(no subject)

[AKress1948@xxxxxxx]

Not to beat the mean piston speed issue to death, but what the hell!
>From the Auto Math handbook:

piston speed in fpm = stroke in inches x rpm / 6

rpm = piston speed in fpm x 6 / stroke in inches

"    Piston speed is the rate at which the piston travels up and down in the
cylinder, and is usually measured in feet per minute.  The rate isn't
constant. At higher rpm, the piston may be going more than 100 mph near the
middle of it's stroke.  It slows as it approaches either end of it's cycle,
comes to a momentary stop as it hits top or bottom dead center, and then
accelerates as it starts back in the other direction.
     In other words, it may be going from zero to over 100 mph back to zero
during each stroke  -- over a distance of only 2.0, 3.0, or 4.0 inches!
     If the piston speed gets too high, the primary hazard is that a piston or
rod -- or more than one of them -- may break from the strain.  Therefore
determining piston speed will also help determine a practical rpm limit.......
    A secondary problem is that a piston may outrun flame front travel --
running faster than the expanding air/fuel mixture is pushing it -- and
effective pressure and, with it, horsepower will drop. ......
All you need to find is the average or mean piston speed.....
....In the early days of hotrodding, when the flat head Ford V-8 reigned
supreme....2,500 feet per minute was considered the maximum practical piston
speed -- not just for Fords, but for all cars......(several examples showing
that @ 4,000 rpm, get 2,500 fpm.).....
with modern advances in metallurgy higher piston speeds have become possible,
but there are still limits.  ...quotes A. Graham Bell book to show some
examples....
     With a stock cast-iron crankshaft and connecting rods, he recommends a
maximum of 3,500 fpm, and with a forged crank and heavy duty rods and main
bearing caps, between 3,800 and 4,000 fpm.  At the outer limit, he believes
that an all out drag racing engine which is equipped with super-duty
components, run on racing fuel with fast flame front travel and revved to the
max for oily a few seconds at a time may get away with  piston speeds as high
as 5,000 to 6,000 feet per minute."

Then goes on to give a few examples for instance, Boss 302 V-8 Mustang Trans
Am engine  using his formula, w/ a stroke of 3"and 8,000 rpm gives a mean
piston speed of 4,000 fpm.

For perspective's sake, keep in mind that 5,280 fpm = 1 mile/min.  Seems like
a lot of distance for all those little pieces to travel in such a short time.
FWIW, a 2 liter Alfa engine w/ stroke of 88.5 mm (~ 3.48") @ 6,000 rpm = 3,484
fpm
crank it up to real race rpms -- say 8,500 = 4,936 fpm.  Now you know why
racers use Carillo rods, and Cosworth pistons!!!
Andy

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End of alfa-digest V7 #130
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