The Spit/GT6/Herald series has a really wide range of ratios available, more
than the TR series. The 'right' gearing depends on power at a given rpm, tire
size, and the type of track. It should be the first thing you think about
when seriously preparing a car.
Like most of this stuff, there is a mathematical formula to use to determine
the right gear for the engine rpm and mph you're looking for. You need to
think about the engine speeds you want to run at and the mph you need to hit
to be competitive for a given track. Really low gears will let you launch
hard but you will lose steam on a long straight where it's easier for the
person behind you to pass. But if the track is tight and twisty, good corner
exit speeds will help you keep others from passing, or allow you to overtake
the car ahead. If you choose wisely, a car with less power can beat one that
isn't prepared as well.
First you need to know the final drive ratio, which is different than the
diff gear ratio: it takes into account the tire diameter. Then, you need to
know roughly at what rpm your engine make the best power or torque. Finally,
find out what kinds of speeds are the other cars in your class hitting on a
given track. The other FOT'ers can help with those specific questions, and
I'm sure you'll get a flood of help there.
Pick up a copy of "Auto Math Handbook" and all the formulas are in there. For
example, to find the correct gear ratio, multiply the rpm desired to the tire
diameter, then divide that figure by desired mph times 336 (a constant). That
will tell you the right gear ratio for a given tire, rpm, and mph. Then find
the closest ring & pinion ratio to that result. The Spit has an advantage
because there are more gears available for it than most cars.
I ran 3.27's in my Vitesse, because the 2 liter six had plenty of grunt to
pull it, and it would accelerate like a jet at higher speeds. You did have to
slip the clutch a bit to get it rolling, but I wanted it to pull like a TR at
65 mph, and it sure did.
A friend with a TR250 put in taller gears (3.45's) in his car and won the
autocross at Triumphest, simply because he didn't have to shift out of first.
In that specific case, lower gears actually added to time.
Taller gears also helped at the old Riverside raceway, where a high top speed
down the long back straight allowed me to pass just about anyone with an
essentially unmodified engine.
Gear selection can make all the difference, and lower isn't always better.
Hope that helps.
Steve Hedke
Team Scrappy
1957 TR3
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