diff ratio's
 

sorry about this guys/girls, i know you have covered this many times before but i just cant seem to work it out

i have a fireblade 919 indy and need to know what diff is in it the codes on the diff are as follows 838g-4030-6a) or sumit like, as few digits are hard to read.
another code on there was (004ba)
the car has a reg mark aswell and old log book to a 1990 g reg sierra saphire lx 1998cc if this helps.

any help would be great

TT is the best one to ask, if not take the back case off and divide the number teeth on the pinion by the number of teeth on the crown wheel and that gives you the ratio to :1.

i think that mite b the best way really at least i know what i definately have then. according to some lists iv found id say theres 75% chance its going to be a 3.36 open diff. if this is correct when i check, would i benefit from using a 3.62 or would it feel a bit slower off the lights? i dont want to change if its going to feel slower

cheers jim

The diff is more likely to be a 3.6 or perhaps a 3.9
The higher the number the lower the diff ratio.
3.6 will be faster accelerating than a 3.3
Top speed will be about 110 in 5th
4 to 1 diff = 4 prop rotations to 1 wheel rotation
3 to 1 diff = 3 prop rotations to 1 wheel rotation

just took the back casing off the diff and on the crown wheel is stamped from ford( 83bg-4210-sd) also it does have 3.62 stamped on it. so i take it, its a 3.62? if so what shud this top out mph. and it seems very clonky going into gears is this with all kit cars or do you reckon the diff could do with a refurb

cheers jim

Top speed will be about 110 but you will get ther quickly.
Clonky gear change is a bike engine/ box standard as the dogs engage, bike boxes can go up the box without dipping the clutch

Why take of the back cover?

Lock one output. Put a pointer on the input.

Turn the output 1 revolution and count the turns + degrees on the input.

By My Calcs.

3.33 = 3 rotations + 118d
3.62 = 3 rotations + 223d
3.92 = 3 rotations + 331d

So it should be fairly obvious at ~106d seperations.

TT

Wouldn't it be half of that due to the differential? In my understanding you either have to rotate both outputs 1 rev each, or lock one output and rotate the other one two times.

(Oh, by the way, here is a classic film about the pripciple of a differential, great if you need to explain it to somebody: http://www.youtube.com/watch?v=K4JhruinbWc . Skip to 2:00 if in a hurry)

I understand what you mean, you can leave one wheel on the ground or lightly apply the handbrake to prevent rotation of the free wheel.
This is not a problem with an LSD but can be an issue with the Quaife ATB

Sierra Diff
 

Hi,
A friendly man at the local ford dealer let me use his microfiche machine and I've managed to get all the part numbers, FINIS numbers, exploded diagrams etc for Sierra, Granada and Scorpios. I'm putting together everything I've found at www.super7thheaven.co.uk
Hope that helps - It's a work in progress