I think this question is course specific. Granted, I don't drive a Miata,
and my V6 has a little more torque, yet I've downshifted to first at least 4
times on one course here in the MS region.
IMO, ideally, you want the highest gear that you're going to use in the
autox to run out of revs just before fuel-cutoff at your highest on-course
speed. My PGT goes around 68mph in 2nd at 7500rpm, and I've never seen
that.
On the course last Saturday, I was able to stay in 1st gear from the start
until the first switchback, so that was good that I didn't hit the
rev-limiter coming up into the curves. Conversely, I've had to go through
cone in the box setups where if I stayed in 2nd gear, I'd be lugging out at
3k rpm which is too slow for me.
Also, you have to take into consider shift times... my 1-2 shift (with 80k
synchros) isn't as fast as when my car was new, so doing the 2-1-2 thing
sometimes might waste more time than it saves, in which case, having a
shorter 2nd and shorter 3rd would help
If it were me, for autox-only, I would go with the 6-speed and 4.30 for max
acceleration. But, since my car is also my daily driver, I probably
wouldn't. Currently, I'm pondering changing out my 5th gear to a taller
ratio for better gas mileage on the highway.
Brian
--
Brian Berryhill
http://www.angelfire.com/ms/brianberryhill/
----- Original Message -----
From: "Burns, James B." <James.Burns@jhuapl.edu>
To: "autox" <autox@autox.team.net>
Sent: Wednesday, August 30, 2000 4:03 PM
Subject: Gear Ratios Question
> 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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