M3 3.0 diff into 1996 318is

[mrklynch]

Hi guys, new to this and need some good info and advice please, I have found a m3 3.0 for breaking and the guy offered me the whole rear end, ie diff, drives etc.,, now i just need to know is this a good swap to my 1996 318is 1.9? what gains or dis-advantages would i be getting? I googled it but it got confusing to me, i know the diff of my own 318 gear ratio is different to the m3 diff but i am confused as to what changes it would make, can someone please fill me in on everything that i would need to know.

Thanks Guys

[KenC]

What's the ratio on the M3 diff?  I know it's not as steep as the 4.10, but I'm not sure what the ratio is. Regardless, you're going to have slower acceleration and lower RPMs at high speeds in 5th.

[deekay]

didn't e36m42 cars come with something taller than a 4.10? i thought they had a 1:1 5th...

[DesktopDave]

I'm pretty sure the USA M3 had a medium case 3.15LSD but I'm told the Euro M3 has a large case & I'm unsure of ratio.  I think it's a 3.23. It should be on the diff somewhere, on a tag or label.  If not, google it.  I found too much contradictory info though.  I'm thinking it a bit too low a ratio for your motor, but you'll get a bump in fuel economy and quieter highway running.

I found this page about it, the US e36 is mechanically the same as the Euro e36 AFAIK.  Should swap right over if you get the entire rear subframe in there.  Since you're getting the whole rear end that won't be a problem.

If you don't understand a diff, here's the skinny:  The ratio is the  number of times the input shaft (off the tranny) will spin for every  output revolution (to the wheels).  So if you have a 4.1, you'll get one  wheel rotation for every 4+ incoming.  Great for little motors like  ours, it's called a 'short' diff (because it runs out of revs quickly).   The motor has a lot of mechanical advantage too, so it'll get the car  moving faster.  However, it'll eat more gas & run high RPMs on the  highway.

As you cut that ratio down ("taller" diffs), each engine rpm gets you  less advantage.  So the car gets slower.  The transmission output has to  only turn three times for each wheel revolution though, so you'll get  more top end and each gear will "last longer."

My e30 318i had an open 4.1.  It went bad so I swapped in a 3.73 LSD.  The car is noticeably slower, but I get better fuel mileage and it runs better on the highway.  The power band is perfect for US driving conditions.  70MPH (120KPH) is maybe only 3500rpm in 5th gear, right smack in the middle of the power band.

School buses are faster from a dead start though...

[deekay]

http://www.unixnerd.demon.co.uk/bmw_e36.html

looks like 3.45 for the M42 car, and 3.23 for the M3.

[mrklynch]

http://www.unixnerd.demon.co.uk/bmw_e36.html

looks like 3.45 for the M42 car, and 3.23 for the M3.

Good link deekay,

will this make a big difference to my car? Had the intentions of doing a little drifting and the odd donut, track near me and they hold a diff and drift day maybe once a month, getting tired of just being a spectator. So would this LSD be ok for me?

[DesktopDave]

If they're that close you wouldn't see much difference.  The bigger M3 diff will add a few pounds.  I don't know how well the LSD will do at drifting though...once it locks up I'll bet it'd be OK.  We don't have a whole lot of torque to play with here.

[mrklynch]

well i suppose i am just gonna have to find out the hard way, no harm in trying it, however i may think that the ratio will definetaly improve my mpg as it does tend to be in sorta high revs all the time, in 5th gear i'm 98% sure that at 60mph im at 3500rpm or very near that and always thought that to be high for a 1.9 engine,but currently doing 30mpg,, i also have a 2.0l vauxhall vectra and at 60mph in that its around 2600rpm.

[Warsteiner]

Sounds about right..... since you probably don't have an overdrive in your tranny.

~Ralph
               
Enter Gear Ratio   :  4.10   
         Width   Aspect ratio   Diameter
Enter Tire Size:   205                60                15
               
     1st    2nd    3rd    4th    5th
Ratio: 4.23 2.52    1.66    1.22    1.00
( E36 M3 ratios)               
RPM   1st   2nd   3rd   4th   5th
750   3   5   8   11   13
1000   4   7   11   15   18
1250   5   9   13   18   22
1500   6   11   16   22   27
1750   7   12   19   26   31
2000   8   14   22   29   36
2250   10   16   24   33   40
2500   11   18   27   37   45
2750   12   20   30   40   49
3000   13   21   32   44   54
3250   14   23   35   48   58
3500   15   25   38   51   63
3750   16   27   40   55   67
4000   17   28   43   59   72
4250   18   30   46   62   76
4500   19   32   49   66   81
4750   20   34   51   70   85
5000   21   36   54   73   90
5250   22   37   57   77   94
5500   23   39   59   81   99
5750   24   41   62   84   103
6000   25   43   65   88   107
6250   26   44   67   92   112
6500   28   46   70   95   116
6750   29   48   73   99   121
7000   30   50   76   103   125
7250   31   52   78   106   130

[deekay]

will this make a big difference to my car? Had the intentions of doing a little drifting and the odd donut, track near me and they hold a diff and drift day maybe once a month, getting tired of just being a spectator. So would this LSD be ok for me?

at this point we're venturing out of the area of hard numbers, into that of opinion- just so you know that's what you're getting here.

given the intent of enthusiastic use, i think you're gonna want to upgrade the clutch- track days alone would warrant it, but going to a slightly taller gear is going to put higher requirements on those parts also.

i'm not saying you have to do this right away, but if you put in the taller diff and take it to drift events, you should plan on a better-than-factory clutch before you need it (especially if this is a daily driver car).

as far as the ratio goes- like Dave said, that's not so big a change as to make the car unusable. if you're willing to take some weight/driven mass out of the car, as i've done with mine (lighter driver/passenger seats, AC/PS/clutch fan gone, rear seat gone, etc), you might not even notice the difference.

[Hodge]

For any larger case diff, you need the 6-cylinder axles, the diff itself, and 6-cylinder driveshaft too, since it is quite a bit shorter than our stock E36 168mm diffs.