I have sourced a 3.91 diff from a 1993 Euro e34 M5 to install in my 1991 US Spec 850i Auto. I am assuming this is the same straightforward swap as the “one-year-only” 1991 US Spec e34 M5 3.91 diff. Is this a correct assumption or are there differences between the Euro and US spec e34 M5 diffs that I am not aware of?
"Asphalt is evil, it must be punished!"
'89 Z1 Dream Black/Alcantara/Camo, Racing Dynamics, the coolest doors known to man -- Sunny days & Sundays
'90 Z1 Red w/custom red/black French leather, Ground Control coil overs, BBS RV-F --Mais Oui!
'91 850i Mauritius Blue/Black --1 of 179
'85 M635CSi Alpine -- "Explosive!!!"
'03 M3 Conv, Imola w/ SMG -- her daily driver
'17 “30 Jahre” M3, Macao Blue -- 1/500 made
Previously owned: 74 2002 (x2), 85 325e, 80 323i Baur, 94 325iC, 03 325CiC, 80 633CSi, 90 325iC, 98 318ti (Budget M3), 01 X5, 02 X5 4.6is (x2), 94 M3, 98 M3/4, 16 340i
Perfect! Thanks, dragon!
What's the best differential for a 1992 Manual 850i ? I know for the auto it's the 3.91 but is it as well for the manual ?
So to be clear , you'd say the 3.91 will be like 3rd best ? Would you say by much or pretty close ?
Why all the questions ? I have two 3.91 differentials , one is going in the auto , second not sure , could go in the manual or you can talk me out of it ? I know the 3.91 is rare ! How rare is the 3.45 to find ? Sorry for all the questions , I want to make a plan
The 3.91 is not an option for the 6 speed manual, I mean you can install it but the first two gears will be pretty useless. The 3.45LSD are as rare if not more rare than 3.91 because they were never an option in any US based cars.
I would suggest selling your 3.91 LSD and then having me build you a new 3.45LSD.
Last edited by dragon850; 03-14-2018 at 12:35 PM.
i disagree. Way overpraised mod.
Hey Mark, some recent info here on output shaft bolts...
https://www.bimmerforums.com/forum/s...PN-33211227279
Thanks again Dragon for the head's up on the diff. too!
2006 R53 JCW (British Racing Green) - 1994 E31 840Ci 6spd (AVUS Blue)
2021 F56 JCW (Rebel Green) - 2000 E38 740i Sport (Titanium Silver)
2017 F26 X4 M40i (Carbon Black) - 2007 E91 328iT (Deep Green)
2012 E82 135i M Sport (Marrakesh Brown) - 2015 E84 X1 (Cashmere Silver)
2005 E53 X5 3.0i Sport (Kalahari Beige) - 2000 E36/7 M Roadster (Oxford Green)
2000 E38 740i Sport (Oxford Green) - 1999 E36 M3 Coupe (Fern Green)
1995 E31 840Ci Canadian (Oxford Green) - 2000 E36/8 M Coupe (Oxford Green)
2004 E46 330Ci (Oxford Green) - 2001 E36/8 Z3 3.0i Coupe (Oxford Green)
2002 E39 525i Sport (Oxford Green) - 2002 E36/7 M Roadster (Oxford Green)
2001 E53 X5 3.0i Sport (Oxford Green) - 2007 E63 M6 (Silver Grey)
1995 E31 840Ci (Oxford Green) - 2006 E86 M Coupe (Sepang Bronze)
2001 E39 530i Sport (Aspen Silver) - 2009 E92 M3 (Space Grey)
2001 E38 740i Sport (Sahara Beige) - 2008 E92 M3 (Melbourne Red)
2000 E38 740i Sport Canadian (Sahara Beige) - 1997 E31 850Ci (Jet Black)
Thanks, Joe. It’s crazy what the BMW suppliers are getting for these...they are used in just about every model, too, including e30, so it’s not a case of e31 part obsolescence and limited supply. The only thing unique about these M10x50mm 10.9 bolts appears to be the torx head. I’m headed to the bolt supply store right now for some generic M10x50mm 12.9’s. They would have had to order the 10.9 grade, but have the 12.9’s (which are even stronger grade steel) in stock. The only difference is the head which is an 8mm hex socket instead of torx. 25 count for $9 TOTAL ($0.36/each)—NOT A TYPO!!!! . That’s two full sets plus a spare bolt for less than the price of one torx head bolt from BMW. I need only 12 of them, so if you buy me a beer you can have the other 13!
