I wonder, why compression tests are never done while the piston is at bottom dead center?
Wouldn’t it, in theory, help identify the cylinder with largest blow by into the crank case?
My theory is that by measuring compression in the compression stroke, only the seal between rings and the upper section of the cylinder wall is measured but the bottom end is completely missed out. So a cylinder could have excellent compressions but could still be blowing combustion gasses straight to the crank case due to wear at the bottom part of the cylinder walls (pitting corrosion, unevenly worn out cylinder barrel..)
Am I wrong?
Baris
It depends on the engine design but are both valves really closed when at “bottom dead center”?
There is also the practical matter of getting compresses air in there at bottom dead center.
If both valves are closed, it works in every position via the spark plug adapter.There is also the practical matter of getting compresses air in there at bottom dead center.
Having said that, tightness at BDC is not that important, as no pressure will be moving the piston there anyway.
Valves are open.
No way to do a leak test.
It would certainly be safer to do this at BDC, because pressurising at TDC runs the risk of the crankshaft being rapidly rotated and chopping somebody’s head off. Someone I know cut his finger off doing this (he wasn’t quite exactly at TDC).
If I wanted to test an OHV car engine like this then I’d just remove the rocker gear temporarily so that the valves stayed closed at BDC. But it is rather moot for that application since a compression test is done with the test gauge in the plug hole and you crank the starter.
Could a lyco/conti not be done by cranking the starter? Does it have to be a leakdown test?
A supposed advantage of the TDC leak down test is that we can hear where air is escaping (to crankcase, exhaust, etc.). That said, some consider that it’s a waste of time and money unless there’s some other indication of a problem.
Jacko wrote:
That said, some consider that it’s a waste of time and money unless there’s some other indication of a problem.
I am one of those people. On my airplanes, I am satisfied pulling the prop through to feel the compression, and thereafter, holding the prop against compression, and listening in the exhaust or crankcase breather for any leaks. I have found exhaust valve leaks this way, and made the required repairs. I have never had an intake valve leak. Many maintainers and owners like the numeric measured compression test, probably because the result is quantitative, and thus objective, and that’s fine. When I sold a plane, the buyer came and did a numeric compression test, and that was fine. When I can feel good compression on each cylinder, and hear no leaks, and its running well, that’s good enough for me.
If a compression test were possible near bottom dead center, it would not be representative, as cylinders are choked. The bore diameter is larger at the bottom, so it would not be as representative as testing at TDC.
Many maintainers and owners like the numeric measured compression test, probably because the result is quantitative, and thus objective, and that’s fine.
Well, it looks quantitative, and precise (like an oil analysis report) but a commonly stated reservation about the hallowed leak-down test is that the figures it produces are usually given to the nearest psi, whereas the test procedure itself is probably only “accurate” or repeatable to about +0/-10 psi.
When I can feel good compression on each cylinder, and hear no leaks, and its running well, that’s good enough for me.
Amen, but I think less practical engineers prefer to see the “figures”, no matter how meaningful. 
