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)?
As a hypothetical (honest!) example, let's take a '99+ Miata being prepped
for CSP.
The standard car has a 5-speed with the following ratios:
1st 3.136
2nd 1.888
3rd 1.330
4th 1.000
5th 0.814
final drive 4.30
The 10AE has a 6-speed with the following ratios:
1st 3.760
2nd 2.269
3rd 1.645
4th 1.257
5th 1.000
6th 0.843
final drive 3.909
The automatic car has a final drive of 4.10.
So, you could use the 6-speed tranny with a 4.30 final drive which would
give some very low overall ratios. You would likely have to shift to third
on most courses, especially if you use the 13" wheels which are popular in
CSP. But if you can shift well you could keep the car in its power band
more often. On the street, you'd be doing more shifting, and you'd be
turning a little higher rpm (I figure about 150 rpm higher) on the freeway.
Another option would be the 4.10 final drive with the 6-speed tranny. This
would give more torque multiplication for autox and the same highway economy
as the 5-speed car (actually a hair better). But you'd probably still need
third gear at times on a lot of autox courses. Would this be an advantage
or a disadvantage?
I know the best way to find out the answer is to experiment with the
different ratios and see what works. But I'm sure others have experimented
with similar options on some SP and Prepared cars and may have some insight
into how to figure out the optimum ratios for autox.
Brad Burns
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