Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Fri, 15 Nov 2002 11:30:16 -0500
Message-ID: <redirect-1871789@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: MP and rpm
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<<..causes the piston rings to slap back and forth in their channels in the
pistons.  This could lead to breakage of the rings.

another post:

..instructor was very clear that BAD things would happen if I ever ran more
RPM's than manifold pressure.>>

I don't know where all these things get started, but I suspect it's because
people have a natural desire for simple answers to complex questions.  Some
even come from otherwise-respected "experts."  Just think of a 4-stroke
engine operating - half of the strokes occur with essentially no pressure in
the cylinders.  In a motoring condition the compression pressures are
probably between 100 and 350 psi, depending on manifold pressure, still a
substantial pressure.  Actual full-load firing pressures are maybe 900 psi.
The only real additional stress from high-rpm running is from the tensile
forces in the connecting rod, but those exist on half the strokes anyway.
Cam lobes typically wear out at the tips and the loads on the tips are
greatest during cranking and idle.  Engines with fixed pitch props spend
most of their life at low MP and high rpm (I would run my old Cherokee at
less than 20" and 2700 rpm all the time).  The TSIO 550 runs at MP way
"over-square" all the time.  Piston rings wear because of forces on the
cylinder caused by pressure and other reasons.  They break for a variety of
reasons, some related to ring-land wear, some from over-heating and often
from built-in inclusions.  I'm not even sure I believe in "shock cooling."
What about "shock heating" when you go from idle to sustained full power in
a couple of seconds?  What about the shock cooling every time you shut the
engine off?  Flying into rain?  On Indy race cars one way to "read" the
plugs was to shut the fuel off while the engine was running flat out - never
seemed to hurt anything.

I think engines just wear out mostly from combined mechanical and thermal
stress and that pretty much comes from horsepower-hours.  One good way to
make the engine last longer is just to slow down, but that's no fun.

Gary Casey
ES