David Brady proposes the formula:
RPM Limit = 4000/( S x .00656 ) where S = Stroke in millimeters
(from an article by Dr. Fred Lanchester.
This may be on the right track, but I'd quibble with it a bit. I don't
have the numbers at hand, so i calculated the stroke of my TR-4 engine
at 92 mm - substituting into the above formula gives an RPM limit of
6628. This is not unreasonable for the factory, since with a 10%
allownace gives you a redline of 6,000 rpm.
David then goes on to say that running the engine at the "RPM Limit"
will cause broken rings, pistons, rods, etc. Well, my TR-4 uses
stock rods (lightened and balanced, but TR parts), normal aluminum
pistons, standard piston rings, etc and I used to shift it at about
6700 rpm or so until I put a rev limiter on it set to 6250 - its
about to get re-adjusted upwards to about 6700 again, since I'm
running into the rev limiter before I'm running out of power. Even
with the higher shift point (ie pre-rev limiter), I've never broken
a rod, piston ring, etc. I also know of quite a few other TR racers
who routinely run their engines above 7,000 rpm. So, I'd have to
say that this "RPM Limit" is a bit conservative if any time has
been spent on tightening up the factory tolerances.
John
HOAN Racing and Rent-a-Racer
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