If you’re considering replacing your stock exhaust system with an aftermarket setup or adding a performance exhaust to a muscle car or off-road build, it’s important to think about which exhaust tubing diameter will work best for your ride. The right exhaust system can unlock additional horsepower and torque by optimizing engine performance, as well as providing a more appealing tailpipe sound. However, bigger is not always better, as two important factors play a role in choosing the ideal tube diameter.
Right-Sized, Not Oversized
First, you need to match the pipe size to the power output of the engine. It’s all about optimizing the flow of exhaust gas pulses. When the pipe is correctly sized, you maintain good pulse velocity, which helps create a scavenging effect that pulls the exhaust pulse completely out of the cylinder. This, in turn, helps draw in a solid intake charge and makes the engine as efficient as possible. It’s important to note that pipe length also plays a role in exhaust scavenging.
If the pipe is too narrow, the restriction causes backpressure – resistance to the desired flow – which will negatively affect overall engine performance.
On the other hand, if the pipe diameter is too large, the exhaust pulses will dissipate heat energy too quickly, cool down, and slow down.
“When people go too big, too soon, without matching the pipe diameter to the power output, it results in a loss of torque and poor throttle response,” explains Mark Emerson, Holley’s business unit leader for exhaust. “It’s because the pulses are slowing down too quickly. But it also depends on the vehicle and how it’s being used. If it’s a drag race vehicle, you can get away with a larger pipe because you’re typically running at higher RPMs, where pulse velocity is at its maximum. For street cars, you typically want to be a little more conservative. And if it’s a custom build, again, you’re just matching it to the power output of the engine.”
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Emerson adds that most street cars can increase pipe size by at least half an inch over stock, even though OEMs typically fit larger exhausts now than in the past to improve performance. However, even with a similar pipe size, there are still performance benefits to be had from a less-restrictive performance muffler that will also aid scavenging and provide a better exhaust sound.
He also notes that for modern vehicles, there is no need to go much larger than stock unless heavy modifications have been made to other engine components, especially with a cat-back system, as the front half of the system – including the manifolds and catalytic converters – remains stock.
For classic cars or race cars, a larger diameter may provide better performance, but once again, the exhaust pipe diameter should be matched to the engine output. The table below shows examples of how pipe size might increase with horsepower in a race application:
