Final drive questions

C

chrisg

Member
There must be a million questions in this but I’m rubbish with search on here - has anyone successfully moved to a 3.91 or 4.1 final drive and how on earth do they did it ? I was told that the rear diff on an x3 2.5si had the right diff and the 3.91 final drive but I’m also told that the connections onto the vehicle aren’t the same - I have a price for a new lsd with 4.1 but that’s 2500 nearly what the car is worth so I’m interested in any ideas from the forum

Oh and if anyone has some nice 18inch wheels I am on the lookout



Thanks

Chris
 
Unsure about 3.91 and 4.1, but I did go from a 3.46 to a 3.64. If you're changing ratios, all you need to do is swap the diff carrier (entire differential housing, without disassembly) from the donor car which had the ratio you want. If you're going LSD, generally companies sell just the piece inside the diff instead of the entire housing, so you have to open up your existing differential and swap the parts then reinstall. I was thinking of going that route, but apparently swapping the stuff inside the diff requires expensive specialized tools that shops have.
 
chrisg bear in that Rsipad has a Z4M and based on your original post I don't think yours is an M.

Ms all had an LSD as standard, so as well as the revised final drive ratio you'd be getting an LSD too. I'm pretty sure a few members have had a Quaife LSD fitted by Birds for less than £2,500!
 
Mr Tidy said:
chrisg bear in that Rsipad has a Z4M and based on your original post I don't think yours is an M.

Ms all had an LSD as standard, so as well as the revised final drive ratio you'd be getting an LSD too. I'm pretty sure a few members have had a Quaife LSD fitted by Birds for less than £2,500!
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Information was from the swap I did on my old non-M 😀
 
chrisg said:
There must be a million questions in this but I’m rubbish with search on here - has anyone successfully moved to a 3.91 or 4.1 final drive and how on earth do they did it ? I was told that the rear diff on an x3 2.5si had the right diff and the 3.91 final drive but I’m also told that the connections onto the vehicle aren’t the same - I have a price for a new lsd with 4.1 but that’s 2500 nearly what the car is worth so I’m interested in any ideas from the forum

Oh and if anyone has some nice 18inch wheels I am on the lookout



Thanks

Chris
Click to expand...
I went from 3.46 to 3.73 FD and it's great. Woke the car up. Paired with a N54 intake manifold and 7600rpm rev limit the car is totally different.

My LSD is a fully rebuilt and upgraded to 6 clutches from an E36 BMW, so an OEM unit. It was built and supplied in a case with a 3.73 FD for about £1500 delivered. This also meant I had my original diff I could sell once swapped. If you would like the contact details of the guy I use, let me know.
 
damocell said:
I went from 3.46 to 3.73 FD and it's great. Woke the car up. Paired with a N54 intake manifold and 7600rpm rev limit the car is totally different.

My LSD is a fully rebuilt and upgraded to 6 clutches from an E36 BMW, so an OEM unit. It was built and supplied in a case with a 3.73 FD for about £1500 delivered. This also meant I had my original diff I could sell once swapped. If you would like the contact details of the guy I use, let me know.
Click to expand...
If you could share the garage - that would be brilliant - why and n54 intake and not the 3 stage manifold ?
 
chrisg said:
If you could share the garage - that would be brilliant - why and n54 intake and not the 3 stage manifold ?
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Already had the 3Sim on the car from the factory, but wanted more top-end power and a more special feeling engine.

Happy to share the contct details, he's based in Portugal, I'll send you hos whatsapp details
 
damocell said:
I went from 3.46 to 3.73 FD and it's great. Woke the car up. Paired with a N54 intake manifold and 7600rpm rev limit the car is totally different.

My LSD is a fully rebuilt and upgraded to 6 clutches from an E36 BMW, so an OEM unit. It was built and supplied in a case with a 3.73 FD for about £1500 delivered. This also meant I had my original diff I could sell once swapped. If you would like the contact details of the guy I use, let me know.
Click to expand...

Hi, sorry to go slightly off topic @damocell are you still to add this intake and rev limit info to your build thread, or did I miss it?
 
RMB said:
Hi, sorry to go slightly off topic @damocell are you still to add this intake and rev limit info to your build thread, or did I miss it?
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Whoops, looks like I completely forgot, its been on for ages now and has completed 4 trackdays with the 7600rpm limit.

I need to update the build thread anyway as there have been quite a few changes recently.

Thanks for reminding me (y)
 
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