Last edited by Budget M3; 03-15-2018 at 06:21 PM.
...fwiw, Most of us don't drive E31's daily, More like weekenders at best, A Dinan Transmission chip will safely make your gears longer even with your 3:91
Going from your Euro spec 3.15 to 3.91 would indeed result in much higher rpm (about 600rpm difference at 80mph/130kph according to my calculations—from ~2300-2900). In my experience, anything less than 3000 rpm is usually tolerable for long trips, but if you’re driving the autobahn at 200 kph I can understand how the increased rpm and fuel economy would be an issue. Going from the US-spec 3.64 to 3.91 is only a 200 rpm difference at 80mph where most long distance driving is done here in the US, so should be well worth the minimal increase in rpm and fuel consumption. Plus LIMITED SLIP for the twisty parts!
"Asphalt is evil, it must be punished!"
'89 Z1 Dream Black/Alcantara/Camo, Racing Dynamics, the coolest doors known to man -- Sunny days & Sundays
'90 Z1 Red w/custom red/black French leather, Ground Control coil overs, BBS RV-F --Mais Oui!
'91 850i Mauritius Blue/Black --1 of 179
'85 M635CSi Alpine -- "Explosive!!!"
'03 M3 Conv, Imola w/ SMG -- her daily driver
'17 “30 Jahre” M3, Macao Blue -- 1/500 made
Previously owned: 74 2002 (x2), 85 325e, 80 323i Baur, 94 325iC, 03 325CiC, 80 633CSi, 90 325iC, 98 318ti (Budget M3), 01 X5, 02 X5 4.6is (x2), 94 M3, 98 M3/4, 16 340i
The increase is 250rpm. I went from 3,64 to 3,91 and speak for my personal experience. I have also driven euro spec car with 3,15 and i base my opinion to the fact that 3,64 is better than 3,15 but 3,91 isn't as much better than 3,64. You guys have best from both worlds.
As far as the limited slip goes. Unless you don't keep spinning your tires in sandy roads you don't benefit from lsd at all. 850i M70auto is so clumsy and underpowered.
Thanks for the additional feedback. What was the increased fuel consumption you experienced going from 3.64 to 3.91? I wouldn’t think it would be too much given an only 200-250 rpm difference so would be interested in your real world experience.
Sorry but I have to disagree with your conclusion on LSD only making a difference on sand or for “underpowered” cars. I owned a 318ti for many years (talk about underpowered!) which I converted from open to LSD. I speak from personal experience that ANY car, no matter how little power under the hood, will benefit from and become more fun to drive with an LSD.
As I’ve already bought the diff for the e31 , I guess I’ll have to wait until it’s installed to really form my own impressions.
Last edited by Budget M3; 03-20-2018 at 08:16 PM.
I agree, They also keep the car centered when tire spin occurs.
Budget M3, I sure would like to drive my E31 to Denver to visit my son and take the car out to High Plains Raceway for some laps.
JDP530, Sir, you have me at a disadvantage as I do not recognize (or remember?) your username. I am always up for a road trip and track time, but my 8’er is currently in SoCal so we’ll need some serious planning if we are to meet with our 8’s in CO. Now my M635CSi would be another story as it’s already primed and ready to go in CO!
I joined forum several years ago when I first bought the E31 but haven't really been active on here until several months ago when I started working on my car. My son lives in Denver so I try to go up there a few times a year. We have both attended events at Pikes Peak Raceway and High Plains. I would like to drive my car up there this year but I am still working on it and having some issues to resolve.
Thanks,
John Patterson
Last edited by 72tpik5; 03-22-2018 at 08:29 AM.
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