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<<Recognizing that the POH is "the law," is there any risk to engine life
running below 6 quarts? What is the "optimal" amount for engine cooling?>>
After collecting info from lots of sources, including Lycoming, and throwing
out what I consider to be not credible, here are my conclusions:
1. The "minimum safe" oil level is that at which the pump will always be
able to pick up liquid in any flight condition. That, according to
Lycoming, is 2 quarts. More oil will likely reduce aeration, so 2 quarts is
really the bare minimum. It probably results in about 1 quart remaining in
the oil pump pickup area during operation, as there is probably 1 quart
spread around the engine. The prop governor is the first to be effected not
because it might be the furthest away, but because the prop requires a
steady source of high oil pressure to maintain pitch. The engine itself
will survive just fine if there is a very occasional and very momentary oil
pressure drop.
2. The oil capacity is determined during the design phase is a compromise
determined by a combination of the maximum oil consumption expected, the
longest flight anticipated and the practical oil pan size. Lycoming has
usually picked 8 quarts for the 4 cylinders and 12 quarts for the 6 cylinder
engines(I might be wrong on this). It is interesting to note that
occasionally they pick 8 quarts for the 6's to make room for the nose wheel.
The longest flight was assumed by some to be ferry flights with extra fuel.
So 12 hours at 2 hrs per quart starting with 8 quarts gives 2 quarts left.
3. There is no technical evidence, except some people's experience, that
leads me to believe cooling is enhanced by higher oil levels. If this is
true, it must be because of more contact area with the oil sump walls or
more coverage of the intake manifold plenum that causes this. Negligible
importance, in my opinion.
4. Some people, even at Lycoming, agree that oil levels higher than 7 will
result in higher consumption. The reason Lycoming has given me, especially
for the 6 cylinder engines, is that during a nose high attitude just after
takeoff oil gets back into the accessory drive section and the gears tend to
"pump" the oil up to the breather. My experience is mixed in that sometimes
I can convince myself that the consumption is higher and sometimes not.
5. Oil level and engine life are not related. As long as the engine
receives a constant supply of oil under pressure the quantity of oil in the
sump has no effect on engine life.
Conclusion: Use 7 quarts as the normal maximum and the minimum depends on
the length of the planned flight, typical oil consumption, and your personal
safety margin (see one of the other posts). My IO-360 burns a quart per 4
hours and I put a quart in when the level is below 6. For a long flight
where I won't want to go to the trouble to add oil at the destination
airport I might put in 8 just for my own convenience.
Observation: I'll bet some of the reported rapid oil consumption occurs
right after an oil change. The sump is filled with 8 quarts and after the
first 2-hour flight there is only 7 left. The pilot might forget that most
of that 8th quart is still in the oil filter and hence it didn't really burn
it that fast anyway.
Gary Casey
ES #157 in my garage (no oil consumption so far)
C177RG N2180Q at the airport
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