Different Differentials

There are a number of ways of identifying a differential - either by counting the crownwheel teeth and dividing by the number of pinion teeth (eg c/wheel 35 pinion 9 = 35 / 9 = 3.89), or by counting the number of turns the front flange requires to turn both output flanges by 360 degrees (or one output flange by 720 degrees) - just more than 3.6 means 3.63, just more than 4 means 4.11.

By just looking at a differential, you can often narrow down the ratio. For convienience, we often split differentials into 4 types:

  • Type A Square front pinion flange. 5/16 bolt holes all round (1/2 AF spanner size). Small quarter shaft diameter inside. Casing no's G, Y, GA, FC to 120,000.
     
  • Type B Square front pinion flange. 5/16 bolt holes all round (1/2 AF spanner size). Big quarter shaft diameter inside. Casing no's GE, FC from 120,001.
     
  • Type C Square front pinion flange. 5/16 bolt holes on front flange (1/2 AF spanner size). 3/8 bolt holes on ourput flanges (9/16 AF spanner size). Casing no HB.
     
  • Type D Round front pinion flange. 3/8 bolt holes all round (9/16 AF spanner size). Casing no's FH, FK, FR, FD, HC, KC, KD.

Type A and B are indistinguishable from the outside, but Type A shouldn't be fitted to a 13/60 as the quarter shafts are liable to break. Type C (which was only used for the Vitesse 6 originally) is very prone to quarter shaft failiure.

Having decided which differential you have, you may now wish to alter the ratio. The following table gives rounded figures for miles/kilometersper 100 rpm.

RATIO

MPH/1000 IN 4TH GEAR

KPH/1000 IN 4TH GEAR

MPH/1000 IN OVERDRIVE TOP

KPH/1000 IN OVERDRIVE TOP

4.87513.5021.5017.0027.00
4.5514.5023.0018.0029.00
4.1116.0025.5020.0032.00
3.8917.0027.0021.0033.50
3.6318.0029.0022.5036.00
3.2720.0032.0025.0040.00

Bigger wheels, non-standard tyres etc, will affect the above figures.