E30 Zone

As I understand it, the B34 and M90 heads are effectively the same in terms of flow capability, and therefore power. If you want to refresh a head, I would either move it to a B35 or leave the M90 head and use a cam gear to change the M90 to Motronic with some other bits (upper timing cover, valve cover, throttle, intake manifold, etc) but keep in mind that you will be converting to Motronic 1.0, which is a good bit more cantankerous than Motronic 1.3. You will also have to change the bellhousing on the transmission,as the crank position sensor runs off the flywheel on 1.0, and uses some other sensors (cold start for one) that are eliminated in 1.3.

Gains really come if you go with a B35 head and custom pistons to bump the CR backup to reasonable levels due to the improved flow characteristics of the B35. Also, don't forget that you will have to compensate with a tune to get much out of this at all, as the stock Motronic system does not have enough elasticity to deal with these changes.

So, really, the work required boils down to your parts choices: M1.0 with a B34 head for minimal gains and a lot of work, or M1.3 with a B35 head for more money (pistons, B35 head, M1.3 harness/sensors/crank pulley) and more power.