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"Poll" on Compressions 74/80-71/80-78/80/-76/80

R

ROBERT DAVIS

Well Known Member
Hi,

I am looking at purchasing a plane with 455 TTSN Aerosport 0-360 Engine & Hartzel prop.

Should I be worried about the compressions?
1 2 3 4
74/80 71/80 78/80 76/80

Would like to hear from you all.

Thank you
 
Last edited:
Check that they were measured...

...with a 0.040 orifice as per Lycoming's SBs.

If it was done with a 0.060 orifice, then the compression is really poor.
 
I see no problems with those readings at all.

Number one, differential compression is a relative measurement. You can go fly that airplane for an hour and get totally different readings. You should be concerned when the compression is greater than 20% between the high and the low reading, or if any reading is lower than 60/80.
 
ROBERT DAVIS said:
Hi,

I am looking at purchasing a plane with 455 TTSN Aerosport 0-360 Engine & Hartzel prop.

Should I be worried about the compressions?
1 2 3 4
74/80 71/80 78/80 76/80

Would like to hear from you all.

Thank you
Click to expand...

To understand the condition of the motor, have your mechanic do a COMPRESSION TEST. The way it is done in the automotive world. It will show you more than just that the cylinder will hold air.
 
Ahh... but...

Mel said:
Number one, differential compression is a relative measurement. You can go fly that airplane for an hour and get totally different readings. You should be concerned when the compression is greater than 20% between the high and the low reading, or if any reading is lower than 60/80.
Click to expand...

Mr. Lycoming says (salient points only - see SI 1191 for full details)....

* Should be nearly equal - 5 psi difference is satisfactory - 10 to 15 psi difference should be investigated.
* Over 15 psi - check again within next 10 hrs.
* All above 70 psi - engine is satisfactory
* less than 65 psi indicates wear and should be tracked with 100 hr measurements
* below 60 psi, or a rapid increase in wear rate, cylinder removal should be considered...

Based on this (the manufacturer's data) - your engine sounds fine...:)
 
Poll on compression

Remove the oil cap, open the throttle and apply the air to the cyl., listen at the exhaust, oil inlet, and air screen. This will tell you what is leaking, exhaust valve, rings, or intake valve.
Walt RV-6A
IO-320 CS finishing up
 
I may be whistling in the dark, but I THINK that engine compression rating affects the expected outcome.

All answers so far are correct for 8.5:1 "standard" compression. I'm not sure how this varies for a "low compression" or "high compression" engine.
 
Compression checks are subjective because you can move the piston 32nd of an inch and get a drastically different compression reading. Compression checks alone are in no way suitable for diagnosing the health of the engine.

We consider mid-time engines that come through our shop that have compression of 60 or less through the rings as marginal and pull them to inspect and re-ring, with possible honing work if needed. Any leak past the valve is unacceptable and requires immediate repair. I believe Lycoming says that 50/80 meets continued airworthiness standards.

Hope this information is helpful to you.
 
Not for the leakage test we...

breister said:
I may be whistling in the dark, but I THINK that engine compression rating affects the expected outcome.

All answers so far are correct for 8.5:1 "standard" compression. I'm not sure how this varies for a "low compression" or "high compression" engine.
Click to expand...

...are talking about here.

It's solely a measure of leakage past the rings and valves.

e2alg.jpg


While the automotive type is compression ratio sensitive -

41Q5HSHJK3L._SS500_.jpg
 
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