Those are a lot of good thoughts and observations. I suggest there is a
good deal of gas pressure over the piston on the compression stroke up
to the top, the plug has fired say 38 degrees BTDC, the pressure really
rises then and is way up at TDC, continues up as you say to peak maybe
17 degrees ATDC so it shoves the piston down hard to maybe 90 degrees
ATDC when the pressure drops off and almost immediately (with racing
cams) the exhaust valve starts to open. But, on the next upstroke the
piston is pushing the exhaust gas out which has some degree of
resistance, and it continues to push it clear up to TDC and maybe 50 or
more degrees ATDC before the exhaust valve closes. Granted, the
pressure over the piston on the exhaust stroke is nothing like that on
the compression stroke, but there is some pressure.
Sure, with most forms of racing conrods break at other times besides
when snapping throttles shut at max revs, perhaps because of cracks
forming from fatigue of extreme stress, letting go; or perhaps because
of the severe shocks of repeated prolonged (inaudible) detonation.
Many years ago in one of the books he authored, World Motorcycle
Champion John Surtees noted that in GP motorcycle road racing he had
observed and found that rod breakage was mostly likely on the over-run
at very high revs when the throttle was snapped shut.
Your observation about 24 hour racing is very true, the rods hold up to
severe and prolonged stress. Here, they are made of the very finest
materials available and sized to endure the prolonged stresses without
failure. In the example my friend cited, people are running Bonneville
with elderly vintage (shall we say 'flathead') engines where for reasons
of nostalgia, economy or just plain stubborness they choose to use OEM
rods. These good old rods cannot be expected to be fully reliable under
extreme stresses they were not designed for, there will be occasional
failures, but all the same, the record books are full of their
successes. Regards, ArdunBill
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