lsd ratios and options

[gchristofi]

Having done a few track days in my 'shed', there are a few upgrades that are definitely on the short list . First up is an LSD.

My car is pre-facelift 1987 325.

1. What are my options and considerations for buying and fitting a limited slip diff?

2. How do I find out what ration my current open diff is?

3. How do you calculate the change in gearing if I were to opt for a different ratio on the replacement diff?

4. What's all this about case sizes? What will bolt straight on my car?

Cheers

[furbster]

Dude, not sure about any of those questions but I do have a 3.91 LSD for sale which I have just removed from my 1989 325i when I fitted an S50.

It was absolutely perfect on track and you will notice the difference between open and LSD straight away and wonder why you didn't change ages ago!

See my for sale thread if interested.

http://www.e30zone.net/modules.php?name ... c&t=176337

[oldroydsr4]

Agreed 3.91 on a 325 is a nice set up.

[gchristofi]

Hi furbster. Cheers for the pointer. Once I understand what it is I'm doing I may give you a shout on your lsd.

Anyone got any info or pointers to info about q's above? Would I get extra benefit from a 4.11 ratio?

[N00b]

Would I get extra benefit from a 4.11 ratio?

Let's say for the sake of argument, your car is currently running a 3.91:1 ratio. As the ratio increases (ie. 4,4.1,4.2 etc) your acceleration will improve and your top speed will drop. It will also mean that at a given speed with your current diff you will be revving higher with the lower (bigger number) diff.
I have a 325 with a 3.9 diff and as I do a lot of motorway miles I'm looking to fit something like a 3.73 as it means my revs will be lower for the same actual speed, hopefully improving the fuel economy.

[DanThe]

If you want to be faster on track then find a 4.1

[gchristofi]

Thanks guys. I have seen a few 4.1 for sale. Now I have a bit more to figure out. i.e. what is already in my car and what the gearing change will mean to speed in each gear.

for arguments sake, if I have 3.73 already, and go to 4.1, is it fair to say that speed in each gear at specified engine speed (i.e. 6500 rpm) will drop by (4.1-3.73)/3.73 = ~10% ??

So approx 10% decrease in gearing which should help acceleration by 10% (whatever that is in units?)

Anyone know what teh top speed in 5th would be and at what revs with a 4.1?

[Motorhole]

pm sent

[Rav335uk]

Try this site.

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

[DHFiS]

Hi Greg, have a look at this link for diff ratio and speed

http://www.juric.org/mopar/tools/topspeed.php

The Restoration Bible says

"Differential: 3.64 and 3.73 on metal bumper cars inc Sport LSD, later Sport uses 3.91"

And look for Ebay seller Geoaxiom on Ebay.com for a USA source of diffs who will ship to the UK.

He has a 3.73 for sale now for $229 (item 320583098884)

[gchristofi]

guys, thanks a bunch for all the helpful info, links and calculations.

[MillRat]

When you do source a suitable LSD (I personally prefer a 4:10 behind a relatively standard M20), I recommend tightening it up (I.e. increase the breakaway torque) from the factory 30ft/lb (~25% lock) to greater than 80ft/lb (or 60% lock).

When I run my 4.10 ratio LSD (with around the factory breakaway torque), I still spin the inside wheel around tighter corners. My 3.71 LSD however (which is far tighter), drives like a sledge hammer out of corners. The only downside, it acts a little more like a locked differential than the 4.10 diff (not a concern of me, however when I use to drive the car on the road, it did grind a bit when cornering).