Euro 3.91 diff install question

[Budget M3]

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?

[dragon850]

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?

Its is the same exact thing. Output shafts and rear cover need to be swapped.

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[Budget M3]

Perfect! Thanks, dragon!

[TedBobo]

Perfect! Thanks, dragon!

It's going to make you smile every time you drive it!

[Budget M3]

It's going to make you smile every time you drive it!

That’s what I hear. Hope I can find the time to get it installed sooner rather than later.

[TheShark850]

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 ?

[dragon850]

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 ?

The most common is 3.15LSD for manual but the best IMO is 3.45LSD.

[TheShark850]

The most common is 3.15LSD for manual but the best IMO is 3.45LSD.

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

[dragon850]

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.

[mikae31]

i disagree. Way overpraised mod.

[dragon850]

i disagree. Way overpraised mod.

Well, it's pretty well established that the diff swap is the biggest bang for the buck you can get on an 8 series.... Sorry you had a bad time with your swap.

[Chamberlin]

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!

[Budget M3]

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!

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!

[mikae31]

Well, it's pretty well established that the diff swap is the biggest bang for the buck you can get on an 8 series.... Sorry you had a bad time with your swap.

Yes it is. And because of all that praise i expected more. Biggest difference was the increased consumption. Not worth it.

You guys allready have the best overall compromise (3,64), so why change it. Driving long distances is more annoying because of the higher rpm.

[8eights]

...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

[Budget M3]

Yes it is. And because of all that praise i expected more. Biggest difference was the increased consumption. Not worth it.

You guys allready have the best overall compromise (3,64), so why change it. Driving long distances is more annoying because of the higher rpm.

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!

[mikae31]

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!

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.

[Budget M3]

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.

[8eights]

I agree, They also keep the car centered when tire spin occurs.

[mikae31]

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.

Yes, consumption increased more than i would have thought.

[Budget M3]

Yes, consumption increased more than i would have thought.

How much? L/100km or mpg is fine. I can do the math ;-)

[JDP530]

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.

[Budget M3]

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!

[JDP530]

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

[72tpik5]

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.

I switched from a 3.15LSD to a 3.64LSD and love it with my 5 speed auto. The best news regarding this mod is it's easily reversible, and the 3.91LSD is now probably worth more than you paid for it. Happy wrenching.