Final drive stuff

Part numbers

A+ FINAL DRIVES

A SERIES FINAL DRIVES

NOTE:
Crownwheel and pinion sets are matched . A+ and A series ARE NOT interchangeable. And within a group you CANNOT change partners (unless in fact they are the same part e.g. pinion for 4.1, 4.2 and 4.3 diff a-series)

Preferred differentials for best of both worlds would most likely be...

1275cc : 10" wheels 3.44, 12" wheels 3.7, 13" wheels possibly a 3.9

As a rule go one diff ratio up (larger ratio, lower gearing) when going from 10 to 12 inch wheels, so state in the table what wheels/tyres you have chosen.

And don't forget that I think many people think a car is quicker because it revs faster i.e. it sounds faster. In actual fact a 4.3 goes through the gears fast but the max speed in each gear is low. A lower diff e.g. 3.44 will rev slower but the max in each gear is higher.

And for each speed on the road each car will just be at different revs and/or gear. Most noticeable factors are A) pulling away from standstill and B) top speed For speeds in between you should be able to find a gear ratio for max torque to accelerate.

Pinion replacement

If you have the diff housing off already, it is easy to stick the gearbox into two gears simultaneously which locks up the final drive.. BE CAREFUL NOT TO PUSH THE SELECTORS TOO FAR! The detent springs may pop out and then you are screwed!

That's how you are meant to do it anyway! (I think)

Ratios

Ratios vs. performance

This subject always causes plenty of discussion amongst hardened mini tuners! There are plenty of final drive ratios to choose from, but it is important that you choose the correct ratio for your own personal driving style.

The final drive ratio (ratio of differential crown gear to its drive pinion) determines the performance characteristics of a car, and denotes how the engine's power is geared to torque at the wheels.

Confusion begins when people talk about 'high' or 'long' final drives. They are refering to a 'high geared' diff, which will have a smaller ratio and a lower first number (eg: 2.95:1). This FD will give less acceleration because the 'mph per rpm' is greater. The torque at the wheels will be less, and the revs at cruising speeds will be lower. This is more suited to a driver who does lots of motorway cruising at ~80mph, and will often get better mpg as the engine will not rev so much.

If on the other hand you want fast acceleration and are not worried about cruising at elevated revs, then a 'lower' diff ratio would be more suitable. This would have a higher ratio/number, but would be lower geared (eg: 3.76:1). This ratio would give very fast 0-60 times, but top speed would be limited by the fact that the engine would be revving high when cruising at high speed. Good for twisty roads, but not too good for motorways.

Obviously, a very powerful engine could accelerate a high diff, and if you are willing/able to rev high then top speed will still be high (racing minis use ~4.3:1 diffs but still reach 120mph at >8krpm!)

So which diff should you go for?

Here is a table which will hopefully aid your choice:

As a rough guide:

Choose a diff which gives your usual top cruising speed at your usual top cruising revs. If you drive at 80mph on motorways and don't like pulling more than 4krpm, then a 3.2:1 would be best for you.

If you want economy and do alot of motorway driving, use a 2.95:1
If you want sporty performance, but still ok for motorway use 3.44:1
If you don't use motorways much and you want great 0-60 use a 3.76:1

Ratios vs. tyres

See also Speedometer Calibration

Engine speed @ 60 MPH in 4th gear (1:1 in the gearbox, no dropgear reduction)

Mph (kph) per 1000 RPM in 4th gear, no dropgear reduction

(TPM = Turns Per Mile)

Therefore, if you have a 3.44 diff and you want bigger tyres (for that sporty look) you'll have to slip in a 4.13 diff and 185 or 195x13 tyres so you will have the same overall gearing. To the fellow who slipped in the Austin America lump without changing the diff, you can see from above why the engine screams all the time. Change your diff or buy a gazillion $ worth of wheels and tyres.

Looking at the above table, I suppose you could draw a couple of bands through the data which show cut-offs for acceptable performance based on usable HP of the engine. Any plonker engine could turn ratios in the top left corner... even a pokey 850 could accelerate nicely, but over on the RH side of the table it takes a lot of HP to turn those ratios.

15 06 2003