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MG MGB Technical - Question for Steve S

Steve, I don't have your email address so I'm using the bbs, although I'm sure others will be interested.

After reading your article, I saw a tranny on ebay listed as 75-80??? There was a chance it was the 75-76 side-fill that you mentioned so I picked it up...but it turned out to be the 77-80 side fill.

You mentioned that the 75-76 is better for performance because of the close 1-2 ratio, well at least the one I bought is second best in that frame of reference.

Now for the questions:
1) You list the 62-67 final ratio as 1.101:1 Is that a typo? I can't seem to figure it out using the manual I have. Does the non-synchro have a lower 4th gear?

2) With the higher 1st gear, the 75-76 would have longer legs off the line, but wouldn't the non-synchro with a 3-4 drop of .2726 be the best for a track, especially one like Road America which is pretty fast? I did a little math and noticed that the mga twin cam optional close ratio gearbox has the same 3-4 drop of .27.

Lots to sit down and figure out with the trannies. I never paid much attention to them until I saw you article.

Thanks again

Fred H

Fred-
1) No, it's not a typographical error. Yes. it is slightly lower than the 4th gear ratio of the 4-synchro gearbox.
2) Gear ratios have to be tailored to the track on which you're racing. However, many racers prefer the earlier 3-synchro gearbox because of its lower weight and the wider range of gear ratios available. The most desirable of these is the March 1967 and later gearboxes because they have a larger diameter (.668") layshaft and beefier bearings which enable them to absorb heavy abuse better than the earlier smaller diameter (.643") layshaft. This heavier duty layshaft was originally introduced as a competition part by the racing department. The non-synchro 1st gear is largely irrelevant on a race track.
Steve S.

All this time, I thought 4th was direct (1 to 1) in all MGA/B transmissions.
Leland Bradley

The 1.101 gear ratio is a real bombshell. This could be the reason no one can agree on a number of mph/1000rpms, even when taking tire size and rear end ratio into account.

The manual I have,(autobook which has been pretty helpful), calls out a 1:1 ration for the early non-synchro. By my calculations - with a tire height of 23.5" and 3.909 diff., a 1:1 4th gear gives 17.9mph/1000rpm and the 1.1 4th gear gives 16.27mph/1000rpm. Quite a diff.(get it?)

Steve, is this gearbox different from the MGA, since those are non-synchro and are also listed as 1:1 4th gear ratio?

I must admit that I don't have an MGB Bently manual, but I should be getting one in the mail any day now.

Fred H

Fred-
I took a peek in my Bentley manual and noticed that the listed fourth gear ratio is 3.909:1. Since that's the differential ratio, the only way that could be possible would be for the power to be coming through with no gear ratio reduction at all, hence a 1:1 fourth gear ratio! It also says that speed in 4th gear is 17.9 mph/1000 rpm. Oops! Looks like my source is in error! Thanks for helping me catch it!
Steve S.

Perhaps if you did a little engineering yourself and left the books alone, your some what poor advice might improve.
very concerned

This thread was discussed between 18/10/2003 and 19/10/2003

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