Getting the maximum airflow into an engine is one thing, but it’s just as crucial to get the gases out as efficiently as possible after combustion. This is exactly where an effective performance exhaust will bag you more horsepower. Basically speaking, out-and-out race exhausts are built to eliminate as much of the flow restrictions as possible, and this comes down to two considerations – design and pipe bore. A good fast-road exhaust simply offers a compromise between the extra power and practicality – considering all the helpful stuff like not making you go deaf or getting you nicked.
In an ideal performance world, most exhausts would be straight, short and restriction free. But that’s not possible on road cars without making them impractical. So, a performance exhaust manufacturer will usually follow the design of the original closely but will smooth out bends as much as possible. They’ll also take out much of the restriction caused by multiple silencers, unnecessary pipework and baffles. These will be sound-tuned so they’re not unbearable – in most cases not that much louder than standard – but, importantly, they will up the grunt.
Increasing bore is another way of increasing gas flow, but it’s a bit more complicated than making your pipe as big as possible. Well-designed performance exhausts will offer a larger diameter but only when it’s needed. On a N/A engine the scavenging effect is needed to help pull through the gases faster. In a turbocharged engine, the scavenging isn’t needed so a bigger pipe is more advantageous. Supercharged engines are somewhere in between. So there you go.
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