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Re: help me select gear ratios

To: DANMAS@aol.com
Subject: Re: help me select gear ratios
From: Zink <zink@pdq.net>
Date: Thu, 20 Nov 1997 23:33:28 -0600
Cc: mgb-v8@Autox.Team.Net, triumphs@Autox.Team.Net, mgs@Autox.Team.Net
References: <971120204135_948459431@mrin40.mail.aol.com>
DANMAS@aol.com wrote:
> 
> Fellow listers:
> 
> The time has come! I need to make a selection for the rear axle ratio
> in my
> TR6/V8. Maybe some of you hot shoes out there with some competition
> experience can give me some advice.
> 
> First of all, the car will not be used in competition (not much,
> anyway -
> maybe a little auto-cross once in a while), but I like to do a little
> spirited driving through the wonderful twisties we have here in east
> TN, plus
> an occasional round of stop light Gran Prix.
> 
> For the transmission, I have the choice of two first gear ratios, each
> with a
> corresponding choice of four fifth gear ratios, as follows:
> 
> 2.95 first, with choice of 0.59, 0.63, 0.73, or 0.80 fifth
> 
> 3.35 first, with choice of 0.68, 0.72, 0.83, or 0.92 fifth
> 
> For both choices, the 2nd, 3rd, and 4th gears ratios are 1.94, 1.34,
> and
> 1.00.
> 
> Rear axle ratio choices are - 2.50, 2.75, 3.00, 3.25, 3.50, 3.70,
> 3.89, 4.11,
> 4.30, 4.57, ......up to 6.50!
> 
> Tire diameter is 26 inches, horsepower is 300+, engine red-line is
> 6000 rpm,
> and the car weight is about 2500 pounds.
> 
> I have calculated speed and rpm curves for all of the above
> combinations, but
> I don't know what to use for selection criteria. There is virtually no
> cost
> difference between the options. The car now has a 3.34 first, with a
> 3.7 rear
> axle, and I am quite happy with that. I might be happier with another
> choice
> though.
> 
> The rpm -mph values, at the extremes, are 34 mph at 6000 rpm in first
> (3.35)
> with a 4.11, and 315 mph in fifth (0.63) with the 2.50 axle (315 is,
> of
> course, ridiculous, but it sure sounds good! For a more realistic
> value, use
> 110 mph at 2097 rpm.). Right now, I'm leaning towards a 3.89 rear,
> with a
> 2.95 1st, and a 0.73 5th. That's 40 mph at 6000 in 1st, and 3103 rpm
> at 80
> mph. I'm assuming I should pick a maximum desired mph in first, and a
> minimum
> desired rpm at highway cruising speeds in 5th - does that sound about
> right?
> and what should these values be?
> 
> HELP!!!
> 
> Thanks,
> 
> Dan Masters,
> Alcoa, TN
> 
> '71 TR6---------3000mile/year driver, fully restored
> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion
> - see:
>                     http://www.sky.net/~boballen/mg/Masters/
> '74 MGBGT---3000mile/year driver, original condition
> '68 MGBGT---organ donor for the '74
Dan, You build it with a 2.50/1 and I'll be the test pilot.

Larry Zink
1964 Spitfire4
Houston (hello,NASA, we have a request)

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