[HBspeed]

The I6 is being used for a few apparent reasons.

1. It will have the lowest production costs. Less parts/materials than any V8

2. It will be the most efficient. Best gas mileage.

3. It makes no sense for BMW to invest even more heavily in any V8 architecture since it is going the way of the dinosaurs sooner rather than later. BMW knows the future of its premium level internal combustion engines is the I6 NOT the V8. By using a more advanced and developed version of their I6 in the M3/M4 they are investing in their future. It may cost a bit more in the short term assuming costs of just slapping the N63 in the car would be cheaper than the added I6 development costs + lower I6 production costs.

The M3/M4 will be the testbed for future I6 development and thus the increased upfront investment makes obvious sense due to its technology being passed along to their mainstream I6 engines of the next 10+ years. Knowing their future is in the I6, BMW has wisely invested in it's further development by taking advantage of the once in 7 year opportunity a new M3 allows. The development of a new halo car allows for more experimental and cutting edge tech to be pushed past the bean counters, which they are willing to allow due to the good will it will generate with the enthusiasts and the later trickle down tech to mainstream products.

It's comparable to the space program. All the massive $ expenditures by NASA over the years have amounted to little to no direct/immediate financial return. They did not result in the mining of unobtainium or bases on the Moon. However, they did result in numerous less exciting advancements across the spectrum. Tang anyone?