Pondering E36 M3 Evo diff

Hi Matt ,

Forgive my ignorance will this make the car lower or higher geared , as I understand it lower geared greater top speed , higher geared , greater acceleration. also what do you propose to do with the diff cooler ? Can it still be used .

I put a 3.91 into my old 840, hated it almost from the 1st drive & is maybe one of the reasons I ended up selling the car as it was way too different, my advice would be ,Don't bother

I also have a 3.91 M5 diff in mine. I have a love hate relationship with it, in that on anything other than a motorway I love it as the car is much more lively and accelerates faster due the revs being up all the time, but ..... on the motorway it revs too high once you go over 80mph or so. 4000rpm at 100mph, and 3000 at 72mph or so. Fuel economy goes out the window too.

Depends on how and where and how far you drive as to whether I'd recommend someone else doing it.

Use the money to buy some new wheels or some Phoenix tweaks to your car. Modding the drive is a guaranteed way to spoil the reason you liked the car in the first place! IMHO.

What about a lightened flywheel , its a csi and therefore manual . This would help it rev quicker

The different posts here are talking apples and oranges. I don't doubt that Steve hated the 3.91 diff in his 840.... gears were way too short from the stock 2.93 and V8 autobox. Now on the 850 with the 4HP-24 those have a 3.64 so a 3.91 rear is a nice little boost. The 6spd driveline I just got has a 2.65 (!) final ratio.... it'd take about a week to get off the line, so I'll be putting in a 3.15 LSD and that should suit quite well. Alan if you do the swap I'll buy the diff and cooler, and rebuild them here......

My opinion (which I give knowing noone wants to read it..) on the matter is:

Don't bother. Really.

With the CSi you already have a 25% locking diff (which is awesome) and the gear ratio will not make a measurable difference in acceleration. BUT it will make a measurable difference in fuel consumption.

See, the point with high ratio diffs is that they take away your option to drive along in low rpms. With a low geared diff, you can choose between low rpms in, say, 6th gear and higher rpm at the same speed in either 5th, 4th or maybe even 4rd gear.

Here's a thing on acceleration from 0 to 60: every time you shift, it will add about half a second to your 0-60 time. and that is only if you can shift well.

And then here's yet another thing to consider: Accelerating from standstill, with the standard diff, you can keep the revs up at around 5000 rpm and you can keep the tires in the "best" slip (highest grip). And by "you can" I actually mean that someone, surely, has the expertise to do so (say the Stig). Here's the catch: You gain absolutely nothing with a different diff.

With the 4spd automatic M70 version, a different diff will, in fact, make a measurable difference in 0-60 time. That is, because you can't rev it beyond 2000rpm while standing still (unless you drop-kick it and kill the tranny doing so). But even then the difference is minor but the added fuel consumption major.

Just my opinion...

Oh and back to the topic: Unless the M3 diff has the same bolts in the same place and the same shaft-diameters all around you will have to have some parts manufactured.

Personally I would. Mine has the 2.93 diff as standard, and I've always found it to be overgeared for the characteristics of the engine. As far as I know, the ratios in all the 6-speeds are the same (840Ci/850i/850CSi) so my overall gearing should be the same as a stock CSi. I dread to think what the automatics are like with a 2.93 diff, as the gear ratios are far higher than the manuals.

Saying that, the V12 is less revvy and more torquey than the V8, so maybe the CSi suits higher gearing than the 840Ci. Still, 6th gear is pointless at anything less than motorway speeeds, and the ability to top out at 175+ MPH equally pointless in the UK.

As for fitting the diff unit -- you can transfer the internals, in which case you can probably keep the oil cooler. However doing so requires swapping in the pinion gear into your case, which needs to be properly tensioned, otherwise it will fail after some time. The rest of the diff unit is easy to install into your case. I fitted an M3 Evo diff into my mate's E34 540i (with 6-speed manual, same as the 840Ci), and I just swapped his existing crown wheel gear onto the diff unit as he wasn't fussed about changing ratio. In this case I didn't need to touch the pinion wheel gear, so the swap was pretty straightforward. I did a similar swap into my 3-series that I use as a daily hack and track car.

Zitat von fuzzifikation;72317

You gain absolutely nothing with a different diff.

apart from about 1.5 secs 0 - 60 over stock. Ask me how I know

Like I said, I love and hate it for totally different reasons.

