KNOW THE DIFFerence

by Ray English

Published in Goblins’ Gazette, Vol VIII, No 4, July 1977

Back in the early years of the Sprite’s competition life, most drivers competing in Club events, and to a lesser extent circuit racing, drove their cars to the event, competed and the drove home. Most newcomers to the sport still do this, but the trend these days is to trailer your Sprite to and from competition events. The advantage of using a trailer is that you are sure of getting home if you manage to hang it on a fence, or rods put a leg out of bed; plus the car can be setup the weekend before the event and all you have to do is roll it off the trailer and into the pits – sounds easy doesn’t it!

When you drive your Sprite to and from the event, especially if it is a long trip (200 miles each way is not uncommon in Australia), you will more than likely select a diff ratio to suit the trip rather than the competition event, and thus the car will not be as competitive as that of your opposition who had their car on a trailer, fitted with the appropriate diff. There are also other advantages – if the car is only used for competition, then registration is not needed – a saving that would buy a reasonable trailer. Anyway, back to the preparation of the car, ratio wise. If you have driven on the circuit before, you will have a fair idea of the ratio required. If not, then ask other drivers in the Club for their opinion.

Right you say, as pick-up out of corners is more important than top speed on this circuit, what we need is a 4.875 (commonly known as a 4.9). Only one problem – where do I find a diff? Well, read on and you will see listed all the ratios that will fit your Sprite and the cars that use them as standard equipment. This does not mean, for example, that if a 4.555 is standard on a Morris Minor 1000 it will have one fitted, as over the years other ratios have found their way into these cars. The most common reason for this being that the local wrecker says, “It’s a beauty mate, fits like a dream”, forgetting to tell the customer that it’s a 3.7 and his 803cc A30 will take about half an hour to get into top gear, and then you can’t stop it. So be prepared for some disappointments when diff hunting. If removing a diff from an A30, remember that the half shafts also fit your Sprite (disc wheels only).

We Sprite drivers are very lucky in the ratio department, as no fewer than seven diffs can be used, catering for just about every need from a very steep, twisty hillclimb, to a long straight circuit – all these ratios are listed with relevant information.

Ratio Standard Equipment Remarks
3.727 Riley 1.5; Morris Major & Austin Lancer Series 1 only
3.900 MG Midget Most 1275cc cars
4.222 Mk 1, 2 & 3 Sprites
4.555 Morris Minor 1000
4.875 Austin A30 / Morris Minor Ute and Van
5.125 Austin A30 Alternative to the 4.875
5.375 Austin A30 / Morris Minor Ute Usually found in 803cc models
Ratios No. of teeth RPM / Mph
3.727 11/41 1000/17.8 4000/71.2 5000/89 6000/106.8 7000/124.6
3.900 10/39 1000/17.1 4000/68.4 5000/85.5 6000/102.6 7000/119.7 7500/128.2
4.222 9/38 1000/15.5 4000/62 5000/77.5 6000/93 7000/108.5 7500/116.2 8000/124
4.555 9/41 1000/14.4 4000/57.6 5000/72 6000/86.4 7000/100.8 7500/108 8000/115.2
4.875 8/39 1000/13.3 4000/53.2 5000/66.5 6000/79.8 7000/93.1 7500/99.8 8000/106.4
5.125 8/41 1000/12.6 4000/50.4 5000/63 6000/75.6 7000/88.2 7500/94.5 8000/100.8
5.375 8/43 1000/11.9 4000/47.6 5000/59 6000/71.4 7000/83.3 7500/89.2 8000/95.2

In some cases you may find that the number of teeth are not stamped on the crown wheel; in such cases, count the number of teeth on both the crown wheel and the pinion. Divide the pinion into the crown wheel and that’s that ratio e.g. 38 divided by 9 = 4.222.

One final thing – don’t forget which diff you have in the car! It’s a lot of work removing it just to have a look. It also pays to write down the ratio you used at various circuits so that next time you can at least have an idea of what to expect.

Additional Note 2010

The above chart is based on a 145/80 x 13 tyre. If you wish to calculate your speed in Km/h per 1000 revs when using modern low profile tyres, you can use the following formula:

26.4R/7 (ratio) where R is the rolling radius of your tyre in cm.

2018-07-20T09:38:13+00:00July 20th, 2018|TECHNICAL INFO|