E30 Zone

I've got an 318iS and im thinking of putting an LSD on it. Only thing is though I hear it will shorten the gear ratio's. Im kinda already unhappy at having to do 4000rpm in 5th @ 90mph. Im currently getting roughly the following:

90mph = 4000rpm
80mph = 3600rpm
70mph = 3200rpm

Can anyone tell me what this will change to if I fit one of the usual 3.64 LSD's?


Many Thanks

a taller final drive will reduce your rpms, but the M42 is not the torqueist engine around so accel will suffer dude

you have a 4.1:1 now ? ( check tag on backplate )
a 3.91:1 is a common LSD, and would offer a good soloution , 3.73:1( one in classifeds btw ) would split the differance quite well

the 3.64:1 is best avoided this side of a 2.5 though

all my opinions only BTW, debate time

Ona 3.64, something like :

90mph = 3700rpm
80mph = 3300rpm
70mph = 2900rpm

Ant wrote:a taller final drive will reduce your rpms, but the M42 is not the torqueist engine around so accel will suffer dude

Now im confused

Thge lower the number the taller the final drive? I thought it was the other way around.

3.64 menas 3.64 turns of the prop for every turn of the road wheels

I have the same problem feel it sucks to be doing such high revs in 5th gear while just crusieing.

Want 3900 at 100mph in top? I have just the LSD for you!!!

3900 @ 100mph - isnt the accel really poor ?? my track car is doin about 6k revs at 110mph but the accel is scary in 1st - in fact i never use first !!! it is all a bit confusing as its a 325 with M3 / sport dogleg box and an LSD of unknown ratio. if i change the ratio will the acceln suffer ? i struggle to do more than 115mph on a long straight but not sure if thats cause of the diff or the size of my testicles !!!

JCB wrote: 3900 @ 100mph - isnt the accel really poor ?? my track car is doin about 6k revs at 110mph but the accel is scary in 1st - in fact i never use first !!! it is all a bit confusing as its a 325 with M3 / sport dogleg box and an LSD of unknown ratio. if i change the ratio will the acceln suffer ? i struggle to do more than 115mph on a long straight but not sure if thats cause of the diff or the size of my testicles !!!

With that gearbox you would normally run a ratio of 3.25.
You've no doubt got the original diff for a 2.5 which on a standard 2.5 is 3.73. Not unless you've changed it?

You should go for 3.64, might be better mate


As for the IS, id go with what Ant said, it's trial and error as i have gone through the same process and have the right combo now

i presume it is stmaped ont he casing somewhere ?? the car is a track car and is changed a lot from standard. i bought it in its preset form so dont really know what has been put on there. it is an LSD (was the standard 325 an LSD ??) i will get underneath and take a peak - where do i look ??. if i do go from a 3.73 to a 3.64 will the accel drop be noticeable ??

If you've got high accelleration then i doubt it's even 3.73!
I'd say 3.91 LSD.
You can most of the time find a tag on the back plate of the diff that will tell you the ratio, this way you can choose what ratio you will need.
It also depend's what sort of track you are going on as the diff you have now maybe suitible for that track. I go with a 3.73 if it ain't already got it?

Hmmm....lemme know if you have a 3.91 diff for sale

dog leg boxes require a 3.25:1 or 3.15:1 to give the correct gearing, same applies to M52/52 swops using the e36/34/39 gearbox

thats said, its not all about doing 150 is it?? limiter on top @ 120 would suck though

Chris-W wrote:Want 3900 at 100mph in top? I have just the LSD for you!!!

Whats that one then ?

3.45.

What will that give me ?

Roughly, depending on wheel size :

Mph Rpm
30 1154
40 1539
50 1923
60 2308
70 2693
80 3077
90 3462
100 3846
110 4231
120 4616
130 5000
140 5385
150 5770

yeah but find me a track wher i can do more than 120 anyway. the longest straights i have found are silverstone and donnington. not many cars that weigh a tonne with 190ish bhp will ever get above 120 before needed to hit the brakes. even a turbo nutter barge scooby only opened up a gap of 50yrds on the straight at donnington, then i caught him on the chicane. i reckon having the sideways inducing acceln is more use on a track (the car is not road legal so no need to worryabout characteristics on the road). going for a setup with longer legs would loose me acceln wouldnt it ?? would it be enough to notice ? if i lost acceln then there would be no point in being able to get to 150mph as i would never have enough acceln to get there anyway ?? (please feel free to correc tme if i am mis-understanding here !) cheers,jonathan

Quite right - as a general rule your gearing for a given track should be such that you rev out in top just before the braking point at the end of the back / longest straight.

the other point i need to remember is that the peak torque & BHP happen about 700rpm before the limiter kicks in so not much point reving beyond 6k as it will not achieve anything. as i have never hit the limiter in 5th yet i guess my diff is ok for most tracks. i wish someone would event a variable ratio diff with electronically selectable ratios so i can change it depending on the track !!! LOL

jonbuoy wrote:If you've got high accelleration then i doubt it's even 3.73!
I'd say 3.91 LSD.
You can most of the time find a tag on the back plate of the diff that will tell you the ratio, this way you can choose what ratio you will need.
It also depend's what sort of track you are going on as the diff you have now maybe suitible for that track. I go with a 3.73 if it ain't already got it?

Exactly what i said! Stick with what you've got mate, you will no doubt get disapointed if you swap and change and then see that scooby losing you!

There is an old saying mate, 'Don't fix what don't need fixing'

fair point !!! at the moment the main concern is getting the engine working again after my catastrophic failure at croft !!! the head is coming off tomorrow then i will know the full extend of the damage. i am guessing its gonna be bad as i took the rocker cover off and found two broken cam followers and the end of the plug was missing and had shrapnel welded to it !!!

Shattered plug? "Electrical problems" in F1 speak

i am more honest - the engine is fuuked !! got a spare one though thanks to "argos" !!