So then, to follow up on the harmonics thing, you'd want an exhaust with a volume that's equal to a multiple of the displacement of the engine. A few basic assumptions being that the exhaust runs more or less straight, an EP91 is about 3.7m long and the exhaust starts about 30cm back from the bumper but also drops about 30cm to get under the block. So L=3.7, V=pi*L*r^2 and you want V to be a multiple of 1.332. Solve for R gives R^2=V/(pi*L) Start plugging in multiples of 1.332: 1.332*1 would give an exhaust diameter of 3.02in. So a 3.02in pipe running the length of the car with decat would have enough capacity to expel 4 cylinders worth of gas per cycle with no drop in pressure (as long as it's all timed up right etc). Course, this could be a lot more precise if I actually went out and measured the exhaust, but should be rightish. Conclusion, 3in straight pipe on a N/A would do nicely