# 3.15 diff swap complete



## 31st330i (Jan 11, 2002)

finnished up the swap this morning. I was able to complete the swap by myself with a floor jack and jackstands. floor jack was also used to lower and raise the diffs. worked well. I recommend taking out the sway bar. I had to remove my muffler (which was cut and now held on with BMW clamps - works well, actually) so it was easy to get the sway bar out. if you have a bolt on muffler, lucky you. I replaced the oil with RedLine. input and output flanges had to be swapped. I marked the input and output flanges then spun the input flange as many turns as it took to make the output shafts spin once to confirm an old ratio of 2.93 and a new ratio of 3.15.

road test: okay so it's a subtle difference at first but I suspect it will grow on me. then again, the last time I drove the car it was cold out so it was running particularly well and today it's warm out. the car does feel quicker. I need more time with it, though, to know how much I really like it. cruising on the highway doesn't add an appreciable amount of engine noise to register on my radar and I'm picky.


----------



## Tanin (Dec 21, 2001)

31st330i said:


> finnished up the swap this morning. I was able to complete the swap by myself with a floor jack and jackstands. floor jack was also used to lower and raise the diffs. worked well. I recommend taking out the sway bar. I had to remove my muffler (which was cut and now held on with BMW clamps - works well, actually) so it was easy to get the sway bar out. if you have a bolt on muffler, lucky you. I replaced the oil with RedLine. input and output flanges had to be swapped. I marked the input and output flanges then spun the input flange as many turns as it took to make the output shafts spin once to confirm an old ratio of 2.93 and a new ratio of 3.15.
> 
> road test: okay so it's a subtle difference at first but I suspect it will grow on me. then again, the last time I drove the car it was cold out so it was running particularly well and today it's warm out. the car does feel quicker. I need more time with it, though, to know how much I really like it. cruising on the highway doesn't add an appreciable amount of engine noise to register on my radar and I'm picky.


Very interested to hear your impressions after more time behind the wheel. Please update this thread when you have a chance.

Thanks


----------



## The Other Tom (Sep 28, 2002)

Why did you only go to 3.15 ? Wouldn't a 3.23 or 3.46 be better (and more noticable ) ?


----------



## SpaceMonkey (Mar 13, 2002)

The Other Tom said:


> Why did you only go to 3.15 ? Wouldn't a 3.23 or 3.46 be better (and more noticable ) ?


And I'm also assuming it's another open diff, and not limited slip...


----------



## flashinthepan (Jul 25, 2003)

Cool !

I am curious as well, let us know your observations as they come in focus. Naturally also if you would want a lower ratio etc...


----------



## KP (Apr 16, 2002)

Nice, how long did it take? I'm planning to switch to redline as well. Getting smoother shifts? Looking forward to the review.


----------



## 31st330i (Jan 11, 2002)

The Other Tom said:


> Why did you only go to 3.15 ? Wouldn't a 3.23 or 3.46 be better (and more noticable ) ?


because I didn't want to go too low and increase RPM/noise at cruising speeds etc. and because from searching the archives here and at phanatics, the 3.15 seemed to be the best choice.


----------



## 31st330i (Jan 11, 2002)

SpaceMonkey said:


> And I'm also assuming it's another open diff, and not limited slip...


correct. didn't want ot pay the extra for a LSD. I think thatputs the price of this upgrade over $2K.


----------



## 31st330i (Jan 11, 2002)

KP said:


> Nice, how long did it take? I'm planning to switch to redline as well. Getting smoother shifts? Looking forward to the review.


having been a mechanic in a past life (haven't lifted a wrench proffesionally since ~1991), I tend to not waste much time and rip right through the task at hand. I haven't had much time so I had to do this piecemeal over the course of a few days. maybe I have 4 or so hours into it total.


----------



## 31st330i (Jan 11, 2002)

I was able to spend a little more time driving the car today. haven't been able to drive it much since the upgrade. anyway, I got to drive the car today a little more with the new diff. the does indeed feel faster and the upgrade is growing on me. I stll need some more time with it to be sure. I need to go blast my favorite canyon road.


