Ok, here's what you need to do. Find yourself a GOOD reputable machine shop that can bore an engine and do it WELL. You have them bore the block to 2995 ccs.
You need a good shop because the cylinder walls will end up being quite thin. I recomend getting some lightweight forged pistons that are aftermarket for the 3.0L M3. Then, you will need a 3.0L crank, again go aftermarket with a nice lightweight one. Unless you're planning on turboing the engine.
Now, I haven't been able to find much information on the head and valves of the non-vanos vs the M3, BUT the 2.5L non-vanos has larger valves than the 93+. To save money, you could have a nice shop just do a nice port and polish job on the head and possibly just bore them out, but if you're going to have them machine it would probably be better and more cost effective to just but a different head.
For cams, the exaust swaps to the intake, and you buy a normal M3 exaust cam.
Now, if you get the engine all built up and slap in on the computer and start it up, if it runs it will probably run like shit. I'd say have somebody like Bavauto, Tuner, etc make you a custom chip for this specific aplication.
I've found very little online other than this however. For a rebuild of this type this info is quite vague so anyone else with some sites to see?
__________________ Kevin (President TBEA)
"WAAAAAAARRRRRIORS, come out to plaaaaay."
the most cost efficient way to go about getting 3 litres out of a 92 M50B25 is to bore out the block to 86mm which alone would yield around 2.7L, and would allow you to use factory S50B30 pistons. then to get 3.0L, use a factory 3L crank (85.8mm stroke) and rods. you cant use factory M3 cams since vanos cams cannot be used in non-vanos motors, but you can buy aftermarket cams with identical performance to the M3's.
in the end your converting your 2.5l (75mm stroke, 84mm bore) to a 3.0L (85.8mm stroke, 86mm bore). you can use a 3.2L crank instead of a 3.0L if you wish to achieve 3.1L. you cant safely bore out the block beyond 86mm on a non-vanos engine, or 85.5mm on a vanos engine.
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