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<< Lancair Builders' Mail List >>
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Mike is "spot-on" (as my British friends put it) regarding minimizimg ground
running of a new engine. The long pole in the tent that causes all the
trouble is that just about all the engines you all are using have a certain
amount of "choke" ground into the cylinders. What this means is, the upper
part of the cylinder barrel is actually manufactured with a slightly smaller
diameter than the rest of the cylinder, starting at about the last (upper)
25 - 30% of the of the bore (though it varies with different engines) and
usually tapering linearly from the start of the taper to the top.
The grand design here is that when the cylinder head temperature gets up to
normal operating temp., the upper end of the cylinder expands so that the
bore becomes nearly constant - top to bottom - and the rings don't get
overly squeezed during normal operations.
But, run your new engine on the ground for ANY extended period, where the
CHT's can't reach their design temperatures (and expansion) and it doesn't
take long to either damage your new rings and/or scrub the choke out of the
barrels, or both! Take the choke out of the bore and when you finally do
start making maximum power and CHT's, the diameter of the upper end of the
cylinder will become larger than the rest of of the bore and ring seal goes
to hell in a hand basket! Can you say "burns oil"? Prolonged ground running
a new engine (that is choked, which most are) will just about guarantee
you'll be needing a top overhaul before you can say "Dagnabbit!".
Another fly in the ointment is the fact that the compression rings require a
lot of combustion chamber pressure to effect a proper seal (the high
pressure of normal or high power actually promotes the proper ring-to-wall
contact pressure). Without that high pressure, your rings will not seat
properly and you'll start using excessive oil. Another way of saying you'll
be looking at an early top.
If you have an overhauled engine with chrome cylinders, even a little bit of
ground running can give you grief as they are even less tolerant of low ring
pressure and low upper CHT's when fresh. I once rode with a friend in his
Piper 6-300 which had just come out of the shop where it had been majored
with 6 fresh chrome jugs. This guy had more ratings (CFI, CFII, plus AP and
a bunch of others) so it really got my attention when we did the run-up
during a short taxi, rolled onto the runway and blasted off! We flew around
for about two hours using nearly max power. It wasn't until we landed and I
mentioned how startled I was that an old gunnie like him had not done what I
thought was a thorough run-up. It turned out that he knew what he was doing
and I learned something.
Hope you have too.
Dan Schaefer
P.S. I think he got close to 1000 hours on that engine before it needed any
cylinder work - which is quite a feat for that particular engine, I'm told.
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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