Monday, October 12, 2009

Swapping your Differential: Serious Performance

A few weeks ago I had the good fortune to help a fellow Bimmerhead add some pep to his car. This time it was in the form of a 3.38 differential swap and Hotchkis sway bars. I have covered the sway bars in subsequent blogs and on the forums so I will focus on the differential swap.

Right off the bat I can say that this was not a hard DIY. Yea, I know, you are thinking "yea, right". But it was truly an easy process. All you need to do is take your time and be mindful of what you are doing and you will be fine. I found a couple DIY's on the forums and read them over and over. I looked at the diagrams on and even crawled under my car for a look around. When I was confident I had done all the pre-DIY research I could do, I waited for the day to come. My friend arrived (came to Bham from Atlanta) with his new differential he sourced from an automatic car that was identical to his car (2001 330i). I found out later it wasn't exactly like his car, but more on that later.

I began by removing his rear sway bar, I did this because I was swapping them anyways. I guess you don't have to do this, but it is only a few extra bolts and with it out of the way, I had all the room I needed to work. Next, I unbolted the lower subframe brace (two bolts) and the heat shield. I also took off the two exhaust hangers so the muffler could hang down. Then it was just a matter of removing the driveshaft bolts and the half shaft bolts (you will need an external torx head for this size E12). Once I had all that done I just removed the 3 bolts that held the actual diff to the subframe and it falls out with a little "persuasion". I am not going into great detail because others already have. The best DIY is found here. It gives you more specifics. We had to remove the outer flanges because they are different. In my case I used a few prybars and a hammer to get them out. They come out easier than you'd think and then the new one's press right back on to the new diff housing.

Here are the little "tips" I picked up along the way:
1. Have a friend help you. Without one, it is next to impossible to lift the new differential into place. It is heavy and akward and I crushed my finger trying to be "he-man".
2. Make sure to do your research so you don't end up with a different input flange (where the driveshaft connects to the differential) because they are different on the 330 than the 323/325.
3. Change the fluid of the new/used diff BEFORE putting it in the car. It was so easy to just tip the replacement unit on it's side to drain it and then just pour in the new fluid. No need to use a pump this way and was so easy. Failure to do this could cause you headache. A recent post I saw where a guy did the swap and he assumed the new unit had fluid in it....and it didn't. Can you say boom?
4. Replace the differential bushing while you are in there. Think ahead......we didn't. Also, if you are going to do a new set of control arms, now would be the ideal time since the differential is out of your way.

Impressions of the swap are simply WOW! What a hoss. It really transformed 1st and 2nd gear (5 speed car) into a tire shredding fun zone. The 3.38 was a good choice IMO over the 3.46 because it is still liveable on the interstate. 70MPH = just over 3000rpm's. If you have a 6 speed (and shouldn't we all) you can certainly do a 3.46 with no problem at all. All in all I think the swap was well worth it. It only took me about 4 hours total (including rear sway bar) and that was my first time doing it and I was under the car while on jack stands. This is NOT a hard job and I would say if you can do brakes or VANOS, you can do this. So if you want a peppy ride, a diff swap is the next best thing to forced induction! Hopefully one day, I can document that process on my own car!

1 comment:

  1. There are various online sources to provide you informative details on this topics, but this is one is very helpful.
    San Jose Auto Repair