Anyone used a 3.91 ratio diff with an M30?

[leeparkes]

M30 brothers,
I think its time to treat myself to a slipper,im currently running a 3.64 but what would a 3.91 be like?
I know 1st gear will be all but null but i rarely use 1st anyway unless im pulling off on a steep hill.
Question is will it make it accelerate quicker than it does with the 3.64?

Oh and i rarely use the motorway so that wont be an issue

Anyone used one?

[gareth]

i have one on at the moment. top speed is 132mph according to mr tomtom (auto ecu limiter may be lower than the manual one?)

to be honest, i'd rather have a 3.64 as otherwise it's hard to get the power down and you spend too much time changing gear which is a totally unecessary excercise on a M30

i've had 3.91, 3.73 and 3.46 (small case) and the gearing felt best with the 3.46 until it went pop

[Ziggy]

IIRC 'mincedbeef' has a 3.91 in his. 3.64 is pretty much spot on IMO - I'd only want something longer if I was either aiming to increase power a significant amount or if I gave two hoots about MPG.

[Rav335uk]

Don't bother lee, not worth it imo.
I have a 3.91 slipper I want to sell as its no good for me either.
3.46 is good for cruising in a cab, but I want a 3.64 to try

[leeparkes]

to be honest, i'd rather have a 3.64 as otherwise it's hard to get the power down

This was what i was worried about.

Cheers

[eta]

A 3.91:1 dif will show a tad over 3000 rpm at 70 mph. Pulling of in second will be no problem. 1st will only be good for hill starts.

[Rav335uk]

A 3.91:1 dif will show a tad over 3000 rpm at 70 mph. Pulling of in second will be no problem. 1st will only be good for hill starts.

Are you sure you got that right 70mph on 3000rpm??

[leeparkes]

A 3.91:1 dif will show a tad over 3000 rpm at 70 mph. Pulling of in second will be no problem. 1st will only be good for hill starts.

Do you run a 3.91 with an m30?

I pull off in second with a 3.64 no problem and only use first for hill starts so there would be no change there but i was wondering if id get a tad more acceleration using a 3.91.

[Rav335uk]

You will get a "Tad" more accelartion Lee, don't know how much but you will.

[E30BeemerLad]

A 3.91:1 dif will show a tad over 3000 rpm at 70 mph. Pulling of in second will be no problem. 1st will only be good for hill starts.

Are you sure you got that right 70mph on 3000rpm??

I see 70mph at blob on 3k with a 3.91 with a M42 gearbox

[Rav335uk]

I only queried it as it doesn't sound right to me, I thought it would have been screeming closer to 3.5-4k with that diff in it.

[E30BeemerLad]

I've found in 5th gear, the speedo & rev counter pretty much run parallel to each other with a 3.91.

It is a bit busy when you cruise at 80+

[Rav335uk]

That's what i was thinking, my M30 with a 3.46 cruises at 85mph at 3000rpm

[leeparkes]

I get 75mph at 3k with the 3.64

[Rav335uk]

I didn't think there would be that much difference between the 2 diffs

[charlE30]

I used to have a 3.91 on but soon removed it as I now mostly do motorway miles and at 80mph it sits way to high up the rev range. 1st gear was completly useless and you could easily spin the wheels up in third if you suddenly planted your right foot while cruising

I'm now using a 3.07 from an E28 535 and it's fookin awsome, the gears are so long and it hits 60 in no time at all with only 1 gear change.

Have a look at http://www.unixnerd.demon.co.uk/revs.html to play around with gear ratio's and work out what's best for you. There are quite a few other ratio's from the E28's including slippers all that's required if the back plate and drive shaft flanges swopping for E30 items.

[eta]

Well 70 mph @ ~3K is what my speadsheet suggests (depending on wheel size). If you tell me your wheel size I can tell you precisely.

My M20B27 powered car using a 3.46:1 rear end pulls of effortlessly in second, an M30 with a more torque from idle will mange the same. With a 3.91:1 it will be easier. However given the very broad torque curve of the M30B35 engine and for most of the rpm range it is above 180ft.lb peaking at 225ft.lb. With this much I would agree with everyone else that going for a 3.91:1 will give you a bit more acceleration but only in third fourth and 5th as 2nd gear will run out too quickly and 1st will be useless.

This is just figured out out from looking at torque curves and knowing the weight of an E30.......

[leeparkes]

Ive done a bit of an experiment today.

I had some 14" non BMW steelies knocking about so i put them on the back (along with spacers to get them on) anyway the idea was to simulate a shorter diff with a smaller rolling radius wheel and it worked!

What a hoot!

1st & 2nd are what i expected but the way it pulls in third is phenomenal and it made a very enjoyable drive so ive decided to run a 3.91 on it till i get bored

Thanks for all your input guys

[Rav335uk]

YOU MAD MAN!!!!!!!

So what is the difference then?

[gareth]

i guess these 14" steelies also had smaller tyres on too?
i'd love to go out for a blat with some 155R13's on the back, would be hillarious! 4th gear drifting anyone?

[leeparkes]

So what is the difference then?

Not sure id like it as a daily, but for fun factor it was a good laugh!

I am tempted by the 3.91 but i can see me getting through back tyres pretty sharpish (and petrol)

Did you sell your 3.91 slipper Rav?

[leeparkes]

i guess these 14" steelies also had smaller tyres on too?

Yep, cant remember tyre size (50 or 55) i put the victim up against my 15" with 50 profile and it was clearly smaller, maybe 30mm or more.

[Rav335uk]

So what is the difference then?

Did you sell your 3.91 slipper Rav?

Amazingly, no, still have it here

[leeparkes]

Did you sell your 3.91 slipper Rav?

Amazingly, no, still have it here

You got it too high priced mate,
£250 is the going rate for a 3.91/3.64 slipper.