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Exhaust systems are a voodoo science. What works for one car may not work well for another. What will work best depends on displacement of the engine, number of cylinders, and operating rpm. Small piping does impose a restriction to flow however it keeps the velocity of the exhaust gasses high which enhances the exhaust scavenging effect. Its a trade off.
Now if you have a turbo it pretty much deadens the exhaust pulses and you want a large diameter pipe to keep back pressure low.
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