I just so happens that I have an entire E30 suspension in pieces in my garage, including a Korman adjustable subframe and some nice M3 and Gruppe N stuff (a friend's car). I'd been meaning to compare parts. This discussion prompted me to getting around to it. I went under my ti (a '96 Sport) so that I could make some direct comparisons.

Note that I made comparison's visually by holding the parts side by side. I also made some measurements with a tape measure and a dial caliper. While I feel my observations are accurate, it would be a good idea to verify the interchangability from another source.

The subframes appear virtually identical. The mounting points and all of the critical dimensions are the same. The only differences I could see were in some of the small brackets for holding the ABS and wear indicator wiring connectors.

I also noted that BMW changed the access on the body to the brake line connections and subframe mounts. These were both buried on the E30 and quite difficult to get access to. On the ti they were all out in the open (happy! happy! joy! joy!).

The rear stabilizer bars have a slightly different shape. The bushing mounts to the body look stronger on the ti (these were a problem area for breakage on the E30 with aftermarket bars). The bar on the ti passes behind the differential instead of over it like the E30. This means the differential doesn't have to be lowered when changing the bar on the ti (more joy!). The attachment points for the stabilizer bar links on the ti's trailing arms are still the same flimsy little tabs. These are fine for the stock bars. Stiffer aftermarket bars almost always cause these tabs to break over time. The only way I know to reliably reinforce these tabs is to weld in some small reinforcements at right angles. (The bolt-on plates that come with many aftermarket bars aren't sufficient).

The trailing arms look the same. Their length and position were identical. I remember hearing somewhere that the angle of the bearing carrier at the end of the arm *might* be different. It looked the same to me, but I didn't have an accurate way of measuring it with the arms still bolted on my ti. If they are different, it's possible that they might be useful for changing the camber and toe settings in the rear.

The upper shock mounts looked the same. These were a weak point on E30's. A common trick is to use the mounts from an E30 convertible, which are stronger. Stiffer still (and obscenely expensive) are the Gruppe N mounts from the E30 M3 EVO II. These could be useful for people running stiff shocks. (If the shock is stiffer than the mounting bushing, then the bushing flexes instead of the shock dissipating the energy. This is an extremely common problem on many types of cars with 'performance' shocks retrofitted. Stiffer bushings will actually make the ride smoother with these types of shocks because the energy will be properly transferred into the shock.)

On the topic of bushings, The Gruppe N bushings for the subframe mounts, trailing arms, front control arms (at the rear of the arm), and differential mount all look like they will fit. These are the stiffest rubber bushings available from BMW. It's really good to know that these are available for the ti for people that need them. They are a great alternative to polyurethane, because they are more reliable and need less maintenance, while almost as good at helping to keep things from twisting out of position. I just wish they didn't cost so much. (My friend's car has a highly modified M20 that is dramatically more powerful than stock. We were having problems with the rear trailing arms and subframe moving way out of position under power due to bushing flex. Maybe someday I'll have to concern myself with similar issues on my ti :-) ).

The differential housings looked the same. (The E30 318's had a smaller differential housing). The output flanges on the ti were smaller. I suspect that the flanges would be interchangeable by popping them out, but I haven't confirmed this. The input flanges had the same bolt spacing, but the ti's flange was a continuous ring, as opposed to the E30's individual fingers. I couldn't tell for sure if they would bolt up the same or not. Again, if they don't, it's possible that the input flange could be exchanged.

The E30's CV joints were also bigger (to match up with the larger flanges). This suggests that the E30 driveshafts (and differential output flanges) would be an easy way to strengthen the ti's drivetrain, for people that have managed to pump up the torque. (I probably would leave them alone, unless I was seeing failures).

Something that surprised me was that the E30's front control arms looked the same as the ti's. The ti's arms have the outer ball joint isolated by a rubber mount, the E30's arms have a solid ball joint. The E30 M3's arms were aluminum to reduce unsprung weight (the other E30 arms, and the ti have steel arms). I think the E36 M3 also uses solid outer ball joints. I don't know if the E36 M3 arms are aluminum - this should be checked out.

Both M3's have the front stabilizer bar links connected directly to the struts. Other E30's and E36's have the links connected to the control arms, which is less direct. The ti could be converted to the E36 M3 style struts and links, but getting the correct calibration for the stabilizer bar and struts could be an issue.

I've read that the 323ti (and the Z3 2.8 and M roadster) use longer rear trailing arms. I don't know if the subframes are different. I don't know exactly how all of the dimensions have changed, so I can't say exactly how they would affect handling. If somebody has some more specific information about the differences, I'd appreciate hearing about them. (Maybe someday I can get a peek under a Z3 2.8). Again, even if the changes are desirable, getting things calibrated correctly for the U.S. ti could be an issue.

On the subject of fitting limited slip differentials to ASC + T equipped ti's, there shouldn't be any fundamental compatibility issues. The two should complement each other very well. I'm sure that there wouldn't be a problem with opening up the ti's differential housing and swapping just the differential gearset.

I was surprised to see on the parts fiche for the U.S. that there are six differential assembly's for the ti. Three for the manual's (all 3.45:1) and three for the automatic's (all 4.44:1). The three version were as follows: open (for non ASC+T cars), limited slip, and open (for ASC+T cars).

Why are there two open versions!? I haven't found anybody that can tell me what the difference is. There shouldn't be any mechanical difference whatsoever. My guess is that there might be a difference in the sensor for the speedometer (perhaps ASC+T requires a higher resolution). At this point this is purely speculation on my part. This really needs a better answer.

I've talked to people who have taken ASC+T BMW's and opened up the original differential assembly to replace the differential inside with both factory disk type units and Quaife's, so it's definitely doable. Especially for people that want to go to a factory disk type LSD, it would be much easier and cheaper to swap the whole assembly from another car. I've heard second and third hand of people doing it this way, but I would like to hear more directly.

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I want to mention that I restricted my above comments to the feasibility of interchanging these parts. I almost completely left out the pros and cons of these changes, and why they may or may not be desirable for certain people. That will have to wait for later.

Brian Brown

Additional references:
Twitching Rear Suspension