Hi Bill,
Interestingly, the rear axle was changed from spiral bevel to hypoid some 6,500 cars before the transmission was changed to four speed with the BN2. BUT they didn't reinstate first gear in the early gearbox at that time.
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Quote, Special Interest Autos, Feb. 1982 interview with DMH:
"How come the first Austin-Healey used a three speed gearbox instead of a four speed?"
Quote, Donald Healey:
"It's no secret now. Austin used that gearbox in a car where they didn't take anything like the power out of that engine that we did. A car called the A90. It was a four speed box, but we couldn't get that bottom gear to stand acceleration starts with the power we were delivering. We used to tear them up. So Geoff (son Geoffery Healey) said, "Shall we lock it out & put an overdrive in it?"
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It's well known that the torque applied to the transmission internal gears is proportional to the gear ratios. It follows that blocking out the original lower first gear would result in considerably less strain on the transmission.
The reasons may be forever lost in the mists of time. I think it's generally agreed that first gear in the original transmissions, if reinstated, was too slow to be of much use except in creeping parades, & the factory DID have to replace a lot of the three speed transmissions under warrenty. True the BN2 transmission isn't exactly a jewel, but it's strong enough to withstand the power of the later cars without trouble.
I once used a BN2 transmission & axle behind a Chevy V8 conversion. They handled it well.
D
PS - To illustrate the torque vs gear ratio difference:
The only difference between these two BW T10 transmissions is the gear ratios. Note the increased torque input rating of the lower multiplication ratios, 375 lb-ft, vs the 325 lb-ft of the higher multiplication ratios.
Another Quote:
"There were two different ratio sets used in the Warner Super T10.
The standard Wide Ratio 4-speed on the L48 and L82 had ratios of 2.64, 1.75, 1.34 and 1.00 to 1. It was rated at 325 lb-ft of input torque.
The optional on the L82 only Close Ratio 4-speed had ratios of 2.43, 1.61, 1.23 and 1.00 to 1, and had a torque rating of 375 lb-ft."
D