3.46 or lower medium case diff

[e30ftw 402]

Looking for a 3.46 medium case or lower ratio e30 diff. Output flanges / rear cover or mount etc don't bother me, mainly after the gears and housing to swap my LSD center over to it.

The 3.64 is driving me nuts, 3300rpm @ 100kmh..

Also a 325i full driveshaft if anyone has one liying around.

[*Glenn* 854]

Is that ratio really the ratio you want ? Calculating a diff ratio you have to take into consideration:

What is the car being used for... road or track

Transmission output ratio ie: 1:1 or overdrive = normally .8

What RPM is the engine power band

Achieveable top speed (calculated)

Power to weight ratio of the vehicle

Engine torque

Wheel & tyre sizes (rolling circumference)

These are some calcs I did using 225/45/17 (634.3mm) for my examples. The RPM will be different dependant on tyres & wheel size. I can change these calcs if I knew your tyre size. These examples I did with my tyre size.

3.64:1 @ 100Kph = 3,050 RPM

3.46:1 @ 100Kph = 2,900 RPM

3.15:1 @ 100Kph = 2,650 RPM

2.97:1 @ 100Kph = 2,500 RPM

To have 3,300 RPM @100KPH you must be running smaller tyres/wheels or your rev counter or speedo is out

My car with 1:1 trans output, 3.15:1 diff and 225/45/17 tyres is reading 2,700RPM @ 100kph. In hinesight I could of gone to a 2.97:1 with the torque produced by the M52 B28 and the power to weight ratio. I don't really need to use 1st gear.

Edited May 9, 2010 by *Glenn*

[BM WORLD 1270]

have a few options .

021433600

092991234

thanks brent

[CamB 48]

To have 3,300 RPM @100KPH you must be running smaller tyres/wheels...

...225/45/17 tyres

I hope he's running smaller diameter than that on an E30!

John - I agree with Glenn - you need to consider what you want and a 3.46 doesn't sound like it. It will only drop you to 3100ish rpm. What's the goal? Low rpm/fuel economy or acceleration/performance?

[e30ftw 402]

205/40 16's

Just after something lower.. Have PM'd you Brent.

[*Glenn* 854]

With 205/40/16

3.64:1 @ 100Kph = 3,380 RPM

3.46:1 @ 100Kph = 3,200RPM

3.15:1 @ 100Kph = 2,930 RPM

2.97:1 @ 100Kph = 2,800 RPM

[e30ftw 402]

What calc are you using?

[*Glenn* 854]

Ooops I was doing my calcs with gearbox output at 1:1 your 240 or 260 should be .81:1

3.64:1 @ 100Kph = 2,742RPM

3.46:1 @ 100Kph = 2,600RPM

3.15:1 @ 100Kph = 2,375 RPM

2.97:1 @ 100Kph = 2,240 RPM

Check your 5th gear output ratio.... are you sure your diff is a 3.64:1 ? 3,300 Rpm @ 100kph seems too high, unless the rev counter is out.

I don't know what your gearbox output ratio is if it is 1:1 the previous rev ranges are correct

[e30ftw 402]

Yes its a 3.64 diff and it's an e36 getrag 220 gearbox so 1:1 5th gear.

Just curious on all gear speeds but I'll have a looksy round.

The 3.64 with the getrag 240 .81:1 was pretty good, maybe a lil too long but something similar (slightly higher) in comparison with the 1:1 box im using now and whatever diff I can get.

[_ethrty-Andy_ 2125]

hes using a 220 with a 5th gear of 1:1 iirc?

[*Glenn* 854]

If its 1:1 the first set of calcs are correct then

[CamB 48]

3.64*0.81 = 2.95 (assuming a 1:1), so you'd want a 2.97 (or a 3.15 if shorter).

Rev calculator (doesn't work very well in firefox):

http://members.dodo.com.au/~wawawa/revs.htm

Edited May 10, 2010 by CamB

[e30ftw 402]

Sweet, Anywhere 2.97 to 3.25 will be sweet.

[*Glenn* 854]

This is a better calculator to use. Load tyre size first, calculate, then put g/box output ratio in the 6th gear box and do the maths. It's usefull for 6 & 5 speed gearboxes

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

[mitchE30 0]

thanks glenn.nice website.

[*Glenn* 854]

Sweet, Anywhere 2.97 to 3.25 will be sweet.

Getting the right ratio in a medium case that will fit your E30 is probably going to be a problem though. I ended up getting a diff from Brent. It was out of an E28 to get the crownwheel & pinion. Then had the LSD, crownwheel & pinion & a full set of new bearings installed and set up by Diff Specs using a solid preload spacer for the pinion instead of the collapsible sleeve. Not cheap to do unfortunately.

Edited May 10, 2010 by *Glenn*

[BM WORLD 1270]

Getting the right ratio in a medium case that will fit your E30 is probably going to be a problem though. I ended up getting a diff from Brent. It was out of an E28 to get the crownwheel & pinion. Then had the LSD, crownwheel & pinion & a full set of new bearings installed and set up by Diff Specs using a solid preload spacer for the pinion instead of the collapsible sleeve. Not cheap to do unfortunately.

was from e34 530i V8 auto .

3.15 ratio . also a comon ratio found in the e36 325i manual's

i have a530i v8 diff that will be coming out soon .

have a 2.93 medium case from a e28 525e auto on the shelf

[HELLBM 1531]

Can do 3.93, 3.15 both in medium case. LSD`s 3.64 and 3.73.

Other options avaliable.