Date: Thu, 8 Jun 2000 10:48:30 -0400
From: Kent Rafferty
<< How does one figure what rear gear ratio would
be "optimal" for autocrossing? Is there a "rule-of-
thumb" for making this calculation?
Kent Rafferty
SM Supra >>
Just like the previous responses, it is a bit course dependant. But the
"RULE" I used with my turboed 83 Celica was to make sure it would go 70 mph
in second gear. The car came with a 3.42 final. With the SP legal motor rev
limited at 6000 rpm, it would go a tick past 70 on 205 55 14 tires. When I
added the turbo and went to EMOD a few years back, the gearing felt good with
all the torque, but it detonated the rear end. I picked up a Supra rear end,
but it was a 4.10. OUCH, I needed 3rd gear 4 times on a 40 second course.
After some searching I came up with a 3.73 limited slip Supra unit, and with
the rev limit upped to 6500 and taller 245 50 15 tires it would once again go
70 mph in second. After a re-build of the motor with beter rods and a cam, it
now revs to 7200 and I am back on shorter tires, 225 50 14's
Basically, 70 mph is second seems like a good place for most regional events.
We had one at Route 66 last year where I needed third, but chose to hold the
7200 rev limiter for several seconds. BIG MISTAKE. The power steering belt
flew off and tangled in the front of the motor. Once I got that out it was
rattling like crap. Turned out it sheared off all six flywheel bolts.
If you are not torque limited (YOU CAR IS NOT) gear it fast enough to not
need the extra shift.
Gary M.
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