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
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