The distance from the start of the Turbine, the point where it starts to turn it, to the point where the pulses of gas conjoin.
The Cross-pipe should be fitted just before the rear wheels and should be a pipe at right angles to the pipework not forming an 'X' but forming an 'H' the 2 longer 'legs' of the 'H' being the 2 main pipes that lead to the rear 'boxes, the cross pipe, the new pipe is parallel with the rear axle line like the rung of a ladder. The position is not too critical but at least 18" from any 'muffler'.