This Is How Performance Exhaust Systems Increase Engine Power

A well-designed performance exhaust system can increase engine power by up to about three percent.

After a flashy set of wheels, there's probably nothing that gearheads wanting to customize their rides love more than the throaty roar of a performance exhaust system. Not only does the exhaust note add character to the car, fiddling with something that promises to unleash hidden engine power is almost mystical.

However, exhaust design is a more dynamic science than stringing together a few pipes and tacking on some loud mufflers and fancy exhaust tips.

In days gone by a performance exhaust system was commonly referred to as a "free-flow" exhaust, with some tuners believing that the best systems were those that did not restrict the flow of exhaust gasses. We now know that this only plays a small role in unlocking horsepower in most engines.

Tuned-length header pipes, often with tortuous routing, that actually scavenge exhaust gases from the combustion chamber are the best performance exhaust systems, for naturally aspirated engines. Turbo-charged engines have their own set of rules.

Tuning an exhaust system to a given application is a case-by-case challenge. The displacement, exhaust valve size, induction system, cam profile, exhaust port design, and RPM range all factor into deciding what form the exhaust system should take. General rules of thumb are easy to grasp, but applying them correctly is where things get tricky.

With the engine running, high- and low-pressure "waves" are manipulated to extract the burned gases from the combustion chamber. To assist in achieving the most beneficial pressure balance between the intake and the exhaust a properly designed header set will exploit two different scavenging mechanisms: Inertial and wave scavenging.

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Inertial scavenging is created by the inertia of the gases being dispelled from the combustion chamber into the header primary tube when the exhaust valve opens.

According to performance exhaust specialists Hooker Headers, the optimum exhaust gas speed for the best power delivery is approximately 300 feet per second, so that if an engine has a 36-inch primary tube, it would take 1/100 of a second for the exhaust "pulse" to pass through the tube.

Even with the exhaust valve closed, the gas, because of its inertia, still continues to move down the exhaust tube at 300 feet per second. However, as the cooling gas loses energy the speed also begins to taper off. This is part of the rationale for placing the turbo-charger as close to the exhaust valve as possible in forced induction engines – as can be seen in the various "Hot-V" engines making their appearance.

Behind this high-pressure pulse is an area of low pressure that continues to expand as the gas moves further away from the valve. Once the last of this exhaust gas pulse reaches the end of the primary tube, all the spent gas in that particular header will be at the same low pressure.

Race-engine constructor, Reher-Morrison Racing Engines, designs its performance-orientated systems based on the relationship between the diameter of the primary header tube and the exhaust-gas velocity.

Key to this is selecting a tube diameter that balances the free-flowing performance of large diameter tubes with the superior scavenging of small, high-velocity headers. The optimum diameters normally range from 1-3/4-inches to 1-7/8-inches for smaller, low-performance engines up to 2-3/8-inches tubes for large displacement, high-power engines.

What is more, by varying the length of the header, it is possible to manipulate the time it takes for the low-pressure area behind the wave to reach the header collector, which is what "tuning" header tubes are all about.

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Unlike inertial scavenging, wave scavenging does not depend on the physical movement of the exhaust gases. Instead, it relies on the pressure of the (sound) wave created by the opening and closing of the exhaust valve.

On opening, a pressure wave begins to travel away from the valve at a speed in excess of 18,500 inches per second - faster than the speed of sound, which at sea level is approximately 13,397 in/sec

Following the same theory that governs the movement of sound waves in an organ pipe, when the sonic wave arrives at the end of the primary tube, a negative shock wave is generated, that is reflected back toward the exhaust port. By tuning the length of the header so that this negative pressure wave arrives while the valve is open, spent gases will be scavenged from the combustion chamber.

If this low-pressure area spills into another primary tube for a cylinder where the exhaust valve is just starting to open, that low-pressure area will help to pull exhaust gases from that cylinder. Here, the internal combustion engine will gain an advantage, because there is less residual exhaust gas remaining in the combustion chamber, which can foul the incoming charge of fresh fuel and air.

This tuned length is specific to a given frequency directly governed by the engine speed. As a result, a tuned header will only increase engine performance over a relatively narrow rev-range, possibly over a couple of different speed bands, and as in an organ pipe, the higher the speed/ frequency the shorter the pipe. Most racing engines work best with primary tubes between 28 and 30 inches long.

In the mystical world of engine tuning where improvements are often measured in single figures, the two to three percent increase in engine power that a performance exhaust system typically unlocks cannot be ignored.

As an engineer with over 40 years of experience in the automotive industry Peter Els sums himself up in one sentence: "Automotive engineer by profession, gearhead by choice." Working from his home on the East Coast of South Africa, Peter spends most of his time analyzing and writing about cars and the exciting technologies driving the future of mobility. He shares his findings, opinions, and experience in several online publications, including a monthly column on the highly respected Automotive IQ portal, articles on FutureCar, Robotics Business Review, and product reviews on the Car Fix Book. When not writing about cars Peter is an avid motorsport follower. Having raced motorcycles for 10 years he still enjoys track-time - although now, on four wheels rather than two.