Limited Slip Differential - please help

Post Mon Sep 25, 2006 6:08 pm

Hi all,

Two questions really,

Just got a replacement diff for my 1990 325 touring. The diff I removed was a 3.91 (non lsd), the new diff label says S 3.84 (once I restored the tag). I assume the S means selective.

1. I've never come across a 3.84 or heard of one before. If my interpretation of the label is right, what will the ratio change in terms of roadspeed/engine revolutions? Sorry to be a bit thick

2. Also, what would a 3.84 LSD have come from originally?

Any suggestions please?

Post Mon Sep 25, 2006 6:09 pm

u sure it aint 3.64

Post Mon Sep 25, 2006 6:12 pm

u sure it aint 3.64

Well it could be but the bottom of the "6 or 8" looks like it forms more of the bottom of an 8 if that makes any sense?

What would a 3.64 mean as opposed to a 3.91 in terms of speed, am I going to be doing 80 mph and hitting the rev limiter

Post Mon Sep 25, 2006 6:14 pm

3.64 will give you more top end

if you were doing for example

80mph = 4k with 3.91
80mph = 3.5k with 3.64

if that makes sense

Post Mon Sep 25, 2006 6:16 pm

The lower the ratio the lower the engine revs for a given speed...

So your acceleration will be slower

but your economy will be better

Post Mon Sep 25, 2006 6:17 pm

80mph = 4k with 3.91
80mph = 3.5k with 3.64

Great, That's what I had hoped for really. Is it more likely that it is a 3.64 then? I suppose the acceleration will be affected though?

Post Mon Sep 25, 2006 6:19 pm

So your acceleration will be slower but your economy will be better

Thanks Mark & Maxfield, I guess it must be a 3.64 seeing as I, or no one else, more importantly has heard of a 3.84.

Would this have come off a sport?

Post Mon Sep 25, 2006 6:21 pm

The early sports had a 3.64 i believe

Post Mon Sep 25, 2006 6:23 pm

Without being an old woman about it

is the 3.64 ok, what would be the optimum choice of ratio?

Post Mon Sep 25, 2006 6:24 pm

Depends what you want really 3.64 is ideal for motorway driving. 3.91 isideal for acceleration

Post Mon Sep 25, 2006 6:25 pm

Bugger,

You can't have it all

Post Mon Sep 25, 2006 6:26 pm

old_skool_2002 wrote:Without being an old woman about it is the 3.64 ok, what would be the optimum choice of ratio?

pass on that one !

The sports had a close ratio box... so i don't really know how this will be on your car, it depends what you want to achieve, better performance or better cruising.

I'm sure someone will be along soon with a definitive answer

Post Mon Sep 25, 2006 6:29 pm

close ratio box

Hmmm....Guess I'm gonna need one of those too

Anyone got one in their shed then?

Thanks for the help guys, I suppose the 3.64 LSD is still better than the open 3.91

Post Mon Sep 25, 2006 6:31 pm

Post Mon Sep 25, 2006 6:32 pm

LSD's are the daddy of diffs! open-diff wrongness!!!

Post Mon Sep 25, 2006 6:46 pm

Glad about that

I could use the old 3.91 as a door stop

Post Mon Sep 25, 2006 7:36 pm

Sport close ratio boxs were only 'shorter' 1,2 & 3, 4 and 5 are the same as a std box IIRC.

Re the diff swap, it'll be fine, the actual real life difference between the two is very little, you'll have longer legs but enough torque to keep it moving along and a much higher top end, should you choose to find out

And remember to use the lsd for it's intended purpose, drive it sideways!

HTH, Mark.

Post Mon Sep 25, 2006 8:09 pm

drive it sideways

That, Bond, is the plan

Thanks Fozzy