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I have received a number of questions regarding gear boxes and speeds. I
am sure that a great many of you have already done your own evaluation
of the possible permutations, and chosen one best suited to your needs.
For those that may yet have some questions, I am forwarding this rather
long set of comments recently posted to Chris Pirouette. Sorry, Chris,
for the repeat.
Steve
--
Steve Laifman < One first kiss, >
B9472289 < one first love, and >
< one first win, is all >
< you get in this life. >
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Message-ID: <347021AB.5349@flash.net>
Date: Mon, 17 Nov 1997 11:51:31 +0100
From: Steve Laifman <laifman@flash.net>
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To: PirouetteT@aol.com
Subject: You asked for it!
References: <971117062648_2105130138@mrin86.mail.aol.com>
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Chris PirouetteT@aol.com asked for my earlier memo on 5 spd OD
transmission comparisons. He just forwarded the following question:
>
> I am curious, however, how the 5 spd. in its first four gears (up to 1:1)
>would relate to the wide ratio 4
> spd. as regards your comments on road race / stop light take-off advantages.
>
Chris
STOP HERE if you already know how to calculate the results, or don't
care, as it's a little long.
Well Chris,
You remember my story on "doing the math" vs "seat of the pants"? Get
ready for "doing the math!" I've done this study for the cases of
"close-ratio" and "wide-ratio" top loaders on Tiger's with 2.88 rear
ends and 185-70/13 tires, as well as what Ford did on the 5 spd Mustang
GT and the case "Stock" Tiger wit the original 590-13 tires (If anyone
still runs them, I can't imagine him changing to anything not original,
but it is here for comparison. I'll give you the m.p.h/1000 rpm for each
case. "It is left to the student" to multiply these numbers by the rpm's
of interest to get the speed vs rpm in each gear. For those who want
automobile formula's for fun and profit, try URL:
<http://www-sci.lib.uci.edu/~martindale/RefCalculators1.html#AUTO>
1) Stock 260 MkI, Mk IA configuration: 2.88:1 rear-end, 4.5 inch J x 13
wheels with 5.90 x 13 Dunlop 'RS5' Tires:
Miles per hour per 1000 rpm.
HEH-F Stock 260 "close ratio"
First: 10.31 Second: 14.17 Third: 18.56 Fourth: 23.92
HEH-B Stock 289 MkII (same tires and rear end)
First: 8.61 Second: 12.41 Third: 17.36 Fourth: 23.92
These are directly from Mike Taylor, "Tiger, Making of a Sports Car",
page 223.
Multiply the rpm (divided by 1000) of interest by the numbers above.
I'll assume that acceleration is your goal, and that your motor has
sufficient pull at 6000 rpm shift points. The entire calculation can be
made, in the same way, for other shift rpm's. For example: Shifting at
6000 rpm, you'd be going 62 mph in the close ratio first, vs. 52 mph in
the wide ratio. A shift to second, assuming you don't take so long as to
slow down your speed, you would be at (62/14.17) x 1000 = 4375 rpm for
the close ratio, and (52/12.41) x 1000 = 4190 rpm in the MkII wide
ratio. A difference of 185 rpm. Not really significant, considering the
added rear wheel torque would be 16.4% greater in first. This is why
MkII's burn so much rubber (along with the 289, of course). The story in
second is (again at 6000 rpm shift point) is 85 mph, going into third in
the close ratio and 74.5 mph in the wide ratio, with a drop to 4579 rpm
in the close ratio, and 4291 in the wide ratio. The difference is 287
rpm. For the final shift, it's 111.4 in the MkI, and 104.2 in the MkII.
The rpm drops to 4657 in the MkI shift to fourth, and 4356 rpm in the
wide ratio, a difference of 301 rpm more drop.
You can calculate the speed vs. rpm for each gear and plot it (say at
1000 rpm increments). These will be straight lines, if you did it right.
Then, you can pick your own rpm shift limit, read the speed in that
gear, and drop to the next gear along the same mph line. Then you'll get
the rpm after the shift (constant mph during shift). Don't complicate
this with "power shifts", "clutch burning", etc. If you do it right,
there will be no change in the results from anything but air and rolling
friction slow-down resulting from how long it took you to shift. It is
safe to assume you wouldn't shift any faster with a different set of
gears. Clutch slippage and tire spinning are just energy wasters, and
should be avoided, by the good driver, by letting the clutch out at the
rpm the gears want it to be (see your chart).
