**Everything you need to know about differentials**

In light of recent noob activity, and as a follow up to the popular eBay headers thread...I shall now discuss differentials.

I can't take credit for most of this information because it was posted on sycE46's extremely handy website (one that I've reference many times). He has created a great site, and I give him a ton of props for it. However, I will attempt to create a more concise version for this site so it's easily accessible via searches. So no more excuses.

Ready? Here we go.

What is a differential? Do I gain horsepower by swapping it?
A differential is what distributes power to your drive-wheels from the engine/tranny, via a driveshaft. You don't gain any horsepower from swapping to a higher ratio, but you do change when the power is put down. Through torque multiplication, it feels like you have more power, when in reality, you're just changing when you put it down. So it feels like you gained torque, but you're just getting the power more towards the low end. Read this if you want a good description.

Well sweet gee willickers Ken! I have an E46, but I is too fail to find what diff ratio I have. Can I findz outs?
Yes, you can findz out.

330 Non-ZHP
Steptronic: 3.38
Manual: 2.93 (3.07 convertible)

330 ZHP
Steptronic: 3.64
Manual: 3.07

325
Steptronic: 3.46
Manual: 3.15 (3.46 convertible/touring)

328
Steptronic: 3.46
Manual: 2.93

323
Steptronic: 3.46
Manual: 3.07 (3.46 convertible/touring)

M3
Manual: 3.62 LSD


Cool beans Kenneth! Now...are all differentials between E46s interchangable? What other BMWs can I pull a diff out of and plop into my car?
Why, excellent question young man! All E46 diffs are interchangable except for the M3 diff, which will require that you swap the rear subframe to even bolt up. You can also pull a differential out of a Z4 or an X3.


Yay! Knowledge! Now, where can I find a differential? I looked it up at the dealer, and gosh! They are expensive! What can I do?
Do not buy it new. You have two real options, A) look on eBay and hope for the best, or B) go onto www.car-part.com and do a search of junkyards in the U.S. You will have to call them and order it (most junkyards have very...unfriendly employees, but it's worth it). You can usually find a diff anywhere from $200-$450. Any more and you're wasting your money/time.


Ken, I have a manual transmission, I want more peppyness, what are most people swapping into their cars?
3.38's and 3.46's are very popular to swap into manuals because it yields a difference, and doesn't destroy your gas mileage too badly. You can pull a 3.38 out of a steptronic 330, whereas the more widely available 3.46 (therefore more popular) can come out of a steptronic 325, steptronic 323, steptronic 328, steptronic 3.0 Z4, or manual 2.5 Z4.


Well poopsticks, what if I have a steptronic? What can I swap in that will make a difference since I already come with a 3.38 or 3.46?
You have two real options to feel any sort of gain on an auto, and it is definitely one of the best things you can do to get some more pep out of your slushbox. One would be to pull the 3.64 out of the steptronic ZHP, steptronic Z4, or steptronic 3.0 X3. Another would be to use the 3.91 out of the 2.5 X3. I did the 3.91 swap, and while it does take some work, it is worth it. Here is another thread covering that.


Oh noes, I want both my rear tires to do the spinny dance. What if I want an LSD on my non-M? What can I do?
Either you spend a ton of money to get the M3 rear subframe and fit the 3.62 out of the M into your car, or you spend a ton of money on a custom built diff from Quiafe or Diffsonline. Either way, you're going to spend a ton of money.


This is all fine and dandy, but what about installation?
A typical shop will charge you anywhere from $150-$300 to install it. You can do it yourself with a buddy if you feel mechanically inclined. Here is a DIY.


What kind of stinky poo smell fluid do I need to use?
Synthetic 75w-90 gear oil. I recommend Redline, you can get it from Turner Motorsport. You will need 2 bottles, and new seal rings (BMW part# 07119963355). You can find a DIY here.


I think that covers the basics, if I missed anything, please feel free to add to this thread.

This is cool

kpeng for mod

Oh and congrats on joining EP

Originally Posted by tblue3cibm

Oh and congrats on joining EP

Thank you good sir.

Thank you, this was a long-overdue thread.

cool...thanks for posting

nice write up like always!

hip hip hooray!

Actually my 5spd 323 rear diff is a 3.46 as it is a convertible and most likely some of the other cic models differ from their coupe/sedan brethren.

Shoot, I forgot to cover the convertible differences! Thanks Genghis! Fixed now.

Thank you for sparing us from further rear differential threads!!!

gee willikers! cool beans!, poopsticks!

do the spinny dance..

when you ask yourself these questions you make it sound like

big ghey al..

and 5speeds please stay away from a "super duper" short ratio.

as much as bmw wants u to save gas and give out performance.. it affects your daily drivability down to the way u you switch cogs during traffic and easier wheel spin on lower gears that can potentially kill you. since your not spreading the power over the gears but dumping onto the diffs.

last thing i want is kids/noobs with super duper short ratios barrel roll-ing off the roads due to traction issues.

MachRc, just keeping with the theme of the other thread.

I think a 5 speed would be fine with the 3.46 or 3.38, I didn't recommend they go 3.64 or 3.91

Gag me with a spoon..you forgot that the Tourings use the 3.46 straight from the factory..

Originally Posted by Arsevader

Gag me with a spoon..you forgot that the Tourings use the 3.46 straight from the factory..

Doh again. Fixed.

Sorry, I was still blinded by rage while writing that. Thanks to you and Genghis for catching the misses.

Originally Posted by kpeng

Doh again. Fixed.

Sorry, I was still blinded by rage while writing that. Thanks to you and Genghis for catching the misses.

No problemo. You should call the thread, 'What's the Diff?"

Can I use the 3.91 out of the 2.5 X3 for my 2001 E39 auto 525i?

Originally Posted by Jason5driver

Can I use the 3.91 out of the 2.5 X3 for my 2001 E39 auto 525i?

No, the E39's use a different differential/subframe. The reason you can use the X3 diff on an E46 is because they share the same subframe.

ken, you used to bug me... now the only thing that bugs me is when i read through a funny thread that you haven't posted in

kpeng for mod