Diff swaps - talk to me

[Mondo]

Afternoon team,

I'm thinking of a little extra poke from my 2004 3.0 and I'm wondering about replacing the diff for one with a "better" ratio for acceleration. I'm not bothered about losing some top end, which is inevitable anyway.

Are there any BMW diffs that are "plug and play" for my car? If not, what's likely to be involved please?

Cheers.

 

[Darkangelv2]

You currently have a 3.07 final drive (assuming you're car is manual).

If you find a diff from the automatic 3.0 Litre, you'd have a 3.46 final drive. Should be relatively easy to find via breakers/ebay.

PS. That's full auto, NOT SMG

 

[Mondo]

So the higher the figure, the better for what I want it for?

Would the diff unit from a 3.0 auto bolt straight into my chassis?

 

[Ed Doe]

All 3.0si manual coupes had 3.46 fd - part number 33107564160.
The 3.0si auto had a 3.64, part number 33107514198.

If you want I have a 3.46 fd spare in my garage, as I had a quaife fitted a while back. It's been kept in oil in a plastic bag so should be perfectly servicable - bit of beer money and you're welcome to it if you want

 

[Mondo]

That's very generous of you, thanks!

Would I be right in assuming that this is the drive itself, rather than a complete housing? So I'd have to have my existing final drive removed from the casing and replaced with this one?

 

[Ed Doe]

Scratch that dude, I'm sorry they obviously would have used the final drive to bolt to the quaife unit - apologies!

 

[Darkangelv2]

So the higher the figure, the better for what I want it for?

Would the diff unit from a 3.0 auto bolt straight into my chassis?

Yes, you are best sourcing an entire unit and yes it will just bolt in.

Rough rule of thumb I was taught, the % change in final drive will equate to the % feel of bhp increase and the speed % decrease.

So, as an example your 3.07 to 3.46 swap

231 bhp + 12.7% = 260bhp
70mph @ 3000rpm - 12.7% = 61.1mph

Speed and RPM you get from your car is a complete guess, but as you can see you’d need to increase RPM significantly to do 70 again.

There is a 3.38 unit that might be worth going for as 3.46 is quite a jump and will need lots of revs for the motorway. I believe it was used on the 2.2

 

[Darkangelv2]

https://www.ebay.co.uk/itm/264519237421

£300 and comes with a warranty

 

[Wookster78]

I changed mine from a 3.07 to a 3.23, whilst some may say not huge I can definitely notice the difference, more urgent and responsive and bit more torque, but the benefit is that the top end is not really effected or screaming in the Rev range, got it for £70
on eBay and refurbed it myself. Came from a 2.5 auto i believe, straight swap