Is bigger automatically better? Do catalytic converters needlessly reduce power? Does a less restrictive exhaust reduce low-end torque? Before you start modifying your car’s exhaust, you should know how it works and what places will see the most improvement.
Why Does Exhaust Have Sound?
When exhaust is pushed out of the engine by a rising piston, it creates a pressure wave: The fast-moving gas molecules from the engine slam into the slow-moving molecules in the exhaust pipe, pushing them along much like one billiard ball striking another. When unrestricted, the result is a series of loud bangs.
Exhaust manifolds and headers merge the flow of each cylinder into one or two pipes that carry exhaust gases away from the vehicle. Those pipes pas the gas through catalytic converters to decrease pollutants while mufflers and resonators absorb the impact of the wave to decrease noise. This increases back pressure, which is commonly believed to be the reason exhaust systems reduce performance. However, the real key to performance is velocity.
Back Pressure vs. Velocity
Improving performance is a matter of balancing back pressure to both let the exhaust move freely while also successfully harnessing pressure waves to push gases through the system, a.k.a. “scavenging.” In engines that run at a steady RPM like those used in generators, manufacturers can simply state a recommended exhaust pressure. However, with cars running at a range of engine speeds, it is more difficult: Go too big, and the low pressure will keep the pulses from pushing out exhaust gases at low engine speeds. Go too small, and the pulses won’t be able to overcome the added pressure. Some manufacturers have gotten around this by designing two-mode exhausts that reduce back pressure based on engine speed, maintaining the velocity of exhaust gases through the rev range.
Getting a little extra top-end power is simply a matter of switching to a less restrictive muffler, increasing velocity at the expense of noise. Increasing the amount of air going into the engine with intake performance mods requires a correlating decrease in exhaust pressure courtesy of larger pipes and high flow converters. Getting the perfect balance for overall performance requires some time on a dyno, which has led to aftermarket suppliers developing full custom exhaust systems designed specifically for a given vehicle, segment and tune.
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