally and cylinders were getting pulled more often than they needed to be. As Tom pointed out, it's a valuable test but you need to interpret the results with a level head and not jump to conclusions. Especially on the larger bored engines like 470's and up 80 psi is really not much pressure to get everything to seat well. We use 80 psi for safety reasons because it could get downright dangerous trying to hold onto a prop with higher pressures. As it is you can get a pretty good whack if it gets loose but it probably won't kill you. The pressures in an actual running engine are far, far greater.

This is why even if you see a compression reading of 30/80 it's unlikely you'll notice it during operation. In fact, you can get a compression reading that may be darn near 0/80 and when you pull that cylinder and look down inside with a powerful flashlight you're likely to see absolutely nothing visual that would clue you to the fact that anything was wrong. But if a valve is leaking badly it will eventually burn up. The periodic compression tests should enable you to catch it in it's early stages.
 
NineThreeKilo said:
60 is really the bare mins. I get p1ssed if mine is any where below 75. Of course if you dont run your engine for a while it will be low and come up after a little run time.
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On Lycomings, it's common to get high compressions, even on old engines. On Continentals, it's different. I was running cylinders that saw 45-52 (legally!) And didn't notice any power loss. Will the new engines on the 310 have mower power and go faster? I'm hoping for a small improvement, but don't expect a huge one. Primary improvement will probably be due to other factors.
 
Sometime in the 50s Bonanza customers complained of lower power in their planes. Beech contacted Continental and they tested a few engines in the field and found no problem. Customers complained some more so Continental built an E-225/O-470 but didn't put any rings in it at all.

The engine made rated power at rated RPM for a few minutes until the plugs fouled.

At my last annual, 3 of 6 jugs were in the 60s. It's a common thing for older Conti engines and I have gobs of power. If there are leaks in the exhaust valve they should be looked into. Other than that, compression in the 60s isn't a problem. The master orifice test for the Conti allows pressures down into the upper 40s, so long as there is no other damage to valves and seats.
 
docmirror said:
Sometime in the 50s Bonanza customers complained of lower power in their planes. Beech contacted Continental and they tested a few engines in the field and found no problem. Customers complained some more so Continental built an E-225/O-470 but didn't put any rings in it at all.

The engine made rated power at rated RPM for a few minutes until the plugs fouled.

At my last annual, 3 of 6 jugs were in the 60s. It's a common thing for older Conti engines and I have gobs of power. If there are leaks in the exhaust valve they should be looked into. Other than that, compression in the 60s isn't a problem. The master orifice test for the Conti allows pressures down into the upper 40s, so long as there is no other damage to valves and seats.
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Dynamic compression.

The engines ability to run properly while it has low static compression.

the concept is based upon how big the hole is, and how much time the gases have to escape during the compression and power stroke. Do the math at 2600 RPM how much time does the piston stay on the compression and power stroke? and how much of the fuel air charge can get thru the leak?
 
Tom-D said:
The FAA advises that 60/80 is as low as they consider airworthy. unless you have other guidance by the manufacturer, like the master oriface continental used in their service bulletin.
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I am aware that Lycoming states this, but where would I find the FAA guidance stating this as a hard number?
 
Continental brought out the "Master Orifice" test for only one reason, to deny warranty claims on their junk cylinders.


I can guarantee that if you remove a Continental cylinder with less than 60 psi, and send it to a cylinder shop, they WILL find a problem.
 
kgruber said:
Continental brought out the "Master Orifice" test for only one reason, to deny warranty claims on their junk cylinders.


I can guarantee that if you remove a Continental cylinder with less than 60 psi, and send it to a cylinder shop, they WILL find a problem.
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How do you define that it's junk or that a problem really causes any harm when the engine makes rated power and fuel consumption?
 
Tom-D said:
Dynamic compression.

The engines ability to run properly while it has low static compression.

the concept is based upon how big the hole is, and how much time the gases have to escape during the compression and power stroke. Do the math at 2600 RPM how much time does the piston stay on the compression and power stroke? and how much of the fuel air charge can get thru the leak?
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Actually, being an engineer, I don't need to do the math. And it's also related to adiabatic pressure more than dynamic, although they both play a part.
 
kgruber said:
Continental brought out the "Master Orifice" test for only one reason, to deny warranty claims on their junk cylinders.


I can guarantee that if you remove a Continental cylinder with less than 60 psi, and send it to a cylinder shop, they WILL find a problem.
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Well, I've been doing this for a while, and I just had three jugs off within the past two years. Since I have the early engine, the jugs are actual Conti, and not aftermarket. If by problem you mean wear, then yes I had some wear. I had no valve recession and no ring or piston damage. They are just looser than similar Lyc engines.

