The Nismo dual exhaust (piping) will outflow a single 3" exhaust by about 15% due to greater cross sectional area. A single 3" exhaust will support 600whp.
However, it is not just the pipe diameter that you guys need to consider. The rear muffle is more than likely baffled a fair bit in order to generate a certain tone, and flow which was purposefully engineered by Nissan
There is mention of scavenging, which does not apply in this case at all. Scavenging refers to one exhaust pulse pulling exhaust gasses from the next combustions cycle chamber. This can only be achieved with the use of OEM, or aftermarket equal length headers.
With turbo applications it is a very simple rule, as the principal of operations is primarily based on pressure differentials. The turbo will spool faster if the pressure differential between inlet and outlet is greater, hence the benefit of a larger downpipe. The pre turbine pressure (depending on the kit/turbo/turbine) can easily be double the boost pressure. So if you are at 12 psi boost, your pre turbine exhaust pressure could be 25psi. If you exhaust (post turbine) builds up a resistance of 5 psi, that is a loss of efficiency.
So with a turbo system, bigger exhaust is ALWAYS better.
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