Ask The Editors: How Does Exhaust Sound Affect Power?

We call this a pipe and it definitely makes music.

They aren’t pulling your leg. Engineers and tuners alike go to great lengths to design exhaust systems that capitalize on sound wave distribution to make the most out of a piece of necessary hardware.

How this works is actually kind of interesting. The sound waves are accompanied by pulses of pressure. Easy enough, right? What we want is a system that cancels out big pressure spikes – not only because we don’t want to be blowing out the ears of bystanders but also because big spikes of pressure can deflect back into the engine. Imagine if for a moment you had a pipe that immediately took a 90° bend – a majority of the energy blasting down that pipe would be bounced right back into the engine. On the other hand, if it went straight out, you would be inviting massive pressure spikes because there would be nothing to cancel them out.

A well tuned pipe will make the most of the fact that high and low pressure from the exhaust note travel in opposite directions. The low is where we get back pressure – so what we want is a design that keeps this back pressure from entering the cylinder right when the intake and exhaust valves are both open. How does one trap this back pressure until the right moment arrives? With bends or in some cases a resonance chamber (that little canister you sometimes see atop the head pipe or, oftentimes, a bulge in the pipe itself). That’s just an empty space that gives the low pressure somewhere to go, stalling out its return trip to the engine.

Where it really gets interesting is that a tuner can use deflection from the bends in the pipe or a resonance chamber to bounce back pressure so that it cancels out (or lessens) the next pulse being sent out. That’s how back pressure works to quiet a machine.

It may sound complex and, realistically, it is a science but the bottom line is those sound waves being expelled carry with them a lot of energy. We want a situation where that energy isn’t working against the rhythm of engine and a good pipe maker understands exactly how to form an exhaust system to make that happen.

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