S50B30 diff ratio recommendations ?

Post Mon Dec 28, 2015 7:13 pm

Well after the last two summers running my E30 with a S50B30 I feel the gearing is not quite right for the road use I do. I've used the standard 5 speed gearbox and had the diff rebuilt and changed from 3.91 to 3.25 to get closer to the E36 M3 set up which was 3.15 if I remember correctly. I also run 17" wheels now after upgrading the brakes too.
At around 60 mph and in 5 the gear the engine is around 2650 rpm and up at 70 mph (max speed limit of course!!!!) it's around 3000rpm plus which is kinda noisy but overtakes easily in top gear ! And I'm not one to go on about fuel economy but it would be nice to make the fuel go a bit further.....
So I just wondered before i try these online diff calculators what everyone else is running and what's tried and tested ? I can have the diff changed again to 3.15 which should be exactly how a E36 M3 but it's starting to get expensive so want to get it spot on this time.

Post Mon Dec 28, 2015 9:55 pm

Intreasting, I'm just about to start this conversion myself and the 3:25 ratio was the one I was going to go for. Should be intreasting to hear people's feedback on these. what kind of top speed are you getting? How much roughly for a Lsd as I will be needing one soon.

Lee

Post Wed Dec 30, 2015 1:44 am

Hmm I'm going 3.64 but have 6 speed box and an audi tdi to do long distance driving.

Depends what you want. If you want less revs then 3.15 any less than that and you will ruin it.

Post Wed Dec 30, 2015 8:44 am

Yeh just really need to get the revs lower so it's better all round. Was hoping someone has or had the same running gear.
I probably will end up using the 3.15 gear and hopefully that will do the trick. My LSD was refurbished so also makes sense to stick with it and have the work done. Iv seen used LSDs from £400 upwards and you don't know what conditions it's in till it's opened up.....

Post Wed Dec 30, 2015 10:06 am

I ran a 3.15 (from a Z3M) on my S50B30 with the 5spd. I didn't run 17"s but depending on your tyre size, the rolling radius will be about the same as the 205/55/15s.
For a road car it was perfect, that's probably why BMW used a 3.15 on the 3.0 M3 lol

Post Wed Dec 30, 2015 10:56 am

Perfect that's just the sort of feedback I was hoping for! I don't want to compromise acceleration but we go to a few car shows in the summer and motorways are unbearable and just end up in the slow lane to keep the revs down.

Post Wed Dec 30, 2015 11:14 am

Revs wise it was just like a 3.0l M3, the E30 weighs a bit less so it picks up a bit better.
From memory there was someone on here who ran a 2.93 and he said that was fine as well.

Post Wed Dec 30, 2015 7:53 pm

3:15 Sounds good to me James just been looking at the Kaaz lsds on drift works website like toadstar said you could Pay £400+ and it could be scrap. The Kaaz ones are £860

Post Wed Dec 30, 2015 8:09 pm

Tyre size will be making the difference, E36 used a 245/40 rear tyre, I doubt you have anything similar on your E30?

Post Wed Dec 30, 2015 8:19 pm

No Dan I have no wheels atm a˜† I will get the diff last after I pick my wheels I suppose was thinking 17"did you get my pm?

Post Wed Dec 30, 2015 8:21 pm

Yes not long replied

Post Wed Dec 30, 2015 8:22 pm

Good point Dan. I can't remember exactly and the cars tucked away right now but I'm sure it's on 205/40/17

Post Wed Dec 30, 2015 8:25 pm

toadster987 wrote:Good point Dan. I can't remember exactly and the cars tucked away right now but I'm sure it's on 205/40/17

You are probably losing about 10mph from that

Post Wed Dec 30, 2015 9:42 pm

going from 3.25 to 3.15 is a 3% change, if you were at 2650@60 then you'd be at 2570rpm after the change.

do you really think you will be able to notice a 3% change?

if you want to use less fuel just exercise more control of the right foot, and yes im being serious

Post Wed Dec 30, 2015 9:53 pm

Haha so either diff will do thanks

Post Wed Dec 30, 2015 10:58 pm

I had an S50 with a 5 speed box & 3.15 lsd, although i did not attempt top speed ( so can't comment), in 5th gear on the motorway it was still pretty lively. I have since swopped to a 6 speed box still with the 3.15, and would say its the best of both worlds, fast low down acceleration ( well pretty much any gear) combined with a civilised quiet cruiser capable of a higher top speed, ( only down side is they weigh quite a bit more).

Post Tue Jan 19, 2016 9:52 pm

Ok so getting back onto this now and after taking onboard everything mentioned above particularly tyres, I'm now leaning towards 3.07 to be in the safe side and make up for the difference between the e36 245/45/17.... The guy selling also has a 2.93 spool available but I'm thinking this could be a step too far perhaps ? Not had much success with the online calculators so just wondered if anyone had anything to add ?

Post Thu Jan 21, 2016 5:40 pm

reggid wrote:going from 3.25 to 3.15 is a 3% change, if you were at 2650@60 then you'd be at 2570rpm after the change.

do you really think you will be able to notice a 3% change?

That's exactly what I was thinking, the difference will be tiny.

Post Thu Jan 21, 2016 6:13 pm

It certainly makes it crystal clear they way Reggid has worked it out. Hopefully he can do the same for the 3.07 and the 2.93 before I take the plunge.

Post Tue Mar 15, 2016 1:06 pm

For what it's worth I'd a 3:45 medium case on mine. As it was mainly used on track so fuel economy was not an issue, but acceleration was not maybe as good as it could have been

Post Tue Mar 15, 2016 1:26 pm

Thanks I'm thinking of leaving the 3.25 in for now or maybe until I come across a 6 speed box anyway.