Compressions

Compressions

I am hoping someone out there can help me here. What would be regarded as good compression numbers for a 2000 182S with 190 hours on the tach ? I've been getting conflicting answers from various local "experts" and I can't make sense of them. Below what level would you feel there is a problem ?

Thanks in advance.

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Re: Compressions

Dave:

The historical answer is 60/80.  It's an incomplete and misleading answer.

As it turns out, an engine can make rated power with very low compressions.  It may run that way for many, many hours--hundreds of hours.  If the low compression is due to leaks past the rings, it may improve on subsequent compression tests.  If it is past the valves, the issue is cloudier.  Just because an exhaust valve is leaking during a static compression test, that is no measure that it is leaking while under load. 

I had a cylinder that was 20/80 with the leak past the exhaust valve.  I was certain that is was NOT leaking when the engine was running and I continued to run it for a looong time.  Eventually, I overhauled the engine and that cylinder got fixed then.  If you do not have an engine monitor and cannot be certain that the valve is not leaking in flight, you might be prudent to change it.

The second paragraph is something few mechanics understand or appreciate so you may well get some resistance, but the data is compelling that it's true.

Walter Atkinson
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Re: Compressions

there are some bits of information put out by the engine manufacterers on this subject.  your mechanic should be able to get a copy of the particular service bulletin for your engine.
Lycoming cylinders are generally considered defective anytime the numbers get below 60/80.  and as previoulsy stated the reasons the number is low can vary as can the readings.  nice thing about a large bore 6 cyl engine is you can run a bad cyl and not really know it until you get into a high gross wieght situation on a high density altitude day and find out the hard way that you are lacking some power.
if the numbers are consistently bad then change/repair the cylinder.  dont compromise the safty of yor passengers for a few dollars not spent on the repairs.
remember that if the necessary repairs are not done your airworthiness certificate is automatically invalid because you would be operationg an airplane that is not airworthy and if anything happened the insurance co would find out and probably not pay.

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Re: Compressions

Mark:

Would you be surprised to find out that an engine with no rings at all and ZERO compression can make full rated power?

It can.  Static compressions are a very poor measure of engine health or capability to make power.

It's not as simple as we have been led to believe over the years that cylinders with low compression are unserviceable.

Walter

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Re: Compressions

AMO #1 did compression test and boroscope, results were 64/66/64/26/64/64 and boroscope shows glazing in all cylinders, recommends all cylinders are rehoned given the relatively low total time and says if not done the compressions can only worsen, leading to possible ring failure and potentially disastrous results.

Sourced second opinion from AMO#2 who says only #4 requires rehoning and the other compressions are "fine".

What would you guys do ? If the other 5 cylinders show "some" glazing, with those compressions, is this situation inevitably going to worsen or could it even reverse itself ? Or is AMO #1 just exaggerating ?

This airplane generally operates in "hot and high" conditions.

Dave Pearce
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Re: Compressions

If the you mechanic did boroscope and determined that the exhaust valve on #4 is NOT burnt, then I'd fly the dog out of it and recheck. They'll all be different, some a little and some alot.
A "compression" check as it is commonly called is not a compression check at all. Your mechanic is performing a cylinder leak check. This check can provide valuable information, but in the millisecond that combustion takes place at 2300 plus RPM, this "compression" check performed at a cooler temp and 0 RPM tells very little.

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Re: Compressions

Ahh, more details.  If the borescope shows glazing, it will have to be re-honed or it will be unlikely to ever get better.  How many cylinders need that is up to the recommendations of the ones looking at it.  Sorry I can't be more specific.

Good luck.

Walter

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Re: Compressions

Walter,
I would be surprised if it were possible to have an engine that can start, let alone make full rated HP with no rings or compression.  if you dont get the oxygen molecules squeezed into the combustion chamber there wont be the explosion you need to get the power etc, etc.  if there were any explosion of the fuel air mixture the majority of the power would end up in the sump hence the term blowby which until now was considered to be a bad thing.  and thirdly the blowby caused by the absence of the rings would blow out the oil thru all the seals due to the overpressurization of the crankcase.  I know for a fact that is a bad thing.  I also know that when the bottom ring on a TCM 0-200 breaks it will pump so much oil into the cylinder that you cant keep the plugs clean long enough to even get airborne.  so how does it work with no rings at all?
now if it were possible for an engine to run with no rings why dont we all have one?  can you imagine the power that could be made by an engine that has had its inherent friction component removed.  I mean that engine would spin over like the plugs had been removed without having the rings scraping along the cylinder walls like they do now..

If you have any data supporting this theory of an engine that has no compression to be an airworthy engine I would really like to see it.

I mean no disrespect by any thing I have said here.  its just that what you have stated flies in the face of any known laws of physics as we know them.
its like the theory I have that we ought to be able to tap into an unlimited source of electrical energy without having to generate it.  after all every atom in the universe is powered by electricity.  so why do we need to generate more?  that theroy doesnt jive with the universal laws we know either.
mark

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Re: Compressions

Mark:

**If you have any data supporting this theory of an engine that has no compression to be an airworthy engine I would really like to see it.**

I knew that post would get someone riled up! <g>

First, I did NOT say a ringless engine would be airworthy.  I made no such claim.  I simply stated the fact that an engine does not have to have compression on a static compression test to make rated power.  That is a proven fact.  That also does not mean that it wouldn't have dynamic compression (which it does).

