BMW's M performance division has devised an exhaust manifold so clever that they've patented it. The manifold helps to reduce turbo lag, and here's how it works.
BMW's N63 engine--the 400-hp 4.4-liter twin-turbo V-8 used in the X6 and 750i--suffers from turbo lag like every other turbocharged engine in the world. Compared to other turbo engines, that lag is minimal. Compared to normally aspirated engines, though, the engine's response to the accelerator pedal is less than immediate.
Instant response has long been one of the trademarks of BMW M engines--they've historically used individual throttle bodies to minimize the delay between stepping on the gas and waking the beast. So when it came time for M to design its first turbocharged production engine, the S63, BMW's performance division set about reducing response times to levels we've never seen in turbocharged engines.
One step in accomplishing this goal was achieved by devising a turbo-lag-reducing exhaust manifold so clever that BMW has patented it. It works by ensuring that each turbo is given a burst of exhaust gas at even intervals--something that's normally not possible on a twin-turbo V-8 engine. Here's why, and a description of how BMW's manifold works.
First of all, the S63 shares the N63's reversed cylinder head orientation--both engines have their exhaust ports vent to the inside of the Vee rather than in the traditional direction, which is out to the side of the engine.
All V-8s with a cross-plane (90 degrees between throws) crankshaft have a firing order that results in two cylinders on the same bank firing in succession once every revolution or so, rather than having the power strokes equally spaced between the two banks. BMW's V-8 firing firing order is 1-5-4-8-6-3-7-2, where cylinders 1-4 are on the left side of the engine as viewed from the front of the car, and 5-8 are on the right. Shown in "L" for the left bank (cylinders 1-4) and "R" for the right bank (cylinders 5-8), the firing order looks like this:
The uneven firing (the double-Rs in the middle and the double-Ls at the end as the cycle repeats) are responsible for the burbly exhaust note that we hear from all cross-plane V-8s.
[In contrast, flat-plane V-8s, like the ones used in Ferraris, have crank throws 180' apart from another. They fire evenly between the banks, or L-R-L-R-L-R-L-R, so they don't burble. But they vviibbrraattee.]
On 'conventional' twin-turbo V-8s, including the non-M N63, each turbo is fed with the exhaust gasses from one bank of cylinders. Using BMW's firing order, let's look at what the N63's left bank turbo experiences as a function of crankshaft rotation:
|Crank pos'n||Left Turbo|
Notice how the turbo experiences the following pulse pattern:
1 0 1 (0 0) 1 0 (1 1) 0...
I put the uneven parts in parentheses for emphasis--the 0 0 interval makes that turbo skip two strokes before getting another pulse, and the 1 1 interval gives the turbo two pulses in a row. These uneven impulses make it more difficult for a turbo to achieve and sustain its operating speed.