"Burns, James B." wrote:
> How do you decide which gear ratios are optimum for an autox?
> Assuming a relatively low torque 4-cyl engine, is it better to have
> lower gearing with closer ratios which requires more shifting, or is
> it better to choose a ratio that allows most courses to be run
> entirely in second gear (except for the start)?
I can tell you what those of us who have completely free gearing do...
First, there's a couple of issues to consider.
(1) Do you really have complete freedom? If you're in Stock or SP,
you're choosing between entire gearboxes, not individual gears. Thus,
you might find one where the gap between 2nd and 3rd is absurdly
large (so you'd want to try to stay in second), or one where first is
absurdly short (so you'd want to row between 2nd and 3rd, or stay in
2nd).
(2) Is the car easy to shift? Due to poor linkages, or engines or
trannys that move too much, some cars can be tough to shift at some
times. Thus, in such cars, you'd want to minimize shifts in order to
minimize shifting errors. Also, note that due to gearbox internals,
many cars are easier to shift between first and second than between
second and third, since you have to switch to another shift fork on a
2-3 shift. Cars that are hard to shift into first are usually because
there's a huge space between 1st and 2nd in terms of gear ratio.
(3) Similarly, can the car be shifted quickly? Closer ratios always
mean quicker shifts, since the gearbox shafts need less change of
speed. But there are other issues. On cars that cannot be shifted
quickly, you want to minimize shifts.
If the ratios are well spaced and the car can be shifted easily and
quickly, then I believe that shorter ratios are always faster. But
some drivers still may not want to devote any of their precious
attention span to shifting, and thus might still prefer to stay in
2nd.
Next, how to choose ratios. First, decide how many gears you're going
to want to use, based on the criteria above. To determine the spacing,
calculate the rev drop when shifted at redline, or perhaps the
percentage difference in each gear. Typically, if you're going to be
using four gears (as I do), you want 3rd and 4th to be very close
together, 2nd and 3rd to be slightly further apart, and 1st and 2nd
to be further still. The idea is that when accelerating, each upshift
results in a slightly smaller rev drop than the one before, because
you'll need the additional power to overcome the additional drag
experienced at speed.
Also, you'd like your upper gears to be close enough to the ones below
that if you do absolutely have to upshift, you might be able to get
away without having to downshift again.
Next, decide what your top speed needs to be. In the Formula Ford, it
looks like that's around 80 mph. Choose your gearing such that top
speed is reached at redline in top gear, then space the other gears
according to the description above. Top speed for my car in each gear
is 41, 54, 66, 81.
> As a hypothetical (honest!) example, let's take a '99+ Miata being
> prepped for CSP.
By my calculations, top speed in gear with each combo, using a
225/45-13 (65.19" circumference):
5-speed, 4.300: 32 53 75 100 123
5-speed, 3.909: 35 58 83 110 135
6-speed, 3.909: 29 49 67 88 110 131
6-speed, 4.300: 27 44 61 80 100 119
So, a quick take on that is that with either 5-speed combination,
you're still going to need 3rd gear sometimes. Given that, with the
4.3, at least you'll be much higher in the powerband when you're in
3rd.
With the six speed, you're always going to need third gear, and you'd
probably need 4th on some courses with the 4.300. If you believe
you'll never be going more than 67 mph,the 3.909 looks like the better
choice for final drive, given this tire size.
As for the sense of the ratios, here's the rev drop -- or more
accurately, the new speed in each gear after upshifting at 7000:
5-speed: 4214 4931 5263 5698
6-speed: 4224 5075 5349 5569 5901
So both have a good progression of gearing. The six speed is indeed a
little closer than the five. With the 6-speed and the 4.3, second
gear is short enough that you'd probably never need to go down to
first, except for those crawl-around-at-idle-in-first-gear pin turns --
and the ratios are close enough that upshifting to first won't be that
hard. Also, at high speed you'll be going between 3rd and 4th,
which is probably easier than going between 2nd and 3rd since it's on
the same shift gate.
My personal pick, assuming an SP Miata can be shifted quickly and
easily, would be the 6-speed with the 4.3 -- but then, I like rowing
through the gears. Someone who wanted to do as little shifting as
possible might go for the 5-speed and the 3.909, and use slightly
larger tires, thus staying in second except for the slowest corners.
If it turns out that top speed in a CSP Miata on fast courses (like
the Devens NT and the NEDivs) is below 67, I might change my mind on
that (or I might compromise with the 4.1 final drive).
Normally I wouldn't want to make gearing adjustments with tire size.
There are just too many benefits to a small tire...
Mark
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