Diff Swap: Research, Swap Procedure and Effect.

[e34.535i.sport]

Diff Swap: Research, Swap Procedure and Effect.

Overview.
For a long time I had my mind set on swapping out the differential for one with a higher (numerical) ratio, and I recently had the pleasure of undertaking the job. (I’m glad I did!) It took me a long time to gather the relevant information I needed to make the change effectively due to it being sourced from a variety of different places. However it was worth it in the end as I learnt a LOT about the workings and application of the e34 differentials. I thought I might try to put all of this information in one place for anyone who contemplates this job in future, to save them some of the hassle I went through. I will say early on it is well worth your while reading through the mass of information available on the web as doing this swap transformed my car, making it more enjoyable to drive than ever before.

For relevance sake, my swap was from the original 3.64LSD to a used 3.91OPEN diff from a 735i Auto (e32). People may question the swap from LSD to OPEN, but I can personally say I haven’t noticed the difference at all between the two so far. The car is my daily driver, and I tend to drive it quite swiftly around town most of the time, with the odd motorway journey now and again.

What is the diff (differential) and what does it do?
To put it simply, the engine spins when you put your foot on that pedal on the right… which spins the gears in the gearbox… which then spins a long shaft taking the power to the rear of the car where that shaft turns the differential gears. The gears in it spin the output shafts (axles) which in turn spin the wheels (and also allow them to spin at different rates as you go around the corner). The ‘number’ on the diff (e.g 3.64) equates to how many revolutions of the input are needed to make one revolution at the output. That may sound like jibberish at first if you are new to this like I was, but think of it like riding a bike. Treat the rear gears on the bike as your gearbox and the front gears as the differential (I know this seems back to front!)… Imagine you keep your front gear in #2 (normally we have 3 gears up front on a bike) and shift through the rear gears normally as you do in a car… When you change the diff ratio it is lik slipping the front gear of the bike to #1 making every gear on the bike easier to pedal in but lessening the top speed you can hit in each gear as your legs end up going around much quicker to reach the same speed. I hope that didn’t just confuse matters!

Why change the diff ratio?
Well if you decide to move up numerically in a similar way to myself (3.64 to 3.91), then you should expect increased acceleration (as the engine is having to use less power per revolution to turn the rear wheels making the revs rise more quickly and getting you into your optimum torque/hp band quicker), at the expense of shorter gears. How much more depends on what ratio you go for. You obviously won’t be increasing BHP/torque figures or anything like that, just making it more easily available to the rear wheels.

People have discussed the swap for a lower ratio on here many times (search the archives) in an attempt to gain more MPG, but the trade off’s involved in the swap are complex and specific, and going to a lower numerical ratio may not necessarily save you money on fuel.

From here on in I will be referring to the swap assuming it is for the purpose of a ‘higher’ ratio.

Theoretical Effects: Pros and Cons.
Increasing the diff ratio will ultimately increase your revs at any given road speed therefore reducing your MPG at high speeds. To put this into context my 3.64 to 3.91 swap increases my revs at any speed by approximately 7.4%, as 3.91/3.64 = 1.07417582 (1.0 would be 100% i.e exactly the same). I hope that makes sense, other people may work it out this way: (3.91-3.64)/3.64 *100 = 7.417582%. As an example… If in the past I would cruise at 70mph turning 2700revs then with the new 3.91 diff ratio I will now be turning 2900revs or thereabouts. As you can imagine, cruising at higher revs for a long period would obviously require the engine to use more fuel.

However, as the engine will need to exert less power to move the car at lower speeds it is possible for the swap to increase your MPG around town if you drive the car sensibly of course! I have noticed an increase of about 2mpg generally around town and a feeling that the engine is ‘happier’ than before!

Research.
The first thing you will need to know is what differential you have in your car in the first place. Try looking at these for general reference, but they are not exhaustive: http://www.vinesauto.com/diffratio.htm and http://www.bmwe34.net/E34main/Upgrade/Recipe.htm .
The best way is to get underneath the car and have a look. There should be a small identification tag (that will more than likely be covered in years worth of dirt and grime) somewhere rear of the output flange on the left hand side of the car. It’s actually ‘bolted’ on with one of the big bolts so the easiest way to read it is to snap it off and clean it up indoors. If there is a ‘S’ before the number on the tag this denotes that you have a LSD (Limited Slip Diff). These seem widely preferred for fast cornering as they increase grip and send power to the wheel that has the most purchase on the ground which is obviously useful in racing applications… Everyday life I’m not so sure you need it. Although people have commented on LSD’s being helpful when driving in icy/snowy conditions...

Once you know your ratio, you can begin to think about what ratio you want to step up to. The above link to bmwe34.net has a useful guide to what might be a sensible change… Too much or too little will have negative or minor effects overall. Some sources have said that in order to maintain drivability you should avoid going further than a 10% difference in ratio’s (as worked out earlier on). I can say from personal experience that my 7.4% increase yielded a noticeable effect on the acceleration from low speeds as well as some shortening of the gears. How far/little you want to go will depend on your driving style. To help you work this out, you can use this gearing calculator: http://xse.com/leres/bin/gearratio?t...le=45&wheel=17 . At the moment it is set up for my particular gear ratio’s (founding the users manual) as well as my tyre size and diff (axle) ratio of 3.91. You can adjust the numbers as necessary to compare different ratio’s… Play around with it.

