Final drive gearing

Update Part Deux:

OK, I now know definitively what I can get and for how much.

New BMW Motorsport 3.45 210mm ring (crown) and pinion set $535 US

Used 210mm 3.15 LS Differentials $550 US

The prices reflect shipping charges to me of $40 and $100 respectively.
The breaker for the differentials will sell to me at $450 all day. The supplier for the gears has 5 sets left after the one I bought and would sell at $495ea. if I get 3 sets. Their normal price is $750 but its old inventory and they're willing to deal.

I will be handing my differential and gears to Korman BMW for a refresh of the seals etc. and inspection. When it's all said and done I expect to have a freshly rebuilt LSD with 3.45 gearing and standard 25% lock ready to bolt into my 840ci for under $1500 US and $200 of that is in shipping all over the place.

With the exchange rate in your favor and the availability of these particular parts here I thought it might be of interest, and I would help arrange a similar setup at my costs if any wanted to follow the path I'm taking. I'm extending this info to all here because we're a global community and I have a sincere interest in keeping these great cars on the road. This is not to be construed as a solicitation of any sort and I have no affiliation with any of the parties I'm dealing with. cheers

I definitely want to get a slipper for mine, it's not easy to get the back end out and keep it sliding without, as the inside wheel just spins up leaving the outside one gripping which pulls the car out of the slide. This is with ASC+T off, I have considered using the "hidden mode" of ASC to act as a poor man's LSD, but I suspect using it in this way extensively will either cook the brakes or cause some other issues with the ASC.

I'm undecided as to which ratio suits my car best (UK 840Ci/6) but I suspect something quite high like the 3.15 would work well, as the ratios are reasonably low to start with unlike the automatics. A 3.91 with my gearbox would be loony, so I definitely wouldn't want to go as low as that. If anyone spots anything that may be suited please let me know.

Thanks for that Martin. I'm wary of going as high as 3.45 or above, I feel that it's geared quite well at the moment, maybe very slightly too high, but the manuals seem to be much better than the autos which are far too high IMO (probably for fuel economy reasons, to offset the lower efficiency). So a 3.23 would be fine, but as you say these are rare and hence probably way overpriced.

Apparently the early E32 750i had a 3.15 diff, some of which were LSD. How many (if any) came to the UK with an LSD is another matter.

My car is a '98 840 Ci Sport (Auto). Last year I had Kevin Bird suppy and fit a Quaif LSD. He has just fitted a 3.45 : 1 final drive and the result is excellent. The car is showing 90 mph at 3,000 rpm.

How much did the quaife cost? I'm suspecting big £££! My car is over 3000 RPM at 90 MPH in 6th with standard gearing, so on a 3.45 I reckon I'd be closer to 4000 RPM! The manual is geared more for performance out of the factory, hence why I'd rather not go for such a low ratio as I think it'd be diminishing returns.

Standard gearing for the 840Ci/6 (courtesy of e31.de):

Standard gearing for M62 840Ci/A (again of courtesy of e31.de):

Note the *much* higher gearing, with the autos being geared more for economy and refinement. Hence why it's highly beneficial to switch to a lower ratio diff on an auto, bringing the gearing more in line with the manual.

Paul, I like how that graph shows a top speed in excess of 200mph @ 6200rpm with standard diff. Yeah, I know wind and rolling resistance gets in the way and I think IIRC that the unrestricted top speed is more like 165mph or thereabouts in reality.

My ahem, official, top speed now @ 6200rpm, is 145mph (on speedo, so more like 138mph in reality) with the loony M5 3.91 LSD ! Only did it once, and my driving habits don't lend themselves to straight line speeding.

Interesting seeing those graphs though. I do like my stats

Martin, I believe my stock diff is a 3.08:1. I appreciate that a lower ratio (numerically higher) will improve acceleration to some extent, but I don't imagine it would offer a huge improvement due to the extra gearchanges required to reach a certain speed.

I think the standard gearing on the 840Ci/6 works quite well, as you're in the engine's power band by around 20 MPH in 1st gear, and after that you're always in the power band assuming you change up at the correct time. So I don't think the acceleration improvement would be massive -- at least in theory.

The sole reason I'd like a diff swap is to get an LSD. I wouldn't mind a 3.15 or 3.23 ratio, but much beyond that I'd need to be persuaded. Mainly because I'd like to retain some "sensible" cruising ability for the odd time I find myself doing a long motorway run, which is the only time I use 6th gear extensively.

Tony, the graphs do indeed provide a very nice insight into the choice of gear ratios made by BMW. It is clear that the autos were designed for cruising and/or economy, as even with one less ratio (or two in the case of the early 850 auto) compared to the manual, they are far higher geared overall and with a wider spread between ratios.

You correctly state that top gear in the auto would theoretically allow a speed of over 200 MPH -- clearly impossible with the power available! The 840Ci/6 theoretical top speed is around 175 MPH, which I think probably isn't far off what the car can actually do. Similarly the 850CSi tops out around 180 MPH (slightly higher geared than the 840, but with a lower rev limit) which it will be able to achieve quite easily due to it having 380 BHP, with the speed limiter removed of course.

These graphs highlight why a diff swap is such an important upgrade for auto cars (assuming you'd rather have performance than refinement and fuel economy), whereas for the manuals it's much less of a benefit -- they are already much more performance oriented from the factory. Of course ratios are one thing, but an LSD is one upgrade every E31 should have, the CSi getting one as standard.

OK here's the dillemma... I've managed to locate a 3.15 LSD from a 1990 750i, which should bolt straight up after changing the rear cover, correct? However this diff is at the opposite end of the country to me, the price is reasonable so I'm happy to have it shipped, but there's always the risk it's got issues that I won't discover until it arrives on my doorstep.

Do you think this is a good idea to buy a diff unseen from an 18 year old car? I've asked the seller a few questions about whether it's all present and intact, and if it ran with no noises etc. But ultimately I'm depending on his knowledge and honesty.

Ideally I'd like to find one locally, but they're rare as hens teeth up this neck of the planet!

Are you a gambling man? if it were me then I would take the risk, as you say they're very rare to come up near to you, but at the end of the day its your money.

I would take the risk if it's under £300 !

Mine cost £250, and definitely worth the money. Prob cost me an extra £1000 in fuel though since it was fitted but then sliding around (deserted) wet roundabouts has never been so much fun !!

Good point Martin, most auto drivers are grandads, and those who aren't tend to drive like they are anyway

Definitely sounds like it's worth a shot, I'm waiting on the seller to get back to me with a price for shipping. In the meantime, does anyone have any suggestions of a courier that is likely to deliver such a heavy item for a reasonable price?

Cheers guys