E30 Zone

Posted: **Sun Sep 25, 2005 3:25 am**

I thought I had a 4.1 LSD in my 323i but I have just been to a dyno where they plotted power against road speed in 4th gear. The guy that did it said he went to the limiter, which I just checked (I have standalone ems) is 6550rpm, and that is at 170km/h on the dyno sheet http://www.members.westnet.com.au/astondg/Dyno323i.jpg (blue line). My tyres are 205/50/16 and 4th is supposed to be 1:1 I think on the standard gearbox? Using this site: http://www.unixnerd.demon.co.uk/revs.html it says that to get to 170km/h at 6550 in 4th I would need about a 4.45 differential ratio. That doens't sound right to me, I didn't know BMW made one of those? I think they made a 4.4 or something but even that wouldn't be right.

Does any one have any ideas? Otherwise I guess I would have to take the differential off the car and seem how many turns and stuff? I don't really want to do that right now and I don't have any stands to put the car on.

Thanks.

EDIT: changed ratio from 4.59 to 4.45, 4.59 was with wrong sized tyres. oops.

Aston

Posted: **Sun Sep 25, 2005 8:39 am**

Sounds like he was using third gear - have you got a dogleg gearbox??

The dogleg unit has a direct drive top (5th) gear which is the same 1:1 ratio as fourth in a standard overdrive unit. In which case 4th gear would give you about 14 - 15 mph per 1000 rpm in fourth.

BMW made a 4.45 ratio diff for 316i - 318i - 320i Auto (plastic bumper).

Posted: **Sun Sep 25, 2005 8:51 am**

Aston.....
Using the calculator you gave the link for, I've just entered the figures for a 4.45 diff and come up with a speed of 169.6320km/h at 6550rpm.
Small engined Tourings (i.e not 325i) were fitted with 4.45 diffs if they had auto trans.
Which gearbox does your 323i have? If it has the dog-leg 'box, then 4th gear isn't direct, 5th is.
If you do have a dog-leg 'box, I don't know what ratio 4th is (I would guess that it'd be similar to 3rd in a normal 'box) but I'm sure someone here will know.
HTH
Stuart.

Posted: **Sun Sep 25, 2005 9:00 am**

Must have been doing the calcs as Andy was typing!
I've worked it out again for a 3.23 diff (323i/dogleg 'box standard fit), guessing that 4th would be the same ratio as 3rd in an overdrive 'box. I've come up with a figure of 166km/h at 6550rpm.
Stuart

Posted: **Sun Sep 25, 2005 9:21 am**

Sorry I should have said this before, I do not have the dogleg box. It is just the normal gearbox as far as I know, big gap 2nd to 3rd. This diff is also definately an LSD but the 4.45 from an auto wouldn't be an LSD would it? Did BMW make a 4.45 LSD? The differential was from a race car, did they have one of these for racing?

EDIT: 4.45 would also explain why it sits on about 3000 to 3100rpm at 100km/h, I thought my speedo was just wrong. Could it really be a 4.45 LSD?

Thanks for the help.

Aston

Posted: **Sun Sep 25, 2005 9:41 am**

I've just done another calculation, but bear in mind there are assumptions involved here.........
If the dyno operator used 3rd, rather than 4th as he told you, and your car has a 3.23 (standard 323i fit) diff, the figure the calculator comes up with is;
170.5865km/h at 6550rpm.
The only way of being certain if the diff doesn't have a (legible) tag on is to raise the car and count how many times the prop turns to turn a wheel once (but you already know that!).
As a matter of interest, what speed does your speedo give in 4th at 3000rpm??
Stuart.

Posted: **Sun Sep 25, 2005 9:42 am**

Must have been typing again while you were editing!! :lol:

Speedos do read fast!

Posted: **Sun Sep 25, 2005 9:55 am**

sorry that's the second time that's happened to you.

It isn't the standard diff 3.23 diff as you can see by my rpm at 62mph (100km/h) in 5th. I don't know about 3000rpm in 4th but I can check tomorrow. With a 4.1 to get 3000+rpm in 5th at 100km/h the speedo would have to be underreading, even with a 4.45 it would be reading correct (not overreading). Sounds like the whole thing is dodgy (diff, speedo, guy behind the wheel, etc.)

.

I'm not sure what's going on. Anyone know about a 4.45 LSD?

Aston

Posted: **Sun Sep 25, 2005 9:56 am**

Just checked on ETK and there is a 4.45 LSD listed. As far as I can make out, there is a slipper option on every ratio and both casing types.

It's looking like 4.45 slipper's favourite!!
HTH
Stuart

Posted: **Sun Sep 25, 2005 10:11 am**

with a 4.45 I think it should accelerate fairly quick? It looks like it is a 4.45 so know I have to find where it's hiding all the acceleration :lol:

.

Aston

Posted: **Sun Sep 25, 2005 12:21 pm**

Did BMW make a 4.45 LSD?

with a 4.45 I think it should accelerate fairly quick?

6th gear.......

Anyone know about a 4.45 LSD?

Some circuit racers use 4.45 LSDs for very twisty circuits, they have an 11 tooth pinion and 49 tooth crownwheel.

There's also a 4.44 which uses a 9 tooth pinion and a 40 tooth crownwheel, but afaik they're only available in small case.

The 4.45 diffs aren't as strong as higher geared diffs due to the small pinion.

Posted: **Sun Sep 25, 2005 1:04 pm**

I think I remember you mentioning that in another post now. Thanks for the info. My differential still has the standard 3.something big case and all the new bits were built into it so I can't tell by looking at it or the tag. Not as great if it is weaker, although I'm not putting big power through it and I stay away from hard launches and stuff, the only racing I do from a standing start is on gravel and I don't go for a super quick start so it's not that hard on the diff. I hope.

Is that an E36 M3? I think my 323i would need a big straight to get to 170km/h, and I don't think it would go much further.

Thanks again.

Aston

Posted: **Sun Sep 25, 2005 1:16 pm**

My differential still has the standard 3.something big case and all the new bits were built into it so I can't tell by looking at it or the tag.

It could have anything in, all the open or LSD large case crown wheel/pinion and LSD units are interchangable.

Not as great if it is weaker, although I'm not putting big power through it and I stay away from hard launches and stuff, the only racing I do from a standing start is on gravel and I don't go for a super quick start so it's not that hard on the diff. I hope.

They're not weak at all, I know of a guy using a 370 BHP M Coupe race car that uses a 4.45 for a couple of circuits. I've severly abused mine without any problems at all. (Standing starts/donuts in 2nd/3rd gear.)

Prolonged heat is the enemy for these diffs, but anything lower geared than a 3.9 isn't quite as strong as the others.

Is that an E36 M3?

3.2 Evo in a Compact.

I think my 323i would need a big straight to get to 170km/h, and I don't think it would go much further.

Mine was sat on a ramp with no wheels on when I took the pic, I was testing the speedo reading.

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Differential ratio problem

Differential ratio problem