They're very possibly limiting torque; it could be to preserve parts (like the tranny which may not be able to handle more than 500lb-ft. But even if they limit torque, and do so equally for both cars, I'm pretty sure the horsepower numbers should still be the same if the torque numbers are, limited or not.
The Mercedes 6.0L TT V12 has always made amazing torque. They've limited it to 738lb-ft, but unlimited it makes 888lb-ft on it's own.
Thinking about it more, I think you're correct. No idea why the numbers don't match up.
Just to clarify, all the numbers in my comparison are taken from the dyno charts provided by BMW in the brochures for each model. How accurate those dyno graphs are is a different matter alltogether...
The M5's exhaust manifold is basically a 180 degree header design. This was used on the Ford GT 40 (without turbos obviously) and got the nickname "bundle of snakes" exhaust due to the exhaust tubes criss crossing from bank to bank.
The idea was to get the bost of both worlds from the two different crankshaft profiles available to a V8 engine - 90degree (cross plane) and 180 degree (flat plane). Allmost every road car V8 engine has a 90 degree crank whereas the racing and Ferrari V8's are 180 degree. As most of you have noticed there is a big difference in the sound a V8 with a 180 degree crankshaft makes compared to the regular V8 burble of the 90 degree crank V8. That has to do with the difference in firing sequence between the two concepts.
The 180 degree header desig gives improved low and mid range torque as well as a sound much more similar to a flat plane V8 race engine. That's why the M5 sounds so different to the X5/6M I guess.
Most modern V-8s have what is known as 90-degree cranks (aka crucified cranks, bent cranks, cross-plane cranks) wherein the cylinder firing order is such that two cylinders on each bank fire consecutively as opposed to alternating banks. The purpose of the 90-degree crank is to minimize vibrations inherent to “flat-plane” cranks. Some racing and high performance engines including Ferraris use a flat-plane crank to optimize the intake and exhaust processes, accepting the additional vibrations. The problem is that for a conventional 4-1 exhaust header, the separation between two firing (or exhaust) impulses is 90 crank degrees in each collector versus 180 crank degrees for a flat crank engine.This gives the engine that inherent “American V-8” exhaust burble, but also causes a “crowded” condition in the collector when two exhaust pulses, separated by only 90 crank degrees are trying to exit through the collector.To accommodate this, the collector for a 90-degree crank motor must be larger than for an “all-other-things-being-equal” flat-crank motor, thereby losing some low and mid-range torque.The “bundle of snakes” design collects two tubes from each bank into a single 4-1 collector resulting in four evenly spaced exhaust pulses as in the flat-crank header design.A broad power band is inherent to the 180 degree header design due to the smaller collector size. This was a great idea, and I am sure contributed to the success of the GT40 “back-in-the-day.”
"First of all, the turbo plumbing both on the intake and exhaust side have been significantly enlarged relative to the previously seen S63 such that engine breathing has been improved and back pressure has been reduced. The cross cylinder bank headers have themselves been increased in diameter for better breathing. The intercoolers are much larger and more pronounced under the hood. Boost pressure has been raised from 1.3 to 1.6 bar.
The valvetronic system has been heavily tuned and modified for the M5 such that turbo lag has been further reduced, and efficiency has been improved. On the test bench at full load the M5 engine shows a 30% improvement in efficiency over the S63 found in the X M cars while making the same power output."