The limitations on the ratio you will be able to swap into your car mainly lie in the different case sizes. As far as I am aware the 525i, 530i and 535i (as well as e32 735i) all have medium cases, whereas the 540i and m5 have large cases. These should be interchangeable with the 740i/750i e32 models. There is not much information on the 518i/520i out there at all so I’m not sure what case sizes/ratio’s they are. It appears that you can make different size cases work in different cars, but whether it’s worth the hassle or not is down to you. For me it wasn’t and I wanted a direct, or as direct as possible, swap.

Light reading.
Here’s some threads I found that might be worth looking at if you’re tempted…
http://www.bimmernut.com/forum/showthread.php?t=41303

http://www.bimmernut.com/forum/showt...ight=diff+swap

http://www.bimmernut.com/forum/showt...ight=diff+swap

http://forums.bimmerforums.com/forum....php?t=1140084

http://www.bmwchannel.com/bb/showthread.php?t=1825


Getting the donor diff…
As well as the above, you must consider the difference in input/output flanges between different diff’s. I was lucky in that mine were exactly the same despite mine being a manual e34 and the donor coming from an auto e32. The output flanges are almost always easy to swap in for your old ones if the new ones won’t fit, although I did read one horror story where they were totally different. This is worth checking out before you purchase the new diff. Inspect your front/side flanges on the diff carefully before going to look at a possible donor is my advice. Here’s a pic of the one I put in for reference:
http://img35.picoodle.com/img/img35/...3m_89f6329.jpg

I would advise anyone who’s going to be putting the diff in themselves to watch the person remove the donor from the car it lived on so you can get an idea of how it is attached. This was a really useful process for me personally.

You should pay particular attention to the front flange as if you need to change this it could lead to trouble as you must but the right preload on the nut when you secure the new flange back on (apparently very difficult to get right). I wanted to avoid this as it has been said that you should have a professional set the preload, which of course means additional expense. That’s besides the fact I don’t have a garage I trust enough to do such an important job. If it is done incorrectly the diff could fail within a year. The good news is the side flanges just pop in and out if you need to change them… Bish bash bosh!
PRELOAD:
http://www.bimmernut.com/forum/showt...ight=diff+swap

Doing the Swap…
PREPARATION: It is recommended from a variety of sources that you replace the side seals and the snap rings that come with them as well as the diff fluid. This is a simple/cheap procedure that is worth doing to prolong the life of the diff and save you hassle in the long run. A lot of the information on this is detailed in my previous thread:
http://www.bimmernut.com/forum/showthread.php?t=41303 (as shown above).

The actual swap of the diff is fairly straightforward. Follow these instructions: http://www.bmwe34.net/E34main/Mainte...fferential.htm

The only thing I would say is it is essential to do this with a friend as it really would be difficult on your own. I was lucky enough to have the Whiskychaser’s assistance when I swapped diff’s - it’s a doddle with two people providing you don’t get any ‘difficult’ nuts/bolts.

The driveshaft nuts are relatively simple to remove, but a ratchet spanner made things much quicker for us. The bolts on the halfshafts (axles) however did not want to come without a fight initially, although we got around that! I read around this topic so much I happened to come across a piece of information that was an absolute lifesaver. The bolts on the halfshafts are the type that require an allen key to remove them… A hex socket is better of course, and the bolts themselves will have acquired a lot of gunk in them over the years. CLEAN IT OUT! Use a tiny screwdriver or whatever you can to get the muck out so the allen key can get right in the hole otherwise you will round it and it’s game over. I also believe our success was due to this magical potion: http://www.halfords.com/webapp/wcs/s...7631_langId_-1 . Whisky is a little sceptical about this as was I initially, but it just seemed to work wonders, literally. I wouldn’t try another job like this without it and I mean it! Get some, simple as. No, I don’t work for Halfords either.

The big bolts holding the diff in (two at the back one at the front) are fairly easy to remove, but you will probably need a breaker bar. Lowering the diff down once all the bolts are undone is the most difficult part, but it just takes some patience and careful consideration. It will be obvious what you need to do when you’re under there doing it.

Once the new diff is in make sure the wheels/driveshaft spin as they should and go and test it. You should feel the difference immediately!

My Results…
I found the difference immediately very noticeable and over the past week or so I have been monitoring the car carefully to try to analyse the differences that the swap has made to the drive of the car. I will discuss each one separately…

Acceleration: The acceleration now feels a LOT quicker from standstill and more immediate with the throttle in terms of response. The power band seems to have definitely shifted to lower down in each gear where I like it, whilst maintaining good mid-range and high end power. Before the swap the car would need to build up momentum before really powering up but now the power is available much, much quicker. The acceleration in 2nd and 3rd gear has improved significantly with the drawback of having to change gear a little sooner due to the increase in engine revolutions. The earlier shifts aren’t that noticeable when driving normally however, only when you hammer the accelerator as the revs/speed rises so quickly you really need to be watching the rev needle! I don’t like exceeding 5500 revs really as a rule and it gets there in the blink of an eye in 1st/2nd gear.

