Thoughts on compression

[jaylw314]

Here's a link to one of Mike Busch's webinars about compression testing vs borescoping.  Of note, go to the slide at 10:48, he references from a Continental study a graph of compressions vs hours in services up to 600 hours, and the compressions fluctuated back and forth from 56 to 75, and are almost uselss.  This is the basis for Continental SB03-3.  The webinar is worth a listen

 

[carusoam]

Continental compressions are a funnier animal than Lycoming ones...

Moving the prop directly effects the position of the exhaust valve.  Slightly open or completely closed...

For the compression test, you want the valve completely closed for best result.  If the valve doesn't close, a good borescope analysis may better indicate the issue... air leaking sounds in the exhaust vs. in the case, are a hint of what is happening.

PP thoughts only, not a mechanic...

Best regards,

-a-

[kortopates]

Continental compressions are a funnier animal than Lycoming ones...
Moving the prop directly effects the position of the exhaust valve.  Slightly open or completely closed...
For the compression test, you want the valve completely closed for best result.  If the valve doesn't close, a good borescope analysis may better indicate the issue... air leaking sounds in the exhaust vs. in the case, are a hint of what is happening.
PP thoughts only, not a mechanic...
Best regards,
-a-

That shouldn't be an issue. Neither valve is open anywhere near TDC on the compression stroke. Both valves are open at the end of the exhaust stroke/beginning of the intake stroke.


Sent from my iPhone using Tapatalk

[MB65E]

My testers gets calibrated annually. Those Eastern gauges suck and only last 3-4 years. Even if you only use them several times a year. I changed all my gauges  to Wika gauges for better operation and reliability.

Compression tests are subjective. Oil consumption i think provides better clue. 

Glad your retest went well,

-Matt

[carusoam]

Paul,

i may have fumbled the valve discussion...  

The valves are supposed to be closed during the compression test. There are a couple challenges that can cause them to not close.  If I have it right...

• A leaky exhaust valve can be heard at the tail pipe.
• Leaky rings can be heard at the oil filler hole.

 

Follow up question...

Why does the mechanic move the prop back and forth during the test?  What is he looking for there?

 

Its been a while since I've seen a compression test performed...

What stands out in my mind the most, was determining engine cylinder health of a Continental engine.  After a decade of consistent or similar compression readings on my Lycoming O360, I was looking at the results of an annual performed on a Missile's IO550.  I was looking into buying a Missile, and trying to learn about in a very short period of time.

The Continental compression results are a much broader range and don't lend themselves to easy interpretation with my PP skills.  Unfortunately, I don't have a bunch of compression numbers to consult this evening. I would love to describe what I'm trying to say with actual numbers... 

I probably fumbled again... But, thanks for your awesome support!

Best regards,

-a-

[Tommy]

http://www.eaavideo.org/detail/videos/webinars/video/5494160522001/webinar--borescope-ascendancy?autoStart=true

[carusoam]

Tommy, that's great!

Lycoming has compressions near 70 and decline with wear...towards 60...

Continental, good readings are still in the mid 40s, where a Lycoming cyllinder would be considered retired...

 I scanned the Mr. Bush presentation just looking for these numbers.  I'll go back tomorrow to see if I can find an answer to why this happens...

Best regards,

-a-

[Tommy]

Just now, carusoam said:

Tommy, that's great!

Lycoming has compressions near 70 and decline with wear...towards 60...

Continental, good readings are still in the mid 40s, where a Lycoming cyllinder would be considered retired...

 I scanned the Mr. Bush presentation just looking for these numbers.  I'll go back tomorrow to see if I can find an answer to why this happens...

Best regards,

-a-

Can't take the credit, just realise that OP's video is the same. 

The thing I like about Mike's webinar is that @jaylw314 video is the same. 

The thing I like about Mike is that he has data and he analysed them in an very scientific and impartial way, so much so that he was able to beat Continental at its game. More the incredible considering it's really not in his own best interest to do so. *Hat off*

 

[Guest]

7 hours ago, carusoam said:

Paul,

i may have fumbled the valve discussion...  

The valves are supposed to be closed during the compression test. There are a couple challenges that can cause them to not close.  If I have it right...

• A leaky exhaust valve can be heard at the tail pipe.
• Leaky rings can be heard at the oil filler hole.

 

Follow up question...

Why does the mechanic move the prop back and forth during the test?  What is he looking for there?

 

Its been a while since I've seen a compression test performed...

What stands out in my mind the most, was determining engine cylinder health of a Continental engine.  After a decade of consistent or similar compression readings on my Lycoming O360, I was looking at the results of an annual performed on a Missile's IO550.  I was looking into buying a Missile, and trying to learn about in a very short period of time.

The Continental compression results are a much broader range and don't lend themselves to easy interpretation with my PP skills.  Unfortunately, I don't have a bunch of compression numbers to consult this evening. I would love to describe what I'm trying to say with actual numbers... 

I probably fumbled again... But, thanks for your awesome support!

Best regards,

-a-

Moving the propeller at the top of the compression stroke moves the piston very small amounts in search of the highest compression reading.

Continenal has an S/B which covers compression testing and a special tool which determines the allowable low value for that day.  

Clarence

[N201MKTurbo]

By moving the prop while doing a compression test you can seat the rings on the top of the grove in the piston which is the situation during the power stroke or the bottom of the grove which is the situation during the compression stroke.

people normally think about the rings sealing against the cylinder wall. The seal against the piston groves is just as important.

[Bob_Belville]

6 minutes ago, N201MKTurbo said:

By moving the prop while doing a compression test you can seat the rings on the top of the grove in the piston which is the situation during the power stroke or the bottom of the grove which is the situation during the compression stroke.

people normally think about the rings sealing against the cylinder wall. The seal against the piston groves is just as important.

I wonder if that (the rings fit in their grooves) in a static condition is a factor for the Continental engines. I assume that the intake and exhaust valves close pretty tight in a healthy engine so pressure loss must be past the piston/rings. 

[N201MKTurbo]

I realize this is about race cars and not airplanes, but it gives a lot of good information about piston and ring design. Enjoy:

http://www.hotrod.com/articles/hrdp-0910-piston-ring-tech/

 

[carusoam]

I assume our ring design is unusually ancient in comparison to the fancy Chevy LS version?

Best regards,

-a-

[N201MKTurbo]

20 minutes ago, carusoam said:

I assume our ring design is unusually ancient in comparison to the fancy Chevy LS version?

Best regards,

-a-

I would assume that is a correct statement. Considering the article states that modern engines have no ring break-in and must be made perfect at the factory. That doesn't sound like SOP for an aircraft engine.

[Joe Larussa]

On 9/9/2017 at 6:24 PM, jaylw314 said:

Here's a link to one of Mike Busch's webinars about compression testing vs borescoping.  Of note, go to the slide at 10:48, he references from a Continental study a graph of compressions vs hours in services up to 600 hours, and the compressions fluctuated back and forth from 56 to 75, and are almost uselss.  This is the basis for Continental SB03-3.  The webinar is worth a listen

 

Excellent information!!

[RLCarter]

on our drag bike we would "gas port" the pistons, but our compression ratio was pushing 17:1