Second, it's not my data, it's TCM's data.  They set up an engine without rings and ran it.  It made rated power.  As you suggested it would, it had one hell of a blow-by problem, the crankcase was pressurized and it was blowing oil out of the breather, BUT it was making power...  not just a little, but rated power. The point to their experiment was that low compressions are not a measure of how much power an engine can produce.  Period.  The point is that we shouldn;t get too wrapped up in STATIC compression readings--especially if an engine is not suffering from excessive blow-by.

The source on this data and the explanation of the experiment was Carl Goulet, former VP of engineering of TCM who conducted the test.

(BTW, a compression below 60/80 does not make a cylinder unairworthy.  See TCM's SB on the issue.)

Walter Atkinson
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Re: Compressions

Mark:

Please allow me to clarify what seems to be a misunderstanding.

The fuel:air charge does not explode.  It burns--smoothly; albeit a fast burn. It will burn on a table-top where there is zero compression beyond one atmosphere.  The purpose of compression is to increase the EFFICIENCY of the conversion of chemical energy into mechanic energy.

Walter

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Re: Compressions

Walter,
thanks for the info.  I need to look that up as I am sure it will be interesting reading.  However, I still stubbornly believe that the better the compression the better the engine will work and be effecient as you pointed out.  yes you are correct . the event at ignition isnt ( or shouldnt be) an explosion as that is termed detonation and thats bad.  unless I am confused on that issue as well.  probably a poor choice of words on my part and as someone who doesnt type that well I sometimes sacrifice accuracy in favor of brevity.
However, I think what the original poster was asking for was some real world data that he can operate with.  I have no doubt that TCM did such an experiement.  However, all labratory experiements are conducted in an enviornment where the conditions can be controlled.  we on the other hand are not able to control the conditions beyond the go-no go decision.  therefore I do not belive that the idea presented should be used as a basis on which to make a decesion in the real world of whether or not to change/repair the cylinder in question.  it should be made using the guidelines in the TCM SB which I cant recall the number of off the top of my brain.  personally I think it would be fun to work in a lab where you get to do all sorts of cool stuff that isnt reality or even possible in the outside world.
additionally I think the bigger question here that the original poster needs to resolve (besides are we all nuts) is why did he get such an unbalanced number on the tests with such a low amount of time on the engine?
some things we dont know.  has there been a history established on this engine and have the numbers consistentaly been low on the #4 cylinder?  and whats that about glazing? where in the cylinder is the glazing?  if he is talking abpout the cylinder head/combustion chamber and the top of the piston its possible that it is operating at too high of cht in the hot and high conditions.  has the cylinder always from day one been low? could it be that there might have been something gone awry at the factory when it was assembled?  is the glazing on the cylinder walls? if so then I will defer to those who recommend rehoning and reringing the cylinders as it possibly didnt get broken in properly for whatever reason when it first came out of the box.  again I am just full of (something)questions and not being there makes it difficult to make a valid recommendation.

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Re: Compressions

I have checked low TBO time (300 hr Factory reman)Lycomings, specifically 0-540s and had the compression checks come in at @ 72/80. Everyone runs around saying the sky is falling but what usually happens is that at the next compression check, the cylinder compressions are back up in the consistant 78/80s.  I use the compression check more as a test for cracked cylinders or for detecting a bad valve system. Glazed cylinders usually give the engine an oil consumption issue.. I did have a 100 hour factory reman 0-540 drop all compressions on all six cylinders at same time.. It was determined, by factory reps to be from a "lightning strike"  The airplanes nose wheel bearings were welded to the cone where the lightning exited the airplane. Warranty replaced all 6 cylinders...

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Re: Compressions

wow lightning strike!!
I have seen lots of damage from lighting strikes but havent heard of it causing all the cylinders to drop like that.  that is good to know in case I ever come across that situation as it will give some idea of what to look for.  I am curious tho what damage was found in the gear train and the magnetos etc.  I have heard of the lightning strike welding the gear train of a turbine engine together.

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Re: Compressions

Mark, The engine was changed out in June with 2200 hours and never any other problems.. The Lycoming Rep who visited the engine had never seen one affected like that with lightning before. Lightning strikes can do strange things..seen damage it done to a Beech Kingair-- splattered metal from the prop to the fuselage and blew a hole in the flap..

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Re: Compressions

With all due respect to the responses, If I had a lycoming with 60/80 , I'd be concerned. In my experience, thse engines carry excellent compression all the way to TBO. What gets you are the valves. If you have a lycoming with low compression, make damn sure you dont have a leaking exaust valve. there are checks for this, ie wobble in the guide, leak down check while listening at the exaust,etc.While it is true that some engines (TCM) are allowed by the mfg to have a dynamic seal leak ( rings ) down to very low limits ( say 42/80 ) no leakage is allowed past the valves.

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Re: Compressions

The definitive test on a leaking exhaust valve is not a static compression test.   It is a borescope inspection of the face of the valve.  A non-leaking valve will have a donut-shaped color pattern on it.  A leaking valve will have odd-shaped, non-concentric heat patterns.  I had an exhaust valve that was leaking during a static compression test with low compression.  It had a normal appearance and was not leaking while it was running.  I continued to run it for many hundreds of hours.  It was not leaking when the engine was running, it was only leaking on a static compression test. (ref: TCM SB on the subject)

Walter

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