Here is what one of my contacts inside Rahal-Letterman racing found out this morning...
The M3 GTR that races in the ALMS GT2 class, uses a P65 4.0-liter engine provided by BMW Motorsport without further modification.
This engine has been homologated by the ACO for use in ALMS GT2 class, and it is using a BMW Motorsport devised flat-plane crankshaft and revised firing order of 1-8-3-6-4-5-2-7.
The 'street' M3's S65 has a firing order of 1-5-4-8-7-2-6-3, and uses a cross-plane crankshaft.
The bore and stroke are the same as the S65 production engine (92.0 x 75.2) for a total displacement of 3999.2 cc. The engine also uses the same double vanos and throttle by wire systems as the S65.
The exhaust system is provided by BMW Motorsport, and uses a small muffler on each side to meet the ACO/ALMS sound pressure rules (no more than 112 dbA measured 15 meters from the edge of the track). The headers and exhaust system are customized for the GT2 car. (proprietary info, no details)
The P65 engine uses a proprietary engine management computer, and the software is constantly tweaked by BMW Motorsport engineers based on the track and the atmospheric conditions.
The P65 engine sounds unique due the combination of a flat plane crank, revised firing order of the cylinders, and the exhaust modifications.
The best is yet to come...