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Anything bolted to your exhaust port that changes the direction of exhaust flow and restricts the exhaust gasses from expanding freely causes back pressure, even tuned headers. This back pressure can be minimized with proper design but always exists. The headers provide scavenging in a specific RPM range which is seen by the motor as reduced back pressure. Scavenging is produced by the expansion of gasses when the pipes join. This expansion produces a momentary pressure reduction and sends a low pressure sonic pulse back up the pipe. In order for scavenging to occur, this pulse must arrive back at the exhaust valve at the exact time the valve is opening. This timing factor is why scavenging is RPM dependent.
This is also why the guy in the Cobalt is not getting any scavenging from his "straight" pipe. Even if his pipe was straight off the manifold and not curved to fit under his car, his exhaust gasses would lose so much energy before reaching the end that any pressure pulse would be minimal and would not be able to travel back to the motor even if it could decide which cylinder to scavenge.
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