Zitat von TallTony;72319

apart from about 1.5 secs 0 - 60 over stock. Ask me how I know

Like I said, I love and hate it for totally different reasons.

well.. here I go: How do you know?
(And I absolutetly do not believe 1.5 sec on a manual using the full rpm-range at take-off, which isn't possible in an automatic).

I absolutely defend the idea that, physically, if you are able to spin the wheels at takeoff (with any gearing, really) you can, theoretically, do so in a good slip-value until you reach a higher rev-band (and you finally let go of the clutch). If this is so, it is solely the engine power and your shift-times that defines your 0-60 time.

IF you are able to reach a satisfying rev-band (which would be around >4000 rpm with the 12-cylinder) with good wheel-slip then it absolutely doesn't matter what gear-ratio you choose (with the diff, that is). And, again, this is not possible with an automatic gearbox. I am talking manual only.

and to be mean-spirited: If one is unable (with a high-powered car as the CSi) to keep a decent slip ratio playing with the clutch until the first gear finally is locked in no sooner than >4000 rpm.. well, then a shorter diff will, indeed help. But the help is solely to veil the drivers inability. (just to be clear: I also share this inability..)

Well.. enough mean-spirited arguing.. going to bed.

P.S.: The 4spd automatic gearboxes have completely different ratios than the 6spd manuals. it is the combination of diff ratio with the gearbox that defines the engine rpm at any given speed (assuming an engaged clutch).

The RPM vs. Speed for all 8-series is given here.

No arguements from me I can assure you, but I have the diff in my car and I know the difference it has made in acceleration, for me. My car is 1997 840ci 4.4 auto.

Before : http://www.youtube.com/watch?v=b-XhYSsG4Wk

After : http://www.youtube.com/watch?v=N8FSDerBy5w

both efforts were simple 'foot to the floor' exercises, and totally unscientific, but since it was me doing t he testing, I'm convinced

As a long ago street racer who has always been autobahn deprived, Gearing is everything.

My Mustang Cobra has 3.73 gears with a 5 speed and it is designed mostly for the street. They claim 153 as a top speed but I would be far less likely to try it than in the 840.

The lower gears (higher numerically) are better for transmitting torque to the wheels--they are Multipliers

The current fad of rev limiter launches doesn't seem to me to be more than a plan to sell more parts. I won't argue the point but high rev launching the weight of an 8 would seem to be one hell of a stress on a drive line tailored for speed. But then again my 9 second '56 Chevy would have ripped the driveshaft into tinfoil chaff launching that way. So would several of my other toys of the day.

If you live in the open and free fast is nice but if you live with traffic lights and cameras Quicker is better.

Older now and just happy to feel gratifying acceleration when driving off from a traffic light now and again. So 3.64 behind a 5 speed auto in the 840 is good enough and the Cobra is there when I need to feel real go power.

When (if?) I finally get around to putting the 6 speed in the 840 it will keep the 3.64 gears and may even get louder too

On the V12 manuals I've heard of some owners complaining that even with a 3.15 that 1st was too short. Personally I've always felt that 3.45 was about optimal for a good balance.

I was surprised to find that the 840ci Sport didn't have a shorter final-drive ratio than the standard model. The standard 740i E38 has a 2.93 final ratio whereas the Sport has a 3.15 final drive ratio and a higher stall-speed torque converter....they go much faster than the standard model!

It is strange that the sportingly-styled 840ci has absolutely no chance off the line against the slab-bodied E38 740i Sport......

Both myself and Wayne (Blackwell) have, and in Wayne's case had, the 3.91 diff. Neither of us thought that 1st was too short, even at the extreme end of the ratio change. Pottering around town in lightfooted D mode is not much different to before since the gears simply change with the revs applied. However IF you want you can make the car more twitchy (another scientific term), and more importantly from a standing start the car flies away from standstill as the power band is reached much more quickly and sustained for longer during hard acceleration.

Whilst there are theories to dismiss the modification, I am talking solely from personal experience (as I have it actually fitted). If my life changed to involve a lot of cross country travelling, at high cruising speeds, then I would be the first to change back, BUT since 90% of my driving is on twisties or around town then I am more than happy to keep my faster version of the car. Not sure that a small change in ratio would be noticeable or even desireable due to the costs involved doing it, but looking at this thread and all the other ones regarding ratio changing, there is plenty of info for you to decide which side of the fence you would prefer to sit on

Since I now use a paddle shifter in S mode, the ratio change is even better. The revs are higher, and the power is on tap more. Seriously, my car even with a standard engine is no slouch now, and only supercars have a chance of embarrasing me