Gear changes/shifts: The common perception seems to be that changing the ratio to one that is higher numerically will increase the number of shifts you will need to do... I have noticed the opposite. The fact that each gear is slightly shorter does not make me change up a gear more often that I always have done, it simpy makes me do it 'sooner'. What is also noticeable is that I'm shifting down LESS than before in a variety of situations because I don't always need to downshift now to get the pull I need, and I can often use second to take off from very slow speeds rather than change to first which I would have done with the 3.64. It's been interesting to analyse this whilst driving as I expected to be making shifts much more often than before. It's a nice surprise.

Engine/City MPG: Although my car has been well maintained and the engine seems in really good health it tended to feel a little ‘strained’ when moving off normally in first gear. I’m not sure why this was but the problem is definitely eliminated now and I could tell instantly that the engine was working less hard to move the car at low speeds as soon as I drove it. My daily routine involves mainly short city journeys in stop start traffic with speed bumps etc… Whilst driving the car ‘carefully’ I have noticed a good increase in my MPG, rising from an average of around 16-17 to now 18-20 OBC indicated.

MPG @ High Speeds: As expected the fuel economy for motorway journeys has decreased as I could average around 28mpg driving very sensibly (I once hit 30mpg on a VERY long trip!) which is now averaging around 26mpg (mainly on short <15mile trips). As I don’t use the motorway for long trips that often this will not bother me in the slightest… Particularly taking into account the fact that overtaking at speed is now even easier than before with the increased final drive ratio or fourth/fifth gear!

+ I have just returned from a 250 mile round trip to the Lake District and the car was still a pleasure to drive at high speeds, the additional revs not noticeable at all. I ended up with 27.4 mpg on the OBC which I later worked out as 27.8 mpg after filling it right back up and doing some sums. However I must concede that I was for the most part following a mini-bus capped at 63mph so obviously at ‘normal’ speeds my mpg would be slightly less than this I would imagine.

Cornering: I haven’t noticed much difference in the cornering habits with the new open diff, although I don’t tend to take corners at speeds anyway. However I am driving the car in the same manner as before and I’m certainly not missing the LSD! The new lowering springs/tyres are probably helping with grip generally…

Overall I’m really pleased with the job, and would recommend people to try it if you’re trying to revive the ol’ e34. For my driving style/habits the swap has worked out really well – I will be cleaning up and selling the 3.64LSD shortly and that’ll be that!

Hopefully this thread will be of some use to someone in future. Enjoy.

[whiskychaser]

Nice write up! I'm still not sure about that magic spray stuff Good tip about cleaning the allen bolt heads out. You dont mention it but I seem to recall the rubber boots getting in the way so you have to be careful the socket is in dead straight? Filling the diff with oil before you put it in was a smart move

[e34.535i.sport]

you dont mention it but i seem to recall the rubber boots getting in the way so you have to be careful the socket is in dead straight?

+ 1,000,000!

[Mordan]

thx for the excellent post

I also have that feeling of strained engine with my M50B20 60 000km 16 years old in first gear. I hate it. it is wierd.

I might do a diff swap but I don't have enough experience yet. your post definitely is telling my brain why not?

[Mordan]

a few things just sprung to my mind

how does a diff wear? how much wear did you see on your diff?
was your LSD stuck? I heard it could happen? Could you provide a pic of the inside? I'm curious.

thx

[e34.535i.sport]

a few things just sprung to my mind

how does a diff wear? how much wear did you see on your diff?
was your LSD stuck? I heard it could happen? Could you provide a pic of the inside? I'm curious.

thx

If you're tempted don't let lack of experience put you off... I didn't! A little research is all you need, it's honestly not difficult.

I'm not sure what happens in terms of diff wear, It seems they last and last providing they have fluid in them from what I've read... I'm pretty sure the LSD was/is fine as it always gripped really well and the unit had different properties when spinning the front flange once it was off to the open diff. Don't think I'll be opening mine up but Jeff B might have some pic's for you IIRC he took his to peices and got it back together again!

[BigKriss]

very nice writeup - thanks!

[632 Regal]

Kinda like this? http://www.thecitypress.com/crap/rearend.html

Don't think I'll be opening mine up but Jeff B might have some pic's for you IIRC he took his to peices and got it back together again!

[e34.535i.sport]

Kinda like this? http://www.thecitypress.com/crap/rearend.html

Is that an LSD? Would it look much different than an open diff?!

[bubba966]

Reminds me I should do something with my 3.91 LSD that's sitting around. Still figure it's probably too deep for my car, but I've yet to get underneath it so see what's in there now. Probably need a 3.64 rather than the 3.91.