Compression test low. So what?
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Veldung
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Re: Compression test low. So what?
Compression test on the DA40 varies quite a bit. Mine went from 74 70 72 66 back to 79 - 78 - 79 - 78 in this years annual.
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rwtucker
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Re: Compression test low. So what?
Normal product of [User Error] + [Instrument Error Test/Retest Same Instrument] + [Instrument Error Different Gauges] + [Parallax Error] + [Ring Position] + [Valve Position] + [Engine Temp] + [Temp of Compressed Air (just compressed in tank, cooling for awhile)] + [Consistency of Gauge Tapping]. There are even more sources of error but these will easily account for your variations.Veldung wrote:Compression test on the DA40 varies quite a bit. Mine went from 74 70 72 66 back to 79 - 78 - 79 - 78 in this years annual.
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Antoine
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Re: Compression test low. So what?
I have had a lot of reasons to investigate compression issues lately and here is my present state of knowledge:
Compression tests as such are not a reliable way of diagnosing engine health. Low compression is at best a hint that something may be wrong.
The reason? The three rings which are supposed to provide the air tight seal are not perfect "closed" rings.
They are open rings so that they can be installed on the piston. The opening in the ring represents a certain leak potential. Since the rings are not glued but merely snapped-in, they can rotate freely. If the openings are distributed randomly, not much pressure can leak through. If all 3 have aligned openings, the opposite is true. All intermediate situations are possible...
The Continental story about low compressions is valid for turbocharged engines. On NA engines a compression ratio as low as 40 would be a serious problem.
Compression tests as such are not a reliable way of diagnosing engine health. Low compression is at best a hint that something may be wrong.
The reason? The three rings which are supposed to provide the air tight seal are not perfect "closed" rings.
They are open rings so that they can be installed on the piston. The opening in the ring represents a certain leak potential. Since the rings are not glued but merely snapped-in, they can rotate freely. If the openings are distributed randomly, not much pressure can leak through. If all 3 have aligned openings, the opposite is true. All intermediate situations are possible...
The Continental story about low compressions is valid for turbocharged engines. On NA engines a compression ratio as low as 40 would be a serious problem.