Which E30 LSD - How can you tell?

[Lloydie38]

Hi,

What is the best way to find out what ratio a LSD is?

I am going to look at a LSD on rear axel off a E30 325isport (19??) this Thursday (8/9/05). I understand that I need to be looking for a 3.9 or ideally a 4:1.

Cheers

Ian

[karlp]

best way to find ratio is to look on the tag (located on the rear of the diff. If no tag then more difficult but as follows.

Jack car up so that both wheels are off the ground then mark a point on the input shaft and the wheel. Count the number of turns on the input shaft that correspond to 1 complete turn of the wheel. Should be easy to accurately judge as the bolts are at every 0.25 of a turn.

If the mark on the input shaft reaches its point just before the wheel does one complete turn then it is 3.91 if the input shaft passes its point then 4.10.

Be careful to ensure it is not something like 3.7 etc but as the bolts are every 0.25 then a '3.75' diff will be easy to diagnose.


With regards choice of diff - depends on what the car is being used for.

On my track car which has a gearbox final ratio of 1:1 - I have a 3.91 (this was determined by loads of race preparation guys).

On you car the final drive is probably 0.88 with 4th gear being the 1:1 but your gearbox wil be more difficult to use as you will be continually crossing the box from 4th to 5th with a 4.10 diff.

Personally I would go for a 3.91 and on track be mainly in 3rd and 4th gear with occasionally into 5th.

Karl

[Lloydie38]

Cheers Karl for this. What can you do with the details on the tag? How can you check this out to find the spec of the LSD ? Is there a site you can log into?

I am going to be using it on a 1.5 mile track, the back straight I can get 120mph in my E36 328is, then there are a few bends mainly 3rd gear and one in 2nd so let's hope it's a 3.9

[karlp]

The tag will be easy to interpret.

If 3.91 will read S3.91 then xxx
If 4.10 will read S4.10 xxx

If you look at this site and put in the wheels/tyres you are using it will give you expected speed etc in each gear

http://www.unixnerd.demon.co.uk/revs.html

Karl

[Taffy]

3.91 should be fine for virtually all tracks, we run an M3 box and only have to change the diff to 3.64 for Snetterton & Thruxton, everywhere else is spot on with a 3.91. This is less of a problem if you are running a standard sport box as it's higher geared anyway

[astondg]

I have the standard gearbox and a 4.1 LSD and I never have to use 5th. I don't get to 120mph though, you have some fast tracks over there and some fast E30s too. The quickest I have been is about 100mph and that was in 4th. The guy I got my 4.1 LSD from raced an E30 with M3 engine, drivetrain (M3 1:1 5th gearbox), suspension, etc., and his differential selection was 4.1, 4.4something and I think a 4.5something. That was all he needed for the tracks here (Queensland).

Aston

[Julian_Jules]

I have the standard gearbox and a 4.1 LSD and I never have to use 5th. I don't get to 120mph though, you have some fast tracks over there and some fast E30s too. The quickest I have been is about 100mph and that was in 4th. The guy I got my 4.1 LSD from raced an E30 with M3 engine, drivetrain (M3 1:1 5th gearbox), suspension, etc., and his differential selection was 4.1, 4.4something and I think a 4.5something. That was all he needed for the tracks here (Queensland).

Aston

Aston,
You are missing seriously ur top end then.....mine's a 3.25 but have a dog leg ( no overdrive ) and can go abt 200kph...

On my local track, Sepang - I do get into 2nd gear which is fine & she pulls sweetly in all gears. ( up to 5th but I have two long straights here...)

I reckon u could be happier if u had a 3.73 with ur current gearbox. The M3 u refered to have a higher redline and much of the power is that end too....hence his choice of ratios. A 4.1 is very sort after here too but for DRIFTING action.

Cheers,
Jules
Jules

[karlp]

In the Uk I have been told that the only track that would benefit from a 4.10 is Brands Hatch, so as taffy says the choice of diff is dependent upon the track it is being used for BUT 3.91 is the number one choice (in the UK).

Karl

[Lloydie38]

Cheers guy's for all your advice. I'll see how it goes on Thursday and I'll report my findings...fingers crossed for a 3:9..

[astondg]

Aston,
You are missing seriously ur top end then.....mine's a 3.25 but have a dog leg ( no overdrive ) and can go abt 200kph...

On my local track, Sepang - I do get into 2nd gear which is fine & she pulls sweetly in all gears. ( up to 5th but I have two long straights here...)

I reckon u could be happier if u had a 3.73 with ur current gearbox. The M3 u refered to have a higher redline and much of the power is that end too....hence his choice of ratios. A 4.1 is very sort after here too but for DRIFTING action.

Cheers,
Jules
Jules

What I meant was the car can get to a higher speed than that but there are no tracks here with long enough straights. If I didn't have the 4.1 my car would accelerate much too slowly, it's already one of the slowest cars in a straight line everywhere I go. I raced with the standard ratio (not sure but either 3.25 or 3.5 or something LSD?) and with the 4.1 I was about 1 second a lap quicker so I think I'll stick with it. There aren't any top speed circuits here and as you know I also do autocross which is much tighter and top speed is about 100km/h so the 4.1 is better for that. With my gearing I think mine should be able to do 200km/h too, although I doubt it could. The way it is pulling at 150km/h it feels like it might get to about 170 to 180 max, it would take a long straight though.

The 3.25 would work great with the 1:1 5th though, I'd like that gearbox .

You are right about the M3s higher redline (about another 1000rpm), that would make a difference.

Aston

[Lloydie38]

The tag will be easy to interpret.

If 3.91 will read S3.91 then xxx
If 4.10 will read S4.10 xxx

If you look at this site and put in the wheels/tyres you are using it will give you expected speed etc in each gear

http://www.unixnerd.demon.co.uk/revs.html

Karl

I went last night and had a look at a rear axel of an alledged 325i sport. It had been left under cover but outside for a year so not in the tidyest condition. The tag wasn't on and I wasn't able to turn the wheels / discs. On the Diff there was the following code:
1 210 433

Any ideas guys??

[Slan]

The tag will be easy to interpret.

If 3.91 will read S3.91 then xxx
If 4.10 will read S4.10 xxx

If you look at this site and put in the wheels/tyres you are using it will give you expected speed etc in each gear

http://www.unixnerd.demon.co.uk/revs.html

Karl

I went last night and had a look at a rear axel of an alledged 325i sport. It had been left under cover but outside for a year so not in the tidyest condition. The tag wasn't on and I wasn't able to turn the wheels / discs. On the Diff there was the following code:
1 210 433

Any ideas guys??

I have the same numbers on mine and also wondering. 4.33 sounds really fucked up. But...

[Slan]

Found some numbers under alot of rust on the plate mounted with a bolt. Turned out to be a 3.46 LSD.

Also talked to my friend who brought the diff to Finland and he said it belonged to a 325i. If that could be any help for others.