Choice of final-drive ratio

Details

Written by Malcolm Eades

With more power at your disposal you have the opportunity to re-arrange the final drive ratio to suit your requirements. You do not generally want to have to change out of first with a screaming engine two seconds after starting off, nor do you want a car that will not run in top gear at anything less than 60mph. There has to be a compromise and there is no single right answer.

Much depends on how you propose to use the car. However, for normal road use rather than racing or sprinting the usual choice is to make the Magnette higher-geared over all the gears (that is to say, there is less reduction between the number of engine revolutions and the wheel revolutions: a higher road speed is achieved at lower revs. It can be confusing because a higher gear has a lower ratio.).  A highly-geared car needs more torque to move off from a standing start but achieves a higher road speed in relation to engine revs. With a more powerful engine you have the torque needed to achieve this.

The use of overdrive also influences choice because it lowers the ratio of third and fourth gears  without affecting the lower gears used to get the car moving. The best of both worlds. Modern cars have been equipped with a fifth gear to overcome the same problem in a different way.

The Magnette’s first gear is generally reckoned to be too low, even as standard. Installing a higher-geared final drive with an overdrive set-up needs experimentation and one ratio lower at 4.3:1 is probably all that is required. Again, it depends on the power output delivered by your modified engine. The overdrive has no effect on first or second, so a lower ratio diff will extend the usefulness of first gear. However, it must not be so much lower that use of the overdrive in top is only practical at very high speeds. Choice of final drive ratio is about trial and error, using road tests in various conditions to see how the car performs. Fitting an overdrive may, alone, give you the desired result if high speed cruising at lower revs is your priority.

Before going much further, I need to explain some definitions. Firstly "differential", This is also referred to as the "pumpkin", presumably because of its generally round or gourd-like shape. It is the set of gears held together on a cradle that is fitted as a unit in the central housing in the rear axle. When a car turns a corner, the wheels on the same axle are turning at different speeds, so the "differential" allows them to do so whilst still applying drive to both.  swapping the diff normally means taking this whole unit out and putting another in its place. However, an alternative is just to replace the crown-wheel and pinion in your existing diff. The "crown wheel and pinion" is the pair of gear wheels that establish the drive ratio. The smaller one is attached to the propellor shaft; and the larger one to the axle half shafts, meaning that the axle shafts turn more slowly than the prop shaft. These components are mated for life and must only ever be changed as a pair. They use a style of curved teeth known as hypoid gears, designed to take heavy forces but run quietly. Accurate meshing of their teeth is essential. The differential has two other pairs of pinions that are part of the system that allows the wheels to move differentially. The half-shafts fit into one of these pairs using splines to transmit the rotary forces. In the Magnette and early cars, including the MGA 1500, only 10 square-ended splines are used, but later, for more strength, cars used 25 or 26 pointed-edge splines with a star-shaped cross-section. 

The rear axle fitted to the Magnette is a very standard BMC item and was used in numerous production models with ratios to suit. This offers the Magnette owner a wide choice, subject to availability of good condition parts. 

Parts from earlier cars are a straight swap and you simply follow the workshop manual procedure for removing the diff and, if required, dismantling setting and re-installing (though this is a complicated procedure that you may wish to leave to a specialist). In some cases the whole diff is a straight swap. In others, you have to rebuild the Magnette diff using the replacement crown-wheel and pinion. If you use parts from later cars there is a complication because, as described above, the differential used a modified half-shaft pinion, sometimes referred to as the “star wheel”. This is the small gear on each side of the diff into which the splined end of the half-shaft fits. In order to use the later diff you need to replace these with the pinions that fit Magnette half-shafts unless you can obtain a Magnette Mk IV axle which had star-ended half-shafts of the correct length.

The following list shows the various cars and the type of diff fitted.

CARS THAT CARRY SQUARE-SPLINED HALF SHAFTS: STRAIGHT SWAP DIFFS

Model                                                                                   Standard Diff Ratio

Z Magnette to car 18100                                                   4.875

Z Magnette from car 18101                                              4.55

MGA 1500                                                                            4.3

MGA 1600 Mk I                                                                    4.3

Morris Oxford Series 3                                                      4.875

Austin A40/A50                                                                   4.875

Wlosley 15/50 from 1958 on                                           4.875 or 5.125

Morris JU and J2 vans, Austin A55 van                         5.125

Morris Oxford Series 5 1959 to 1961                             4.55

Wolesley 15/60                                                                   4.55

Austin A55 & A55 Mk2                                                        4.55

MG Magnette MkIII (1959 – 1961)                                    4.3

Riley 4/68                                                                              4.3

CARS CARRYING DIFFS WITH STAR SPLINES: FIT MAGNETTE GEAR WHEELS

Morris Oxford Series 6                                                      4.3

Wolseley 16/60                                                                   4.3

Austin A60                                                                            4.3

Riley 4/72                                                                              4.3

MG Magnette Mk IV                                                              4.3

Austin A60 van and early Sherpa van                              4.55

MGA 1600 Mk2, MGB to 1965 (roadster only)                4.3, 4.1, 3.9 or 3.7

If you obtain a diff from an unknown source, the ratio is usually marked on the edge of the crown wheel with the number of teeth on the crown-wheel and pinion (e.g. 43/10). You can also count them. Dividing the larger number by the smaller gives the ratio. Be sure to check used parts for wear ( see the Magnette Workshop Manual for guidance). It may be cheaper to find a new crown wheel and pinion and have your diff set up by a specialist.

 Relevant Calculations

You may find it helpful  to calculate your over-all ratio as it stands, so as to approximate the desired new ratio. To do this you will need to calculate your over-all gear ratio in each gear to compare it with the standard Magnette. This will give you an indication of whether you need a higher or lower diff ratio. The following formula shows how to calculate your road speed per 1000 revs:

S = E x W x 60

Where:

S         is road speed ( see W below)

E         is engine speed in revolutions per minute

W        is the distance travelled by a fixed point on the circumference of the wheel during one revolution.

(If you want S in MPH you must use W as miles. For KPH use W as Kilometres. For a 165 R 15 wheel,W = .001239 miles or .001962 kilometres. For absolute accuracy, put a chalk mark on the wheel at ground level and a similar mark on the floor. Then, push the car forward until the mark is again at the bottom of the wheel, make another mark on the floor and measure the distance between the two floor marks. Normal tyre pressure and loading assumed).

G         is the gearbox output ratio, e.g the standard Magnette first gear is 3.64, top is 1.0

D         is the differential ratio e.g. the standard ZB figure is 4.55, the ZA 4.875

For those of you who are computer-literate and can use a spreadsheet it is a simple matter to put this formula into a chart that will do the calculation for you and enable you to change the variables.