The Myth of the Bottleneck May 03 2017

Before I ever cracked open a fluid dynamics book or performed flow analysis in a professional setting, I falsely believed that if you were to improve the flow of a system, you would always be limited by the most restrictive feature in the system so that improving the flow anywhere besides the most restrictive feature would be fruitless. So in other words, if your muffler was the most restrictive element of your exhaust system, deleting your catalytic converter wouldn't yield any gains. This idea of a bottleneck was reinforced by internet "experts".

What actually happens, is that every part of the system restricts the flow in a cumulative manner. When dealing with flow inside pipes or ducts, it is a difference in pressure between the inlet and the outlet that drives the flow of the fluid. Each element in the system creates a pressure drop, and they all add up. The more the pressure drops, the less pressure there is to drive the fluid through the pipes, and the flow decreases.

Diagram showing gauges installed upstream and downstream of a catalytic converter. The downstream gauge will show less pressure than the upstream gauge.

This hypothetical illustration shows an exhaust system, with pressure gauges mounted along the way. In this example, the catalytic converter is responsible for a 5 psi drop in pressure. Now, there is only 25 psi remaining from the original 30 psi to drive flow through the catback pipe and muffler.

Diagram shown with catalytic converter removed and replaced with a straight pipe. The upstream and downstream gauges read virtually the same.

With a hypothetical catalytic converter delete, now there is 30 psi to drive flow through the catback pipe and muffler (assuming negligible losses through the straight pipe). Intuitively, we can see that a 30 psi differential across the muffler will flow more than a 25 psi differential across the muffler. Even though the muffler is the most restrictive element, improving other elements in the exhaust system still yields better flow.

For those of you familiar with electrical circuits, fluids behave in very similar ways as electrons. An analogy is that pressure is like voltage, restrictive plumbing elements are like resistors, and flow is like current. If you put several resistors in series, reducing the resistance of any of the resistors will result in more current.

There is a shred of half-truth to the bottleneck myth. If one element is extremely restrictive, to the point where flow slows dramatically, improving other elements would yield very small gains. However, this is almost never the case with exhaust or intake components. So don't write off improving flow anywhere in the intake or exhaust plumbing just because there is another element that is more restrictive. 

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