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MG MGB Technical - Overdrive on MGB Automatic

Hi all,
I'm considering buying an MGB GT with an auto box, but have heard they are quite low geared so are not that good on a motorway.

Question is: has anyone attempted to fit an overdrive unit to the back of the auto gearbox? Is it possible?

Cheers,
Marcus
Marcus UPTON

Third gear on the auto box is 1:1, the same as fourth gear on a manual.

The automatics had a taller final drive ratio, so they are better on a motorway than a non-overdrive manual, but not quite as good as a manual with overdrive.

Fitting an overdrive unit to the auto box would not be a straightforward job.
Dave O'Neill 2

According to this, the final drive on the auto was same as on the manual (3.91:1) http://www.middlebank.co.uk/MGBspec.html
Geoff_MGB

The autos had 3.3:1. Same as early MGC.
Roadwarrior

Thanks for your replies everyone.
I've had a look in Clausager and it seems to match up with Dave's posting.

Clausager says the overall ratio in third for the 69-73 auto (which is the one I'm looking at) is 3.7:1, which is between direct fourth and o/d fourth on the manual. For the manual fourth is 3.909:1, o/d fourth is 3.205:1 (post 67 gearbox).

I'm living in Australia now where the freeway speed limits are 100kph (60mph) and very occasionally 110, which is lower than the UK 70mph limit, so maybe the higher top gearing will not show up as such an issue.
I may take the auto for a test drive on the freeway and see how revvy it is?
Cheers,
Marcus
Marcus UPTON

I have an auto. it cruises well at 60 (~3500 rpm but the counter reads high) but 70 just feels that the car is close to the comfort limit, however, the wind noise is one of the bigger contributors, the suspension whilst all new is still crude in design and so the gearing is only part of the story.

the engine revs well (i have just reconditioned it) and has enough power to make driving a lot of fun. to be honest, it goes better than some manuals i have been in with older engines.

it would be great to have an extra gear, but i tend to try and avoid long sessions on 'bigger' roads.

if you like driving autos (like I do, i did want an auto) then i don't think you will be as disappointed as a lot of folks make out. in the 70's, auto boxes were not very popular on *any* car of this level of power so i can imagine on a sports car like the MGB that not many people even TRIED them to see how they drove and hence the lack of popularity. The BW35 gearbox has a reputation for solidity and reliability and has been used on many marques.

try it and make your own mind up. it's the only way. I understand it may be possible to alter the diff to change the ratios and if you put enough horses under the hood then i imagine the car will cope admirably. i expect that would be easier than the overdrive. I don't know enough about the BW35 to know if internal mods will change ratios, but as like I said above, it was used on many cars of varying power levels so that may be a possible route to explore (or transplanting one from a volvo, stag, rover (?). It i were embarking on a world tour i may look into it further....

do keep the thread going as i would be interested in your quest should you buy one.
G
Graham Moore

The BW35 is also known for excessive converter slip and for reducing the performance of any smaller engine it is attached to significantly. Modern automatics experience far less power reduction.
Bill Spohn

One reason for the taller rear end gear ratio on the old autos was to compensate for torque converter slip. No lockup clutches like modern autos have. Also the converter stall speed had to be high enough to compensate for only 3 gears.

The BW was pretty much bullet proof if kept cool and built up correctly. I suspect hard parts may be difficult to come by these days.
Robert McCoy

This thread was discussed between 08/03/2015 and 12/04/2015

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