The cylinder shop replaced the rings, lapped the valves, and put it back on the engine. It gained a few psi, but was still around high 60s, or about 70-ish.
 
docmirror said:
Actually, being an engineer, I don't need to do the math. And it's also related to adiabatic pressure more than dynamic, although they both play a part.
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You don't need to be an engineer to understand it doesn't have time to leak.
 
Jaybird180 said:
I am aware that Lycoming states this, but where would I find the FAA guidance stating this as a hard number?
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chapter 8 of the AC 43-13,1b

8-14. COMPRESSION TESTING OF AIRCRAFT ENGINE CYLINDERS. A test to determine the internal condition of the combustion chamber cylinder assembly by ascertaining if any appreciable internal leakage is occurring is compression testing of aircraft engine cylinders. If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in para- graphs 8-15b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected. To determine the cylinder’s problem area, have someone hold the propeller at the weak cylinder’s top dead center and with compressed air still being applied, listen. If air is heard coming out of the exhaust pipe, the cylinder’s exhaust-valve is not seating properly. If air is heard leaking out of the air cleaner/carburetor heat box, the intake valve is leaking. With the oil dipstick removed, and air is rushing out, the piston rings are defective. Remove and repair/overhaul the defective cylinder.
 
Compression tests tell you very little about your engine compared to other measurements. It is mostly a regulatory requirement dating back decades and provides a litmus paper type result.

Better indications of your cylinders' health can be found in two ways:

1. An engine monitor... nothing tells you more about your cylinders than CHTs and EGTs...

2. A Borescope... shows you the health of your intake valves, exhaust valves, cylinder wall, and even status of your rings...

pc
 
pcorman said:
...
2. A Borescope... shows you the health of your intake valves, exhaust valves, cylinder wall, and even status of your rings...
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Disagree, as I said earlier you can have darn near zero over 80 and with the cylinder removed and looking down inside with a powerful flashlight you very likely will not see anything visual that would indicate a problem. By the time you are able to visually detect valve or ring problems with a borescope you are WAY beyond the point where they will clearly present themselves with a compression test.

I agree that an engine monitor is a great instrument but we certainly don't want to have a regulation stating that every airplane has to have one.
 
Jaybird180 said:
At what point does low compression manifest with a noticable lack of performance?
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Continental did tests and found no appreciable power loss on engines down to 40/80 and IIRC state that as long as it's not leaking past the valve, it's good to run.
 
Many times the poor compression is a valve guide issue, and nothing will show in a borescope inspection.

That is why Lycoming came up with the wobble check.
 
Tom-D said:
Many times the poor compression is a valve guide issue, and nothing will show in a borescope inspection.

That is why Lycoming came up with the wobble check.
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And even then it's no guarantee...had compressions of 75+/80 on all 4 jugs, good power, and at 1500hrs, 1 valve guide was so wide open the valve moved all over the place, 1 had a crack in the exhaust port behind the valve in a second jug and the other 2 jugs valves/guides were just barely in nominal range.

Can you spell top overhaul?
 
DutchessFlier said:
And even then it's no guarantee...had compressions of 75+/80 on all 4 jugs, good power, and at 1500hrs, 1 valve guide was so wide open the valve moved all over the place, 1 had a crack in the exhaust port behind the valve in a second jug and the other 2 jugs valves/guides were just barely in nominal range.

Can you spell top overhaul?
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This is why I tend to advocate new cylinders at overhaul...

Although, as Tom has correctly pointed out, for certain engines you may be better off with overhauling your old cylinders.
 
I use rebuilt jugs when replacing a cracked/failing jug. However when doing an OH you can't beat a new one as the price is not significantly more but the value (some of it) carries through to the value of the plane to some degree or another.

I think it a waste(most of the time) to put a new jug on a half run out engine as you are going to get new ones at OH and have to discard a still good jug..
 
Wow, 2 banned members and one deceased. Almost all the rest have been active since this thread first took life. Just an observation about really old threads, carry on.
 
Who's the deceased? I see two banned and two who voluntarily left.
 
Tony_Scarpelli said:
I use rebuilt jugs when replacing a cracked/failing jug. However when doing an OH you can't beat a new one as the price is not significantly more but the value (some of it) carries through to the value of the plane to some degree or another.

I think it a waste(most of the time) to put a new jug on a half run out engine as you are going to get new ones at OH and have to discard a still good jug..
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What discard, someone will want those cylinders and you should be able to get at least half of you price back if they are still in good condition.
 
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