>From this simple comparison, with all other things being equal, I don't
think an additional drop of a 200 rpm, or so, would be significant with
the large engines we have, as the torque cure is pretty flat there. The
horsepower available is the rpm times the torque at that rpm (plus
multiplication by a few constants). The flatter the torque curve, the
more directly the available horsepower relates to the rpm difference. In
the given example, assuming flat torque over the 300 rpm range, the
horsepower drop would be the rpm drop percentage, or 287/4291 = 6.7%.
This is assuming the same engine in both cases, and does not allow for
the higher 289 torque, or a modified 260. Shifting at a lower rpm lowers
the % crop as well.
An example of the influence of tire diameter can be seen from the same
MkI, using 185 x 70R13's. I believe the "diameter" is 23.78 inches, vs.
stock, calculates First = 10.59 mph/1000rpm, Second = 14.54; Third =
19.04; and 4th = 24.56.
The "average" rpm drop with the "close ratio" is about 1200 rpm drop at
5,000 rpm. The "wide ratio" is about 1443 rpm drop between gears. The
exact value can be calculated from the above examples. The "spacing"
between the gears is pretty consistent for the "wide" ratio, compared to
other transmissions which tend to make you think you've skipped a gear.
On the subject of 5 spd OD, the comparisons I made were for my '66
Mustang, so the tires were P195/70-14's and the 2.78 rear gears I had,
so you'd have to adjust for the Tiger. The results for the T-5, in mph
per 1000 rpm, were:
First: 8.07 Second: 12.27 Third: 17.77 Fourth: 23.81 Fifth: 37.79 (!)
Speed in gear at 6000 rpm:
First: 48.4 Second: 73.6 Third: 106.6 Fourth: 143 Fifth:
227 (!)
As you can see, you'd never get to 6000 in 4th with anything I'd drive
on the street.
_________________________________________________________
The Tremec gave:
First: 43.7 Second: 72.15 Third: 106.6 Fourth: 143 Fifth:
210 (!)
__________________________________________________
AS Ford geared their 1996 Mustang GT, 16 wheels with tire diameter of
24.4 inches, and 6000 rpm:
First: 47.3 Second: 80.2 Third: 120 Fourth: 160 Fifth:
238 (!!!)
At a more reasonable 5,000 rpm (never seen anyone drive one like this)
First: 39.5 Second: 66.8 Third: 100 Fourth: 133 Fifth:
198 (!!!)
At 70 mph, with this set-up, you'd be cranking only 1760 rpm. As I've
discussed previously, you can only cruise at this speed, not accelerate
appreciably, and would significantly "lug" the engine. At 60 it's even
worse.
I hope this answers your questions, Chris. If you want the formulas, see
the referenced URL, or reply to me and I'll write a "how-to" book for
you. If you have an Exel Spreadsheet, I could zap you the calcs, and
graphs.
My recommendations:
If you drive around town, and want some zip from a dead stop to get up
that short on-ramp, the "wide-ratio" top loader is a good choice. If you
are road-racing, the "close ratio" is better because you only see that
long first gear once. For auto-cross ask Ramon. If you've got lots of
money, I'd pick the Tremec with the "racing option" 0.8 overdrive they
are rumored to offer for a small fortune. The T-5 has a reliability
problem with high-torque motors, as it's really only good for about 300
ft-lbs of torque, and the "World Class" version is only a little better.
See piles of them at the repair shops catering to lead foots. The Tremec
is strong, and their T-3550 is even stronger. But non of them can hold a
candle to our top-loader, which is used in 427 Cobra's (with some few
differences). Tremec bought out the T-5 rights and produces both
transmissions now. Both take some tunnel work to replace the top-loader,
and some adapters for the bell-housing as well as the 5-6 bolt problems.
I know some will argue with that, but they've probably lowered the rear
motor mount. This will increase the drive-shaft angle, and can cause
some serious reliability problems. As I recall, 3 degrees is the MAXIMUM
angle you want to put this thru, including suspension travel. Our shaft
is so short already, that the same movement at the axle is significantly
more angle than a Mustang. Dropping the rear mount just makes it more
critical.
Steve
--
Steve Laifman < One first kiss, >
B9472289 < one first love, and >
< one first win, is all >
< you get in this life. >
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