|
-
POSTING RULES

-
Donate yearly (please).
-
Advertise in here!
-
Today's Posts
|
Insert Pics
|

08-08-2017, 03:43 PM
|
 |
|
|
Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
|
|
Quote:
Originally Posted by dtw_rv6
Mike Busch claims that compression readings have no correlation at all to an engines ability to make full rates power. It only tells you if the engine will consume oil.
Continental says to borescope the cylinders to determine their condition. If that is okay then fly for (??) hours and then redo the compression test.
Don
|
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.
|

08-08-2017, 03:57 PM
|
|
|
|
Join Date: Jan 2016
Location: Southwest
Posts: 1,119
|
|
What?
Quote:
Originally Posted by bjohnson1234
See https://www.aopa.org/news-and-media/...ance-borescope
A compression test measures static compression. You slowly bring up the pressure and hold it, but that's not what happens when your engine is running. During the compression stroke the piston is moving very rapidly and while I'm sure the underlying physics are more complicated, there essentially isn't time for enough air to leak. 5/80 is pretty low and if you can't bump it higher by pushing on the prop then you might have a more serious leak (like the loose plug that someone mentioned), but if the borescope looks good and the engine runs fine then you are probably just going to go through a lot of oil. There might another problem that someone more knowledgeable can point to, but a compression test is not a reliable or repeatable test and shouldn't be used as the basis for airworthiness.
|
Are you saying to Push on the prop during a compression test? I hope not this could be dangerous. But I had a bad test because the rings were not seated as if they were on a normal compression stroke, but never have seen 5/80. I am really curious what caused this. Eating my popcorn while I wait.
__________________
John S
WARNING! Information presented in this post is my opinion. All users of info have sole responsibility for determining accuracy or suitability for their use.
Dues paid 2020, worth every penny
RV9A- Status:
Tail 98% done
Wings 98% done
Fuselage Kit 98% done
Finishing Kit 35% canopy done for now
Electrical 5% in work
Firewall Forward 5% in work
www.pilotjohnsrv9.blogspot.com
|

08-08-2017, 04:07 PM
|
|
|
|
Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
|
|
Quote:
Originally Posted by PilotjohnS
Are you saying to Push on the prop during a compression test? I hope not this could be dangerous. But I had a bad test because the rings were not seated as if they were on a normal compression stroke, but never have seen 5/80. I am really curious what caused this. Eating my popcorn while I wait.
|
Is it dangerous? Are you just worried about the general dangers of non-grounded magneto accidental start or is there something else? By "push" I just mean move the prop slightly to add some pressure.
Last edited by bjohnson1234 : 08-08-2017 at 04:09 PM.
|

08-08-2017, 05:22 PM
|
|
|
|
Join Date: Jul 2017
Location: Oceanside, California
Posts: 106
|
|
With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.
But.
As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.
I agree with doing a very slight bump. It takes very little to make the gauge jump a bit. Just to make sure the gauge is repeating and not sticking. Anything more than a "tiny slight bump" is asking for painful and scary injuries, or worse.
I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.
|

08-08-2017, 05:43 PM
|
|
|
|
Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
|
|
Quote:
Originally Posted by rackley16
With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.
But.
As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.
I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.
|
I think you forgot that the prop is a long lever arm. 1600 lbs at the crank journal, which is offset, what, maybe 4", from the centerline?, will turn into 160 lbs at the end of a 40" radius prop. Still nothing to sneeze at. But not 1600 lbs. In fact, 80 psi is standard for these tests because it was thought that that was the maximum "safe" pressure one should use.
|

08-08-2017, 05:48 PM
|
|
|
|
Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
|
|
Quote:
Originally Posted by rv6ejguy
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.
|
IIRC Mike B was talking specifically about leakage past the rings. He has long advocated that NO leakage should be tolerated past a valve (this was at a time when TCM was having exhaust valve issues, and came up with the idea that some leakage was not grounds for a warrantee claim).
|

08-08-2017, 06:08 PM
|
|
|
|
Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
|
|
Quote:
Originally Posted by rv6ejguy
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.
|
You would see a burnt valve on the borescope so I'm not sure that there is a qualification. He's not saying that you should throw out the compression check and just hope for the best, he's saying that a borescope is a much better tool.
|

08-08-2017, 07:07 PM
|
|
|
|
Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
|
|
Quote:
Originally Posted by rackley16
With 80 psi in the cylinder there is about 1650 lbs of force being applied to the small end of the connecting rod. At TDC everything is in a straight line as far as force application. It takes very little to hold the prop steady and safe.
But.
As the prop turns the force will be applied at an angle to the crankshaft throw and will increase quickly and forcibly, at the prop, the farther it turns toward 90 deg. of crank rotation. Which would be maximum force applied of 1650 lbs.
I agree with doing a very slight bump. It takes very little to make the gauge jump a bit. Just to make sure the gauge is repeating and not sticking. Anything more than a "tiny slight bump" is asking for painful and scary injuries, or worse.
I am sure you guys know this. I just thought I should mention it in case someone is not versed in this type of check.
|
Since it's been pretty well established that the raw compression values don't have a lot of meaning, is there a compelling reason to continue to use 80 PSI? Maybe 40 or even 20 PSI would work just as well?
|

08-08-2017, 07:14 PM
|
|
|
|
Join Date: Sep 2007
Location: Central IL
Posts: 5,516
|
|
Quote:
Originally Posted by rv6ejguy
I hope Mike doesn't really claim this without some qualification. Got half of a valve burnt off or a quarter size hole in the piston, you'll have poor compression, leakdown and little or no power from that cylinder. A burnt valve may not affect oil consumption at all.
|
I agree, these articles are not gospels but some technical content written for interesting effect. The do contain some interesting facts. The Conti test increased gaps - we both know the ring pressures are highest on the top and decrease for second compression and the oil ring pressure is little affected by the chamber pressures. Meaning, the wear and gap increases (and bore wear) are not the same for all rings. If the Conti test evenly increased all the gaps, then the oil consumption could easily have increased due to uncovered area by the gap in the oil rings.
If one wanted to follow a diagnostic flow chart it would likely fill a double garage door with 10 point type and enough lines to make the reader blind. There are few simple truths in engine diagnostics. PRB technology is just a part of the whole engine.
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
|

08-09-2017, 06:02 AM
|
|
|
|
Join Date: Apr 2010
Location: Royse City, TX
Posts: 143
|
|
Lycoming SI 1191A
Instead of all the conjecture about how and what your compression test indicates, possibly consult Lycoming's service instruction SI 1191A. While not regulatory, Lycoming probably has a better understanding of their engine design than most other conjectures.
Do your research - Lycoming and TCM cylinder compression tests have significantly different processes.
Again, not regulatory, but Lycoming and AC 43.13B have similar compression test processes which recommend that if a static compression is below 60 psi, further investigation should be considered. 5 / 80 psi is a dead give away for a massive leak somewhere. Possibly a valve is burned or crack in the cylinder. Leaky rings show up as air hissing out the oil dip stick or breather, air out the exhaust or intake. Spray the cylinder head / barrel with soapy water solution to look for cracks in the head / barrel.
|
| Thread Tools |
Search this Thread |
|
|
|
| Display Modes |
Linear Mode
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
All times are GMT -6. The time now is 04:28 AM.
|