For decades, countless hot rodders have extolled the virtues of minimizing exhaust back pressure to increase engine performance. Conventional wisdom says that an engine’s efficiency can be improved by reducing the amount of work required to expel exhaust gases, and that efficiency improvement will increase the engine’s performance in turn. That’s inherently true, but the problem with conventional wisdom is that often boils things down to their most basic elements for the sake of simplicity, and doing so typically excludes caveats and context that tell a larger – and more accurate – story.
But before we dive into the nitty gritty, we need to take a closer look at what exhaust back pressure is. Although the concept may seem straightforward, this term is often confused or conflated with other exhaust system behaviors that also have an impact on engine performance.
“Back pressure is essentially a buildup of pressure in the exhaust system that’s caused by a restriction of flow,” explains Mark Emerson of Holley Performance. “That restriction causes the exhaust ‘pulses’ to push against the outside atmospheric pressure, and that means the engine has to work a bit harder to get those exhaust gases out of the system.”
Due to the engine’s four-stroke cycle (intake, compression, combustion, exhaust), the flow of the exhaust is never constant, even though it may seem like it is as engine speeds increase. “Picture a gas ball of heat and sound energy leaving a cylinder once the exhaust valve opens,” Emerson continues. “Because the engine is cycling through that sequence of events, it’s not a constant rush of exhaust gases flowing out of the system. And in order to maintain efficiency and performance within the engine, you have to maintain the velocity of those exhaust pulses as they pass through the exhaust system.”
Restrictions can be caused by things like kinks in the exhaust system tubing or constrictive muffler designs, and these restrictions slow down the velocity of those gases as they pass through the exhaust system. That reduction of velocity soon creates a situation that’s akin to a “backlog” of exhaust gases in the system, which basically forces the engine to “push” those exhaust gases out, thus creating more work for the engine.
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Emerson tells us that in order to eek out all of the engine’s efficiency, the scavenging effect of the exhaust system must be optimized.
“Scavenging is essentially a pulling effect that happens between the exhaust pulses. There’s a change in pressure between the exhaust pulses; ideally, you’ll have a low-pressure area between one exhaust pulse and the next. And when you have that low pressure area between the pulses, it helps pull the next ball of exhaust gas out, which means the engine doesn’t have to do extra work to push those exhaust gases out. So you’re getting all of that exhaust gas out as efficiently as possible, and you get a complete air and fuel charge coming in for the intake.”
Exhaust restrictions that create back pressure will negatively impact that scavenging effect, but Emerson points out that this scavenging effect can actually hurt performance if it’s taken too far. “For example, if you have a camshaft that has a lot overlap and a super-efficient scavenging setup, it can hinder that efficiency. If the system over-scavenges, it’s going to start pulling intake charge into the exhaust stream, and we don’t want that either.”
The relationship between exhaust back pressure, scavenging, and engine performance gets trickier when we start to look at specific use-cases. While the presence of back pressure will never improve an engine’s efficiency, there are situations where it can yield a benefit.
“This typically comes into play in situations where people install an exhaust system that’s too large for the application,” says Emerson. “If you remove all restrictions by replacing a two-inch exhaust with a four-inch exhaust, it might be fine for wide-open throttle performance. But with a street car, the reality is that the vast majority of driving is done at lower RPM. And because of that, the exhaust pulses will lose heat and sound energy – which slows down the exhaust pulse velocity – when they’re in a pipe that’s too large for the engine. That situation has helped create this false notion that switching to smaller diameter pipes is adding back pressure which is improving performance. But what you’re really doing is just sizing things correctly, and the pulses are maintaining the velocity needed to keep things moving at the right pace.”
While oversized tubing might not negatively impact the engine at the top end of the powerband, the lack of proper exhaust pulse velocity at lower engine speeds can result in sluggish throttle response and other performance issues. Ultimately, the exhaust system needs to be dialed in for the engine’s specific requirements, and that’s where engine size and power, vehicle weight, use-case, and a multitude of other factors come into play. “You’re essentially ‘tuning’ the exhaust system for the requirements of the engine to maximize performance across the RPM range,” he says.
Generally speaking, this tuning balance is achieved by minimizing restrictions and optimizing the exhaust tubing size for the engine in question. “Stock-style mufflers, catalytic converters, and any odd exhaust routing in the system can add restrictions. So to minimize that, you’d want to use the appropriate exhaust tubing size for the application and avoid exhaust system routing that has a lot of sharp bends. The straighter you can get it, the easier it’s going to be for the exhaust pulse to travel. Using performance mufflers that with a straight-through design – or a well-designed chambered muffler – can help reduce restrictions over an OE-style system as well.”
Tube sizing is also highly dependent on how the vehicle is going to be used. “If it’s an all-out, high-horsepower drag car, you’d lean on the bigger side for the exhaust tubing because the engine is going to be operating at higher RPM most of the time,” Emerson adds. “But if it’s a street car, it’s beneficial to be a little more conversative on the tubing size because the engine is going to spend more time in the lower RPM range, and still you want to maintain the proper exhaust pulse velocity at those engine speeds. It’s like choosing a carburetor – you’re tuning for the needs of a specific application. Just because one is bigger than another doesn’t mean it’s going to be the better option.”
