Originally Posted by JCL

If you consider that the 3.0 twin turbo uses the same technology as the 4.4 litre, the V8 is actually detuned from the inline 6. The 3.0 is rated at 300 hp, but appears to be making about 320 based on chassis dynos of stock vehicles. Even using 300 hp as a base, for the same specific output (hp/litre) it would give 440 hp in the V8. There are lots of chips or maps available for the 3.0 that are adding 60 hp simply by changing boost parameters and fuel maps. Even the Dinan ECU flash for the 3.0 is getting into that range, probably a little lower than 60 hp.

Factor all that together, and it would be easy to get 500 hp from a 4.4 twin turbo, completely smooth, same redline, same exhaust, same air intake. Play with exhaust and air intake, and 550 hp will be available in a few months from introduction, at the same redline, IMO. Increasing the engine redline is unlikely unless the turbos are also changed, as they are small and limited by maximum turbo speed.

The 400 hp rating is BMW conservatism, while they produce a few and get some real world data. Expect even the factory version to climb significantly in hp a year after introduction. This also fits with the marketing department concerns; BMW officially rated the 3.0 tt 300 hp which kept it well under the M3 rating, and it will help if the 4.4 tt is kept under the M5 ratings until the turbocharged V10 is in production.