Have found an LSD for my car, is it suitable?

[DUBGT]

Hi there all, hoping you can help. My daily/track 325i is starting to take shape with the chassis now sorted with Z3 rack and LSD next on the list.

My car is a 325i non-sport model so currently has an open diff. This isn't really an area of expertise for me so could do with a little advice. I know that the Sport models were fitted with a 3.91 mated to their different ratio Sport gearbox.

I am not planning to replace my gearbox but have been offered a 3.73 LSD for my car. Would anyone be able to help explain to me whether this would be suitable for my car when mated to the standard 325i gearbox bearing in mind my car is my daily ride and will most likely do 4-5 track days in the next 12 months.

Many thanks in advance,

James.

[UweM3]

if you have a 3.91 fitted a 3.73 will make your gearing LONGER. As higher the number as SHORTER as diff get's. For a track car you want to go shorter i.e 4.10
A shorter diff will rob you top speed but acceleration will improve. But I am not too familiar with the 325 ratios available.
There is usually a tab on the diff with the ratio stamped in. Back cover right hand side IIRC

[bramley]

I've got a 4.45 LSD in my 325. Used on the road and on track. Very short ratios! 4.10 would be good I reckon. Forget travelling on a motorway if you get a 4.45, if I had a long journey I'd probably stay below 70mph for the sake of the noise and revs. My top speed is now around 110-115 ish, but the acceleration is pretty good.

[Ziggy]

3.73 is perfect for a road 325i.

[MrBenn]

I've just put a 4.1 lsd in my track 325, standard engine and box, and my friend is running a 3.9 lsd (same setup), we did a track day at the weekend and there wasn't much difference except for a few corners he was in 2nd and I was having to use 3rd but getting a bit bogged down. I wouldn't want to go far on a motorway either as the revs @ 70mph are about 3500-4000. I'm going to try a 3.9 or 3.7 next. Depends on the tracks you drive and how important fuel economy/noise/comfort are.

[DUBGT]

Ok guys that's really helpful. I have spoken to a couple of friends who are more in the know about this than me also and think that I'll go for it. Given that the current project will be my road car for this year also the 3.73 seems to be a good level between standard and sport for me. It will a good upgrade from where I am but not too obtrusive when cruising on the motorway.

Thanks again,

James.

[GeoffBob]

Hi James,

I've already done this calculation for someone else to day, so I might as well do it for you while I still have the spreadsheet open.

With your Getrag-260 gearbox, the proposed 3.73:1 differential, and a pair of 215/40/16 rear tyres, your road speed at 6500RPM will be:

31mph in 1st gear,
54mph in 2nd gear,
84mph in 3rd gear,
118mph in 4th gear, and
146mph in 5th gear (if you have the power to make it here. Sorry, but I don't know what mods you have made to your engine. I suspect that you may find your top speed around 130mph at 5500rpm in 5th gear with this diff).

Most importantly, since this is a part-time track car, if you shift up a gear at 6500 rpm your revs will drop to:

3734RPM from 1st to 2nd,
4136RPM from 2nd to 3rd,
4643RPM from 3rd to 4th, and
5265RPM from 4th to 5th.

Alternatively, to land your revs on exactly 4500 rpm after a shift up you will need to rev to:

7900RPM in 1st gear (37mph, hmmmmm?),
7100RPM in 2nd gear (59mph),
6300RPM in 3rd gear (82mph), and
5550RPM in 4th gear (101mph).

At 70mph on the motorway in 5th gear your revs will be exactly 3100RPM.

I could be wrong but I suspect that, with this diff, this will amount to a 4-speed track car, with a 5th gear for off the track. If intended for use purely as a track car I would not recommend you use this diff, but you should still be able to have a lot of fun with it using 1st to 4th on the track, with a nice 5th for on the motorway. Note that, given that you are not running the most powerful engine amongst the E30's, you will likely see your acceleration times most notably compromised in 3rd and 4th gear compared to your old diff, but I don't think this should deter you.

Regards
Geoff

EDIT: Values have been updated to reflect the tyre size shown below.

[DUBGT]

Geoff many thanks for that, it's really helpful. Like you say with my car being used more on the road than track this diff seems to be suitable as for cruising I'll still maintain a quiet and relatively economical gearing.

My tyre size is 215/40/16 so won't be a million miles off what you have worked out. If it is easy to do it would be interesting to get the calculations for ths size but please don't put yourself out on my account as you have been most helpful already.

Regards,

James.

[GeoffBob]

Hi James,

I've updated the values shown above to reflect your 215/40/16 tyre size. This basically results in a slightly lower road speed at a given RPM due to the slightly smaller rolling circumference of the 40-profile tyre. At 70 mph your revs will be 3100 RPM in 5th gear, which sounds fairly comfortable to me.

Good luck with the installation.

[bramley]

At 85 I'm doing 4,250rpm in 5th

[Jon_Bmw]

It is an interesting issue this. You kind of need to match the diff to the characteristics of the car. Has anyone got a standard 325 graph to hand?

My 205, is pretty different to the power graph of a 325. It really comes on 'cam' from about 4900 and makes maximum power at the rev limit of 7100rpm(yes I know I ideally need to increase that to 7400rpm)

This is the rpm I get when changing into the next gear at 7100rpm. It is half the reason in is pretty relentless, it doesn't seem to get significantly slower, in terms of acceleration, as you go up the box.

1st-2nd
4033

2nd-3rd
5136

3rd-4th
5586

4th-5th
5773

It means that it has a useable 4 gears on track, you never use 1st, so that can be as long as you want really. The slight issue, is the car takes an absolute hammering never seeing below 5000rpm! But that is what it was built for.

I'm always inclined to fit the shortest FD possible, but thats the nature of a revvy 16v engine. It might not suit the 325i, although I suspect it would.

[GeoffBob]

At 85 I'm doing 4,250rpm in 5th

You're fast for your age