The exhaust system is one of those key areas of a vehicle that affect performance as well as visceral appeal. A properly chosen exhaust system can improve performance by reducing exhaust gas back pressure (resistance to flow) while providing a more aggressive or substantial sound The two aren’t necessarily tied together, though, as there are plenty of loud exhaust systems that rob power, while there are very quiet systems that improve performance.
Again, let’s go back to the premise that an internal combustion engine is a relatively complicated air pump. Think of a piston at top dead center in the cylinder. The first stroke sees the piston moving downward in the cylinder while the intake valve opens. This downward motion creates a suction that draws air and fuel into the cylinder. As the piston reaches bottom dead center on the intake stroke, the intake valve closes and the piston rises in the cylinder, compressing the air/fuel mixture (stroke two). When the mixture is compressed, the spark plug fires and the mixture burns rapidly, increasing cylinder pressure considerably and forcing the piston back down in the cylinder (stroke three). Once the cylinder reaches bottom dead center and begins to rise again (on stroke four) the exhaust valve opens and the rising piston pushes the exhaust gases into the exhaust system.
With these four strokes recurring over and over, and multiplied by the number of cylinders in a given engine, it’s easy to see how important the exhaust system (and intake system) is to the performance of the overall package. If the exhaust system is restrictive, the piston has difficulty pushing the exhaust gas out of the engine (backpressure). By reducing backpressure the engine doesn’t have to work as hard pushing exhaust through the system, and more power is available to move the vehicle. By the way, exhaust backpressure and intake restrictions are known as “pumping losses” to engine engineers.
The exhaust system is similar to the intake system in that pipe diameter and length plays an important role in the efficiency of the system. Smaller pipe diameters maintain good exhaust gas velocity and improve low-end and midrange performance. Larger pipe diameters work well for large engines and those that see high engine speeds. For this reason, putting a set of headers with very large diameter primary tubes on an otherwise stock engine probably won’t improve the efficiency of the package a great deal, and of course the opposite is true also. Header sizes should be matched to a given engine’s existing power characteristics and usable rpm range for best effect.
A well-designed set of headers also has the ability to scavenge the exhaust out of the cylinders. Think of what we discussed earlier: the piston pushes the exhaust gas out of the cylinder into the pipe. This “column” of moving exhaust gas has mass, and therefore inertia, which means that for a certain amount of time the column of gas will remain in motion even though the piston is no longer pushing it. By tying cylinders together through a header collector, it’s possible to harness this motion energy in such a way that it can create a low-pressure area in the exhaust primary tubes after a column of moving gas has passed through it. This way, when the exhaust valve first pops open on a given cylinder’s exhaust stroke, a slight vacuum will exist in the pipe and help scavenge exhaust gases out of the cylinder. This makes it easier for the piston to evacuate the cylinder. Scavenging typically only works at certain engine speeds, and depends greatly upon pipe diameter and camshaft timing, but this should serve to reinforce the need to size a set of headers so that they complement a particular engine where that engine works the best.
Past the headers, the pipes that run into the muffler(s) and out the back also play a role in performance. Again, pipe size is important, although not quite as important as with headers. As the exhaust gas travels through the system it cools off and contracts, meaning it takes up less space. Mufflers and catalytic converters (which re-ignite the exhaust gases to ensure no unburned fuel particles escape out the tailpipes) often prove to be restrictive. High-performance mufflers and high-flow catalytic converters are available that will keep the sound down and the exhaust clean while not hurting performance.
Exhaust systems range from simple dual-pipe sets for early cars up to complete header/muffler/resonator kits for a variety of vehicles. Late-model vehicles also benefit from aftermarket exhaust kits, which typically consist of a system that replaces the factory pipes and mufflers from the catalytic converter back. There are headers and Y-pipes available also for late-model vehicles, and these have proven quite popular with owners of late-model performance cars.
Remember, though, that exhaust system modifications on pollution-controlled vehicles are restricted. Components installed on these vehicles must be approved to be legal.
Like so many other systems on a vehicle, optimizing the exhaust can increase performance when done properly. Conversely, it can hurt performance if not matched properly to the rest of the engine’s systems. Choose exhaust system upgrades carefully and both performance and aesthetics can be improved – making your vehicle even more fun to own and operate.