318iS revs for speed with 3.91 LSD???

[MikeeMiracle]

Hi

I have just had a 3.91 LSD fitted and the acceleration has suffered quite a bit together with the gears feeling much longer. I was wondering if I could compare revs/speed as I feel I might have got a 3.73 LSD instead.

Previously with the standard 4.1 diff I was getting....

1st @ 6krpm = 30mph
2nd @ 6krpm = 50mph
3rd @ 6krpm = 80mph

70mph in 5th = 3200rpm
80mph in 5th = 3600rpm
90mph in 5th = 4000rpm

Im currently getting....

2nd @ 6krpm = 60mph
3rd @ 6krpm = 90mph
4th @ 6krpm = 120mph

70mph in 5th = 2800rpm
80mph in 5th = 3200rpm
90mph in 5th = 3600rpm
100mph in 5th = 4000rpm

Does this sound right? I was expecting the revs/speed to be affected but adding an extra 10mph to each gear seems a touch high.

Thanks

[kevin316i]

Going from a 4.10 ratio diff to a 3.91 ratio diff means about a 4% reduction in revs at a given speed. While this sounds very little, it can be noticeable when driving. I had a 316i and put a 3.91 ratio LSD into it and did notice a difference. I decided not to put it into the 318iS for that reason. If I decide to hold onto the iS long term, then I'll start looking for a 4.10 or 4.27 ratio LSD.

However, you should balance having a 3.91 LSD against a 4.10 open diff.

[Brianmoooore]

Definitely not a 3.9. Your figures suggest it could even be a 3.64.

[MikeeMiracle]

Definitely not a 3.9. Your figures suggest it could even be a 3.64.

That's what im afraid of also. It's not just a touch slower, it's very noticably slower, especially in 1st and 2nd gears. Beforehand it would push you back into the seat much more.

[MikeeMiracle]

Been out down some twisty country roads and you can tell when the LSD kicks in so overall im happy with the purchase.

Anyways, bump

[Bob_S]

I put a 4.1 on my 316i and noticed it wasnt as quick off the mark but the economy went through the roof lol!

[E30Mark]

You have definitely got a 3.64 diff fitted!

Take your 3rd gear speeds

3rd @ 6krpm = 80mph (4.1 diff)
3rd @ 6krpm = 90mph (unknown diff)

divide 80 by 90 = 0.888 * 4.1 (original diff) = 3.64 (new diff ratio)

Just think about the economy !

[stuartgallafant]

i put a 3.91 lsd in my iS, i didnt really notice a difference with the accelration, certainly not enough to make me complain. and its less revvy on the motoway, so an all round winner for me

[dark_sounds]

the motorway miles im racking up is annoying me, as the IS ratio is way to high for motor ways tbh, i think i need a 3.91, will pay for itself tbh two meets a month, 120-150 miles on the motor way a month for the meets, it will save me a few pennies!

[Brianmoooore]

I went down just one ratio on my wife's 325 touring because a) the original suddenly started to whine MUCH louder, and b) to improve fuel economy a bit.
Had a much bigger effect on performance than I thought it would. Easily noticable.

[MikeeMiracle]

i put a 3.91 lsd in my iS, i didnt really notice a difference with the accelration, certainly not enough to make me complain. and its less revvy on the motoway, so an all round winner for me

That was my plan also and what I was expecting, a slight decrease in acceleration. I have spoken to the person I got it off, he basically got it out of another car which aparently came with 3.91 as standard (tech1 sport?)

Anyway the deal was done in good faith, there was no way for the seller to know if one of the previous owners decided to upgrade to a 3.64 so I do not see how I can complain about that.

I am enjoying the benefits of having it fitted, im wondering if I can find someone with a 3.91 who possibly wants to swap. The other alternative is more power. From what I have read on here a good intake, BBTB, & chip should see me close to 150 horses which will hopefully get me back upto the speed I was before. £250 worth of mods I hear which isn't breaking the bank.

[Alean_Mofo]

"The other alternative is more power" -

I'm telling you, the answer to most problems is more POWER