----------



## Raffi (May 18, 2002)

Did you have any issues with the speedo sensor, as others have when swapping out diffs on E46s?


----------



## Jspeed (Dec 23, 2001)

The Other Tom said:


> Why did you only go to 3.15 ? Wouldn't a 3.23 or 3.46 be better (and more noticable ) ?


In addition to the higher cruising engine rpm, with too short of a differential, you'll also be doing a lot more shifting. For example, 3.46 is 18% shorter than 2.93. This means the maximum speed in each gear would be reduced by 18%.

e.g. Going from 0 - 60 mph would take 3 gears instead of 2, suppose in slow city traffic you normally use 4 gears but now you'll need all 5, etc.


----------



## 31st330i (Jan 11, 2002)

Raffi said:


> Did you have any issues with the speedo sensor, as others have when swapping out diffs on E46s?


I haven't read or heard about these problems and I did search the archives here and at phanatics before performing this upgrade. What sort of problems. Kaz mentioned something but it didn't make sense.

What sort of problems should I be having?


----------



## 31st330i (Jan 11, 2002)

Jspeed said:


> In addition to the higher cruising engine rpm, with too short of a differential, you'll also be doing a lot more shifting. For example, 3.46 is 18% shorter than 2.93. This means the maximum speed in each gear would be reduced by 18%.
> 
> e.g. Going from 0 - 60 mph would take 3 gears instead of 2, suppose in slow city traffic you normally use 4 gears but now you'll need all 5, etc.


yeah. I think 3.15 is about right. not sure I would go any lower. maybe a 3.23. as it is I can feel that the 3.15 is low.


----------



## Kaz (Dec 21, 2001)

31st330i said:


> I haven't read or heard about these problems and I did search the archives here and at phanatics before performing this upgrade. What sort of problems. Kaz mentioned something but it didn't make sense.
> 
> What sort of problems should I be having?


Well, I got this from the info that was posted earlier about diff swaps in the Z3s. The sensor and sensor wheel are different, so it would either not connect up to the car without swapping, or the readings would be wrong, I forget which. If BMW decided to do more cost-cutting and do this without part incompatibility, then great.


----------



## 31st330i (Jan 11, 2002)

Kaz said:


> Well, I got this from the info that was posted earlier about diff swaps in the Z3s. The sensor and sensor wheel are different, so it would either not connect up to the car without swapping, or the readings would be wrong, I forget which. If BMW decided to do more cost-cutting and do this without part incompatibility, then great.


but there's no sensor on the differential as there was say on my old e30's. perhaps they're using one or more of the ABS wheel sensors to detect speed. and that wouldn't change anything since it is measured at the wheels (after the diff anyway).


----------



## Mr Paddle.Shift (Dec 19, 2001)

Aye..haven't been reading the forums for a while...

anyway, great job 31st330i! Any pics of your install process?


----------



## 31st330i (Jan 11, 2002)

Mr Paddle.Shift said:


> Aye..haven't been reading the forums for a while...
> 
> anyway, great job 31st330i! Any pics of your install process?


nope. no time. I wanted to do a full write up for my web page but no time.


----------



## Raffi (May 18, 2002)

Kaz said:


> Well, I got this from the info that was posted earlier about diff swaps in the Z3s. The sensor and sensor wheel are different, so it would either not connect up to the car without swapping, or the readings would be wrong, I forget which. If BMW decided to do more cost-cutting and do this without part incompatibility, then great.


That's what I was referring to. IIRC, Ron Stygar posted about it when he did his diff swap in his Z.


----------



## 31st330i (Jan 11, 2002)

Raffi said:


> That's what I was referring to. IIRC, Ron Stygar posted about it when he did his diff swap in his Z.


so where is the sensor on the Z? is it in the diff like E30's? if so, then it's not a problem for us since there is no sensor in the dif on our cars.


----------

