Tangent about an Eventful Differential Swap!

[Rsipad]

Hi everyone,

I just wanted to share my experience on swapping out an E85 differential. This is not a guide (I'm just bored and want to share a story ).

For a few months, I've been wanting to swap my 3.07 differential out for a 3.64 from an Auto 2.5i. After doing some research on this forum, I went to a local scrapyard which was advertising a 3.64 diff with 130k miles for 350 USD. When I went in person, and they had a 74k mile 3.64 (my car has 76k miles so its an upgrade!) and I was able to get the price down to 200 USD, so I bought it.



My friend has an NA Miata and has taken his differential out to weld it in the past, so I contacted him. A few days later and the day comes to put the new differential in the Z4.

Here's what we expected:
6 hours max
3.07 out
3.64 in
job done, go get ice cream (105 degrees F that day)

In reality:
10 hours
3.46 (huh?) out
3.64 in
put everything back together
exhuast leak (blown gasket)
fix leak, put everything back together
job done, go home and sleep



So I saw a thread previously on this forum which said that 3.0i manual e85s had a 3.07 ratio. However, it turns out that after 2005, Z4 3.0i manuals have the 3.46 ratio. After going back to that thread I saw one key thing: this was for 2003-05 3.0i's (I'm an idiot). So, instead of the change in torque being about 19%, it was much lower, about 6-7%

It has been 5 days since we did the differential swap and since then I've gone on backroads and shopping runs. The car definitely feels more powerful and is a lot more prone to oversteer during hard cornering and way more wheelspin off the line. I have noticed a change in gearing on the mountainous backroad as where I would normally be in 4th, I'm now mostly in 5th. Overall, I'm glad that the DIFFerence (sorry) was not as big as I thought it would be as I would be off the side of a cliff by now. However, the difference is still big enough to make it hard to resist the urge to go sideways every time I pull out of a parking lot :lol:

Recap: 2006-2008 BMW Z4 3.0i manuals have a 3.46 differential ratio not a 3.07 ratio. An idiot accidentally makes a major torque gain to a minor improvement, but doesn't regret their mistake.

 

[brillomaster]

good stuff! i remember many moons ago swapping a 2.93 diff to a 3.38 diff in the back of an E36 track car... that was a very noticeable improvement.

given how easy it is to get hold of different ratio diffs, i think its a very worthwhile mod for cars driven hard.

 

[plenty]

Nice post and congrats on the successful swap. Due to the need to hit fuel-efficiency reporting targets most modern cars are far too high geared for proper driving on twisty roads, so a diff swap makes sense if you can. No such option exists for the M-diff, sadly, so all I can do is run 35-section rear tyres instead othe recommended 40.

My Impreza which sits at 3,800 revs at 70 in top gear is a revelation on the road.

 

[Mr Tidy]

There must be a way to do it.

I've read of a couple of M owners on here who have fitted a lower-geared differential to their cars. Here's a link to a topic Ed Doe posted on - his signature says he has a 4.1 Final Drive:- https://z4-forum.com/forum/viewtopic.php?t=102312&start=150

Good to know the OP is happy with is anyway.