doesn't matter what you get... a 318 will be harder to install as it will take some custom welding to be done along with new mounts. any 6 cylinder will work but you'll have to strip the motor down to just a 32 longblock and then use all of the parts from the current engine to make it work on models from 92-95. so basically, convert the engine to obd1 and then get a custom chip to make the timings work properly so the engine will be like a 94/95 m3. on obd2 cars it will basically go into place and not much would have to be changed. might need to get your car sharked with the info for an m3 but that should be it. I did a lot of research on this crap and this is what I gathered. personally, if I were you, I'd get a 95 325i and do the conversion... you'll add 20+ more horse to the engine doing this.
I would personally after doing some research... if you have an obd1 engine you can do the conversion why not? more horse and torque with older parts on a m52 engine or s52. and people like Bill Arnold's BMW say doing this conversino to s52 motor is WELL worth it.
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