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MG MGA - BANJO DIFFERENTIAL 3.9 4.1 ???

First and foremost I'm not too well versed on the technical aspects of gear ratios which will likely become obvious as you read on. Could someone explain the reationship of the 3.9 or 4.1 to the actual tire revolution. I would assume that one revolution of the drive shaft is converted to "x" revolutions of the axle thru the differential ?? Which ratio was standard from the factory and which is preferred assuming that one will give you lower RPMs at highway speeds and how would I check to know what I have at the present time in my 1960 MGA. Hopefully my question will make sense to someone ...... thanks !!
Michael Hosier

Both units are original. The 4.1 will allow you to have lower RPM's on the open road while the 4.3 will cause you to have higher RPM's on the open road. The 4.3 will be quicker off the line from the start and works better with a lower powered engine. There should be a chart in the archives on how many miles you will do per 1 thousand RPM.
JEFF BECKER

The ratio is the number of teeth on the diff's ring gear to those on the pinion gear. Earlier motors had higher ratios of teeth on the ring gear to the pinion gear.
In one revolution of the pinion gear, a ring gear with less teeth with move the car forward more. The engine and trans will run the same, but the car will travel further (faster) at the same rpm when using a smaller-ratio ring & pinion. When cruising at the same speed...the rpms will be lower.
Jon Bachelor

In your 60 A there should be A 4.11 Diff. It is a very popular conversion to get a diff from an early MGB and use the "pumpkin" (center houseing) with the gear. You will also have to change the spider gears as the splines to the axels are different from the MGB to the MGA. The early MGB diff used a 3.90 gear ration. This will give you lower RPMs at highway speed, however off of the line times will suffer as the gear ratio is lower. This is a common conversion.
Jim Pennoyer

To do your calculations use the Microsoft Excel spreadsheet at:

http://www.angelfire.com/amiga/mga/index.html


Mick
Mick Anderson

Jeff, the ratio of a rear axle gear set is usually shortened to 4.1 or 3.9, but in reality should be expressed as 4.1:1 or 3.9:1. The first number indicates the number of revolutions the pinion gear rotates in relation to one revolution of the axle. The gear reduction also multiplies the torque applied to the pinion, so a lower numerical ratio (3.9) will result in lower torque applied to the axle and correspondingly slower acceleration, but also give lower engine rpm at cruising speed. You can often determine the ratio by jacking the rear wheels off the ground and marking the inside of a tire with chalk. Then manually rotate the pinion and count the revolutions until the chalk mark makes one revolution.
Make sure that both wheels are turning, if one stops the measurment will not be accurate. The difference between a 4.1 and a 3.9 is fairly small, so watch closely and perhaps repeat a couple of times to check.
Bill Young

I'm working from memeory on this, but wasn't the 4.1 rear only standard on the Deluxe? I would have thought the '60 MGA would have come standard with the 4.3 diffy.

And I thought the splines issue was only a problem with the early (1500) MGA's.

I converted my 1959 1600 from 4.3 to 3.9 with a straight swap of the pumpkin.

Joe Cook

I think 4.1 was on the 1600 Mk2.

When working out diff ratios, only jack up one wheel and then divide by two. Then you don't have to worry about whether the wheels are turning at the same rate.
Dan Smithers

If you can read the numbers stamped in the edge of the casing divide the higher number by the lower number and you have the ratio, too. 10/39 = 3.9, 9/38 = 4,222 etc.
Willem van der Veer

Willem,

correct, assuming a previous owner hsn't already swapped the pumpkin!
Joe Cook

Joe, that's true! Maybe a quality swap should be finished by stamping in new numbers....
Willem van der Veer

This thread was discussed between 04/10/2004 and 07/10/2004

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