On the N/A motor, too large of a dia. pipe will reduce backpressure, but in doiing so will also reduce exhaust gas velocity, which is what is important to making power in a N/A motor. This is why there are X-pipes, to create a scavenging affect. The scavenging effects creates a negative pressure at the crossing of the Xpipe which sucks the exhaust gas and increases exhaust velocity, making more power and torque.
In contrast, on a FI motor, The larger the pipe, the better, and the least backpressure is whats important.
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