Speed vs Rpm - Question to 3.91 diff owners

[M5pilot]

Those of you with 3.91 Diffs, what rpm are you doing in 5th gear @ 100mph?

Mine is showing 4250rpm.

Using 205/55 R15's

[zaust]

4k- 4.2 sounds correct to me depends how used the box is, diff too, although it,s problay 96-97mph ish.(due to tyre profile)

[Karan]

same.... its crap.... get a 3.73 if u plan on doing mway miles.... if not then stick to 3.9 cos its super cool round windy roads

[M5pilot]

I went on the motorway yesterday on a 120 mile trip and it just revs too high!

I really would be interested to see ihow much a 3.64 LSD would affect acceleration. 3.64 is sublime on motorways with a 2.7.

3.73 LSD......find me one!

Karan,

what u using on the m5 e30? 3.64? The Hartge H36-24 used something really long but cant remember what the exact ratio was. 3.15?

[Karan]

ive got a 3.73 lsd to go on the sport........

on the iS i have put a 3.64 lsd on it already which will give 70mph in 2nd gear which is good enough

[Karan]

I went on the motorway yesterday on a 120 mile trip and it just revs too high!

I really would be interested to see ihow much a 3.64 LSD would affect acceleration. 3.64 is sublime on motorways with a 2.7.

3.73 LSD......find me one!

Karan,

what u using on the m5 e30? 3.64? The Hartge H36-24 used something really long but cant remember what the exact ratio was. 3.15?

oh yeh.... a 3.64 would kill a 2.7 cos even a 3.73 dampens acceleration

[japcarkiller]

erm . so what is the normal open dif? and what would a 325 sport have as it left the show room?
i need to learn about diffs.

[Richy325iTouring]

cant remember off hand sal but i will check on way home tonight
and im running 225/45/16

[M5pilot]

Richy, that would be great because Im also running the same tyres.

Karan,

you sureeven a 3.73 really slows down a 2.7 compared to a 3.91?

You sureits not the case of it takes longer to rev that makes it feel like your going slower? Rememeber you are picking up more speed also due to the longer gearing so you might not actually be going any slower.

2.7's should have enough torque to make any difference negligable.

[Jimbob]

2.7's should have enough torque to make any difference negligable.

Therefore, on Wednesday when I have both my 3.91 diff and BTB fitted I shall actually notice no difference in acceleration?

2.7 + 4.10 open diff @ 100mph = 5000+ LOL!!

[M5pilot]

You have a very valid point!

Im just wondering what my old Alpina would have been like with a 3.91 diff rather than the 3.64 it was fitted with. It went like the wind with a 3.64.

Oh well, I will never know!

[Simon13]

Sal i run 225/45/16's aswell and the difference between these and 15 inch bbs is minimal at m way speeds so betwwen 4 and 4.5k is about right

[Richy325iTouring]

right at 100 mine does 4500 rpm

a 70 its sat slightly over 3000 rpm
any help to you sal

[M5pilot]

Thanks Rich,

Yeah thats about right. Mine shows 4250 and 4500 sometimes @ 100mph........maybe the speedo's arent that accurate but its in the same region.

Conculsion - 3.91 diff crap for motorway driving!

Need to build a mechanism which holds 2 diffs in the car and changes them over at a press of a button. Now that would be good! Let me get on the phone to NASA and see if they can make such a thing!

[320Touring]

Thanks Rich,

Yeah thats about right. Mine shows 4250 and 4500 sometimes @ 100mph........maybe the speedo's arent that accurate but its in the same region.

Conculsion - 3.91 diff crap for motorway driving!

Need to build a mechanism which holds 2 diffs in the car and changes them over at a press of a button. Now that would be good! Let me get on the phone to NASA and see if they can make such a thing!

i'd speak to Glenn about that (he could probably do it in 15mins!!! )

as for the diffs for maotorway, the 2.97 off the 525e has to be the daddy for that

[Simon13]

it's only a little extra noise at those revs, i can't see the problem with them personally!

[Karan]

it's only a little extra noise at those revs, i can't see the problem with them personally!

its cos u cant cruise at 120 comfortably with the 3.9 as the revs are too high...u can with a 3.73

[Simon13]

but were can u drive at 120 in this country comfortably?!?!

[Karan]

but were can u drive at 120 in this country comfortably?!?!

i do a lot of mway miles on the m4 and u can easily do these speeds safely at night

[Simon13]

but were can u drive at 120 in this country comfortably?!?!

i do a lot of mway miles on the m4 and u can easily do these speeds safely at night

I know i've heard u go past my house!

[zaust]

but were can u drive at 120 in this country comfortably?!?!

every day, and in rush hour!!

[jitvv]

!20mph at 4.5rpm i think i must be running a 3.64 LSD not a 3.73 as i first thought.

[Wilson325i]

mine does 70mph@ 2700ish

sometimes i find myself doing 90/100mph and i dont notice it until i look on the speedo, and am like Sh!t.

[billgatese30]

i think that cabs had a differnt ratio guys

about 3.6something i think