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GEARBOX - Final Drives, standard
FD - Final Drive/diff ratio
FDs are ultimately responsible for the way your Mini goes after engine, gearbox, and under carriage tweaking has been applied. The aforementioned and the degree to which it has been done will affect the decision as to what FD is required. They’re also responsible for much discussion between many tuning freaks, and confusion to the less informed. There’s nothing weird or scientific about it. Maximum acceleration requires a low final drive, top speed a high one. And these two terms confuse most. The confusion being the LOWER number denotes a HIGHER gear. Likewise the higher number denotes a lower gear. Largely because the lower the ratio, the slower you go and vice versa. That’s how I remember it. So a 2.95 FD is a HIGH gear (high top speed, reduced acceleration), a 3.76 a LOW gear (low top speed, increased acceleration). That was too simple, so we need to add just a little ‘spice’ to it by introducing everybody's favourite term ‘COMPROMISE’. To establish the FD ratio, just divide the tooth count on the crown wheel (big gear in diff housing) divided by the pinion tooth count (little gear on end of mainshaft that engages the crown wheel).
Unless you specifically want maximum top speed or maximum acceleration you are going to have to make a compromise. The main influences are what you use your Mini for most, and what you find acceptable. Mostly driven in town with the odd foray onto a motorway, 3.44 gives a good general performance on smaller-bore engines, 3.105 being a better choice where higher output/torquier big-bore engines are concerned. When Motorways are the largest chunks of your habitat then a 2.76 is the way to go. Rover plumped for a 3.105 FD as it’s do all in the A+ engined cars, with the odd change for ‘Specials’ - like a 2.95 in the 998 E (Economy). It was Ok in the big-bore engined cars, but killed the small-bore ones. Since 1996 Minis have been endowed with a 2.76 FD in a last-ditch attempt at keeping noise levels way down, and better economy on long motorway runs. It has killed engine performance on the road though as the engines barely have sufficient mid-range power to pull it. Unfortunately there aren’t any hard and fast rules for selection, but there are a number of influencing factors. Below is a table depicting the standard, helical-cut ratios that have been made for the A series.
Final drive ratios and part numbers
Tooth count Part nos - A+ Part nos - Pre A+
- Although some gears have the same tooth count for different ratios, they are not interchangeable - hence the need for distinguishing part numbers. Therefore use in known pairs only.
- A+ gears have a different tooth profile to pre A+, so they are not interchangeable.
- A+ pinions can be identified as they have flat machined surfaces on either side, pre A+ ones have a shoulder on one side.
- A+ crown wheels have casting numbers stamped into them that are different to their actual part numbers, so refer to table.
- Pre A+ gearboxes, and A+ gearboxes having cast centre main bearing retainers will have to be modified when fitting 3.1/2.9/2.76 FD. This is because the pinions are much larger on outside diameter than the others. Some later sintered retainers may also need modifying. Always carefully check for clearance before finish-assembly of these parts. The modification is a simple filing out procedure.
There are, of course, a number of influencing factors over which is the best FD to fit to suit your particular application. We’ll ignore the obvious (like the one you’ve got/can easily get/was given free by a mate). The three most important to consider are engine performance envelope, wheel/tyre combination fitted, and - as previously mentioned - main vehicle usage.
An engine built with a specification akin to a circuit racer for blasting around twisty ‘B’ roads will not take kindly to a 2.95 FD. The fact that the camshaft doesn’t produce any power worth mentioning until the rev counter sees 3,000rpm will make it an absolute pig to get it moving from a standing start. Much clutch-slip would be necessary, putting excessive strain on it and associated components. Consequently they won’t last long at all. On the other hand, a 1380cc sports-tourer engine spec built for demolishing motorways isn’t going to do so with a 4.33 FD. You most definitely need to consider the useful working power-band your particular engine build is going to give.
The over-all size of the wheel and tyre combination used influences the FD performance. Ten-inch wheels with 165/70/10 tyres will do more revolutions in a mile than thirteen-inch wheels with 175/50/13 tyres. This increases acceleration as it lowers the over-all gearing. An interesting side effect is that it may also make the car faster at the top-end by allowing the engine to pull more revs to over-come drag. Fitting a high FD doesn't automatically guarantee maximum top speed - after-all, the latest 13-inch wheel-shod Sports-pack Minis are about 6mph slower at top speed, and 0.6seconds slower 0-60mph than the 12-inch wheel-shod cars!
Usage. This is where you really have to be either all or nothing, and damn the consequences, or be sensible. Difficult I know, but can make or break your love affair with your Mini. A low FD may make the car accelerate like a scolded cat around town and those twisty lanes, but the din created by a high revving engine/induction/exhaust will make your ears bleed at anything over 60 mph, and crucify your hard earned pay packet too! A very high FD may do wonders for the mpg when out on open roads, and make it lovely and quiet, but can be a real pain in town when you’re rowing the car along on the gear lever - and seriously adversely affect your fuel-economy.
Apart from the usual advice of talking to specialists and other like-minded folk at shows to try and assess which way to go - how can you do a self-assessment? The simplest way is to consider what Leyland/Rover have used over the years on their relevant models that appear to be what you’re trying to achieve. For instance, Rover fitted the 2.76 FD to the latest cars as it achieved a number of things - but mainly driver comfort. It provides reasonable economy and low engine rpm on motorways and therefore a sensible noise level, and the engine just about has sufficient torque/power to pull it. The Coopers and Ss were equipped with 3.65 or 3.44 FDs (along with a set of close-ratio gears) to allow spirited driving around twisty roads, where it excels. Noise levels were therefore compromised, and economy depended on frequency and depth of ‘pedal to the metal’ activity. Early 10-inch wheeled small-bore engines were given 3.76 FDs for nippy-ness about town and country. Get the picture? It's really only recent automotive fashion and customer expectation that has pushed them into the 2.76FD silly-ness.
For those wanting a little more scientific approach see the 'Formulae for car speed, etc.' for calculating mph per 1,000 engine rpm. And for those who really want to stretch their grey cells, the formula for calculating gear ratios and transmitted engine rpm are included too. Using this and a calculator you can compute the most satisfactory FD for your engine type/usage - guaranteed hours of fun!!