Diff ratio's and wheel sizes

Post Wed Nov 07, 2007 10:32 am

I am building an e30 rally car so i only really need a top end of 100mph,

What sort of top end would i be looking at with a 3.91 diff and 13" wheels with say, a 35 or 40 wall tyre?

Can you fit any higher ratio diffs to an e30?

Post Wed Nov 07, 2007 10:34 am

Sorry, the car is a 316 m10 with the standard box......

Post Wed Nov 07, 2007 10:35 am

I don't think 13" wheels fit, maybe on the back but not the front...

Post Wed Nov 07, 2007 10:41 am

oh, i have not researched this in full yet!

How about 14"?

Post Wed Nov 07, 2007 10:42 am

A number of different places I've seen on the net do much shorter final drive ratio for E30 diffs.

Post Wed Nov 07, 2007 10:58 am

Are they expensive?

Do they just fit into the same housing?

Post Wed Nov 07, 2007 11:07 am

There is small case and medium case diffs. The ring and pinion don't fit in either, so it would have to be both small cases you were swapping parts from and same with large case. You can change the large case diffs and small diffs between any E30, it's just a slight prop adjustment.

You can pick open diffs up on here for Ã‚£50 and LSD's for about Ã‚£300.

This might help you with what diff ratios are:

http://www.e30zone.net/modules.php?name ... engine.htm

Post Wed Nov 07, 2007 11:14 am

Thanks for that!

So my car already has a 3.91 open diff, if i wanted a higher ratio lsd that would fit in the same casing what can i do??

Does anyone know what the top speed of an m10 316 was?

Post Wed Nov 07, 2007 2:43 pm

4.45 CWP from a 1988 520i auto and an E30/E34 medium case LSD unit.

Post Wed Nov 07, 2007 3:12 pm

Thanks mate, any idea what sort of top end that will give me?

Post Wed Nov 07, 2007 3:37 pm

Trise30rally wrote:Thanks mate, any idea what sort of top end that will give me?

Gearing calculator.

Post Wed Nov 07, 2007 4:01 pm

Thanks for that!

Right then, so at 6000rpm (red line for a standard m10) your idea of a 4.45 diff ratio and a 5th gear ratio of 0.73 my top speed will be 115mph.... assuming the little engine has enough power to get the red line in 5th!

Does anyone have any idea what an m10 with 292 degree cam and twin dceo carbs will rev too???

Because i am guessing i will get atleast 7000rpm which will give me a top end of 135mph so really i need an even higher ratio diff than that......6.0 according to that site, is that even possible in a standard case??

Please let me know if i am missing anything as i am fairly new to all of this!

Post Wed Nov 07, 2007 5:08 pm

Anyhting lower than a 4.45 is Ã‚£Ã‚£Ã‚£Ã‚£Ã‚£.

Use an E36 gearbox with a direct 5th and save first for the trailer.

Post Wed Nov 07, 2007 5:26 pm

lol that sounds like going too far

Maybee i will stick to the 4.45 then..........

Will that ratio improve my exceleration allot?

Post Wed Nov 07, 2007 6:54 pm

wont be much quicker than a 4.10 diff as you will need to change gear more to hit a certain speed.

Post Wed Nov 07, 2007 7:47 pm

You can get a 4.45 diff from a 318i/316i Auto

Post Wed Nov 07, 2007 9:18 pm

maxfield wrote:You can get a 4.45 diff from a 318i/316i Auto

Small case, not as easy to uprate the LSD. (If you can find one.)

that sounds like going too far

I've been using a direct 5th 6-speed with 7.5K revs (640mm diameter tyres), I'm ditching the 6-speed because 125 MPH in 5th is enough. (Proved to be more than enough.

The gearbox swap is part of the re-shell.

)

Post Mon Nov 19, 2007 5:36 pm

What size diff casing does a 4.45 have?

Someone has one for sale but he says its a small casing.........

Post Mon Nov 19, 2007 7:17 pm

Tris, in your position I wouldn't bother playing with diff ratios; just stick to using 2-3-4 and forget 5th exists.
With the standard 4.1:1 diff and standard 195/65/14 tyres, 4th gear at 6,000 revs gives 104mph, just what you want.
14" BBS alloys are light, cheap and easily available; Colway do proven rallying tyres in 185/65/14 from Ã‚£25 each. Why pay more?
Oh, and weld the diff

Post Mon Nov 19, 2007 7:17 pm

Tris, in your position I wouldn't bother playing with diff ratios; just stick to using 2-3-4 and forget 5th exists.
With the standard 4.1:1 diff and standard 195/65/14 tyres, 4th gear at 6,000 revs gives 104mph, just what you want.
14" BBS alloys are light, cheap and easily available; Colway do proven rallying tyres in 185/65/14 from Ã‚£25 each. Why pay more?
Oh, and weld the diff

Post Tue Nov 20, 2007 9:56 pm

hi guys got a question ive got a 320i 89 plate and i think the bearings in the diff have gone ive also got a 316i 90 plate now the question is wot differance is the gear ratios from both cars and could i put the 316i diff in my 320i will it be quicker of the line but use more petrol or wot? i aint got alot of money so wanna do it cheaply as poss any advise would be apprciated thanks
ross

Post Wed Nov 21, 2007 3:50 am

What is the diff doing? Usual wear for a diff is on crown wheel and pinion (CWP) and will show up as a whining which is loudest at high speed and light/neutral throttle. If you have the rumbling noise normally associated with bearing wear, it's far more likely to be your wheel bearings - check for up/down and sideways play with the wheel off the ground.
You can swap in the 316 diff (which I think is a 4.45:1), but ChrisW has the original 4.1's for sale cheap. Personally I'd fit a 3.91 like the early 320i had as I think it suits the car better.
Oh, and post a question in 'Technical Help' next time, rather than hijacking someone else's thread