The OBDI manifold came on the M-50 2.5 liter and S-50 3.0 liter motors from '93 to '95. While the OBDII manifold came on the M-52 2.8 liter and S-52 3.2 liter motors from '96 to '99 in the e-36 sedans. In the e-36/7 Z3, it only came with the OBD II motor. So by this change in the rules the sedan 328 can use either manifold, while the Z3 is limited to the OBD II manifold. The OBD II manifold is the main difference between the M-50 and M-52, same head, cams, valves, even the throttle body, and yet the M-52 is 300cc larger and makes the same peak HP at 600 less rpm.
I decided to just swap to the 2.5 to get the better manifold, and also to keep the weight down. But if someone wanted to build one, it seems like it deserves a spec line exception, because the sedan can swap between manifolds.
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