3.91 LSD Diff

840?
Thought your mpg figures were a bit low.

I had a 3.45 in my 850 and when we drove to Sweden this year a friend with a stock 3.15 diff was in the group and we compared consumption after about 700km of country roads and motorways at speed from 60-120kph.
I had 13,7l/100km (21,6mpg UK), and I believe he had 12 or 12.1l/100km (23.3mpg UK).
I do have a supercharger so the cars are not directly comparable as such, but I have recently fitted a 3.15 LSD to my car, calculations indicates an RPM drop of about 200rpm at 100kph(60mph) and that should have an impact on consumption.
I'll be looking forward to next years Euro tour so that I can see how much it has improved.

The best I ever managed to get from the 3.45 diff before the supercharger was 11l/100km (25.6mpg UK), but my average was 17l/100km (16.6mpg UK)

In your case I think you will find that your city consumption will not have changed much, but your motorway cruise consumption will have increased quite a bit. you have after all gone from a 2.93 to a 3.91, I would not be surprised if your rpm at 60mph is 400rpm higher than before.

Edit: I ran the figures through my gear calculator.
Stock your car should have had 1850rpm at 60mph, with the new diff is up to 2460rpm, 600rpm higher.
At 100mph its 4100rpm with the new one and 3080rpm with the old one, 1000rpm higher.
Old theoretic top speed was 205mph, new is 153mph
Think your high speed consumption has increased a lot
Re-edited figures because I initially used the 4-speed not 5 speed auto gearbox ratios.

looking forward to your updates.

Blimey, I bet that leaves the line pretty smartish, I'm not surprised you got to 100MPH in a hurry!

P.S. I make the theoretical top speed @ 6200RPM 153.MPH

Nice to see that you succeed in the process Angus.

Yes, I enjoyed it also very much to swap the diff, you get a lot of nice performance...(but when it wore off, than you want to look futher and end with a supercharger???)

Timm True, I was playing around with the 4 and the 5 speed gear ratios, and during my top speed calculation I was using the 0.72 ratio of the 4 speed instead of the 0.79 ratio of the 5 speed.

153mph it is

Here's a great tool for calculating speed, rpm etc. on the E31s: http://e31.net/GearSpeed.zip

I drove the disabled swedes car with a 3.91. It was bloody quick, but not something i would fit to a daily driver. It was like driving a 6speed diesel with regards to rpm changes between gears, but with only the middle four gears to choose from.

At the time I loved the 3.91 in my Supercharged 840 but the pleasure was replaced increased noise levelsI remember 100 mph was 4000 revs
Not good

Zitat von stevep840;103637

At the time I loved the 3.91 in my Supercharged 840 but the pleasure was replaced increased noise levelsI remember 100 mph was 4000 revs
Not good

That isn't a good combination, but without a charger a good one Steve!

Zitat von DickB;103638

That isn't a good combination, but without a charger a good one Steve!

I fully understand what you mean Dick but would not advise this in a V8.

The Old Green Sc 840 would hit 60mph in 5.28 secs but was wayyyyyyyyyyyyyyy to busy under the bonnet, ie not worth it really,hence one of the reasons I sold it

Fill your boots mate , if you like it that's fine & Dandy, my comment was obviously based on experience & is my opinion on a 8 series internet forum so no real need to be silly.

I'm in with the "tick over at cruise speed" lovers on this one

But yes, we are owners, not custodians, so we can do what we like.
It doesn't matter if others have different views, as I don't think we're trying to evangelise on this forum

Zitat von anguswolfendale;103704

It is a matter of personal choice, as is any modification. I cannot see the point of louvres in the bonnet, but that is my preference. If you are doing a lot of high speed long distance cruising on the motorways/autobahn then I can agree that the low ratio standard diff gives best economy and quiet low-rev running. My infrequent drives are around country lanes, where fast cornering and the need to overtake quickly when infrequent opportunities to do so arise, are the main characteristics of the trip.

On the back roads your car is going to be 100% fun all day! I would love to have that kind of acceleration.

Thanks for updating us - it sounds as if you are happy with the new diff! As you say, cruising in the UK at 70 MPH you are not going to notice any difference in engine noise - I've often been trundling up the motorway in Sports mode for miles before I notice and drop into 5th gear.

Zitat von anguswolfendale;104637

Running Report

I thought I'd give an update on the fitting of the 3.91 LSD and CSI anti-roll bars.

Cornering:
The car is much more secure in cornering - no roll at all and the LSD means the car feels glued to the road when you accelerate through the bends. I have EHK suspension, but in Sports mode the car is really firm. I had occasion to accelerate hard from a slip road and felt a slight slip on loose gravel but no loss of grip or power.

Engine Noise and Consumption:
The previous posts were correct - 100mph = 4,000 rpm. However, I don't drive around at that speed so 70mph is around 2,750rpm. There is no discernable increase in noise. The car is so well designed in aerodynamics and insulation, there is no interruption to driving pleasure from engine noise. Even at 100mph I don't have to turn up the radio.

Previous to the mod I was getting 22.2mpg for all round driving including motorway cruising at 90mph and spirited side road driving.

Acceleration:
All I can say is WOW! The gearing creates a whole lot more torque which powers the car easily up to 100mph and beyond with no drop in acceleration. I haven't dared to top it out but I think the calculations would be correct at around 148mph. It always seemed ridiculous to have a car geared for 184mph then limit it to 150mph. It has transformed it from an autobahn "smoocher" to an exciting performance car with bags of acceleration should you need it. I know its not to everyone's taste and it's not a Ferrarri (being 2 tons in weight), and I can't afford a 850CSI - but it has made a great difference.

Gear Change:
I do notice a slightly clunky gear change when slowly moving off from a standing start and 2nd gear engages, but nothing I would class as unacceptable.

Overall:
Well, for now I am having fun and enjoying the car all the more. I've kept the original diff in case I grow bored of the acceleration and feel the £1 a day in fuel is hurting too much!

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I'm glad you are happy with the performance after the diff swap!
I know exactly how awesome it feels, as i did the same swap