As an enthusiast, you probably know that an aftermarket exhaust will add power to your car, or at least that’s what the internet says. The idea is that with a less restrictive exhaust your car gains power. What confuses many people is why a less restrictive exhaust gains power. Things like thermal efficiency, scavenging, velocity, and total flow all matter and can separate a great exhaust from a bad one. In this short article, we’re going to dive in-depth on everything you need to know about aftermarket exhaust systems and how they increase power.
Before we dive in, let’s quickly cover the basics of exhaust gases. After the engine goes through the intake, compression, and ignition cycles, it now has hot gases it needs to expel in order to continue the cycle. The exhaust valve opens and the piston comes up, forcing the air out of the engine and into the exhaust system. These gasses are extremely hot as they are the byproduct of an explosion and they come through the exhaust system in rapid pulses. The purpose of the exhaust system is to route these potentially harmful gasses away from the cab of the vehicle and to the rear or sides.
Flow and Scavenging
The headers are the first point of contact that the exhaust gasses run into. Typically, OEM exhaust manifolds are made of heavy cast iron and don’t have the greatest design. The OEM headers can be swapped out for tubular headers with a larger diameter and a better design. When an exhaust pulse comes through the header it creates a low-pressure area behind it. With a good design, this low-pressure area can help pull the exhaust pulses behind it. This is known as scavenging, and a well-designed header will greatly improve scavenging efficiency with leads to much greater flow.
Past the headers, we have the rest of the exhaust system. Depending on the size of your engine and how much power you are making, the optimal diameter of the piping will vary. The whole idea with a larger diameter is increasing the flow. With increased flow, the piston has to do less work when pushing the exhaust gases out of the cylinder which ultimately leads to more power.
In case you didn’t know, heat is energy. As exhaust gas cools, its velocity decreases. Harnessing this energy isn’t extremely important for a naturally aspirated vehicle, but it’s important for turbocharged vehicles. With hot, high-velocity exhaust gases entering the turbocharger, the turbine can be spun faster resulting in better spool times, more power, and ultimately much better thermal efficiency. You can harness even more of the energy by using an exhaust wrap which is designed to keep the heat inside the header instead of it dissipating to the air under your hood.
The Back Pressure Myth
All over the internet, there is a massive misconception that four-stroke engines need a certain amount of back pressure to run properly, and very little back pressure will decrease power. This is entirely false. Decreasing the pressure as much as possible results in greater flow which allows the exhaust gases to be forced out of the cylinder easier. With too large of piping or a poorly designed exhaust system, however, exhaust scavenging can be decreased which will result in decreased flow. If back pressure is so important, why do high horsepower drag cars use open header exhaust systems?
To summarize all of this information, there are a few key points to remember. A good exhaust system will increase flow, possibly increase velocity, optimize exhaust scavenging, and use heat to its advantage. The whole idea is to simply make it easier for the piston to push the exhaust gases out of the cylinder. If it’s hard to push the gases out of the cylinder, that will essentially resist the rotation of the crankshaft. If it’s easy to push the gasses out of the cylinder, there will still be resistant to the rotation of the engine, but with less resistance comes more power.
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