The specification decided by engine bench testing & actual racing.

Engine bench testing & actual racing
Exhaust manifolds manufactured by TODA racing are used in competition. Ideal lengths, diameters, and shapes are determined using the feedback data from racing and continual bench testing.
Ideal 4-2-1 design 
For ultimate power the 4-1 type exhaust manifold design is the one to use. The problem is that power is only made from 4,500 rpm in the process removing drivability. TODA racing have designed 4-2-1 system which restores drivability 15 % more torque* with only 1.5% loss of power*.
*TODA 4-1 bench testing manifold used for comparison.
As used on all the latest 4-cylinder racing engines the 4-2-1 gives best result.