Ever since internal combustion engines became a thing, we were addicted to the noise and power of exhausts. If we go back to the dawn of automotive sports, we’ll find cars such as the 1910 Fiat S76 with its open manifolds, spewing flames and thunder even at rough idle. Despite becoming way more civilized since then, modern cars still owe a lot of their character to the exhaust sound they produce.
However, exhausts do much more than just satisfy our sonic cravings. Upgrading the exhaust system on your car can often increase its power. Today we are going to touch upon exhausts, how they work and how upgrading them can improve much more than just the sound of your car.
How do Car Exhausts Work?
Car exhausts may look like nothing more than a bunch of pipes welded together to an untrained eye. However, there is a lot of science that goes into these systems. The main purpose of an exhaust is to efficiently and safely move exhaust gasses from the engine, away from the cabin and towards the rear of the vehicle.
Every exhaust system on modern cars consists of several key components. Let’s take a closer look at each going from the engine and moving towards the back of the car:
Exhaust manifold – Exhaust manifolds or headers are the first components that take on exhaust gasses as they are forced out from cylinders during the exhaust stroke. Manifolds can be quite complex since their geometry dictates the velocity of gasses passing through them. As we’re going to discuss later on, this single fact is quite important if you want to get better performance out of your engine.
O2 Sensor – The next piece of the puzzle is the O2 sensor that sits right between the exhaust headers and the catalytic converter. This little sensor reads the amount of O2 in the exhaust gasses as they pass through the system, and sends that information back to the ECU. From there, the ECU can use the info to adjust the air/fuel mixture if necessary.
Catalytic Converter – Next up is the catalytic converter. Although many claim that this portion of the exhaust reduces the overall performance of their cars, cats are a necessary component. Burning fuel in internal combustion engines produces a number of toxic gasses such as the NOx and CO. A catalytic converter takes on said gasses and converts them into much less harmful gasses that are somewhat safe to be released into the atmosphere. Removing the catalytic converter, also known as de-catting, is the easiest way to gain a few horsepower, but it’s also quite illegal in most places.
Tubing – After the cats comes the tubing. Despite being essentially just a simple tube, this part of the exhaust is extremely important as its diameter plays a major role in the whole exhaust process of combustion. More on that later.
Resonators and Mufflers – Lastly, we have the resonators and mufflers. These two components are designed to reduce the noise coming out of the exhaust so your car wouldn’t sound like the Fiat we’ve mentioned earlier. When people tune the sound of their cars, these are the exhaust components they swap.
Exhausts and Performance – Pressure Zones and Exhaust Gas Velocity
Improving engine performance and horsepower on the exhaust side of the equation looks fairly simple on paper. All you really need to do is remove any restrictions that might affect the flow of exhaust gasses and you’re good. Is it that simple? Not really. If it was, people would just run a massive straight pipe from the headers all the way back to the rear of the car and call it a day.
As it so happens, you can go overboard with exhaust diameter and cause backpressure. In other words, you can have a situation where gasses are flowing back into the cylinder. Needless to say, that’s no bueno.
Exhaust gasses are released from the cylinders in impulses. As you probably know, internal combustion engines run on four different strokes, the last of which is exhaust. That means that there’s idle time between exhaust strokes.
These pulses of exhaust create a pressure differential in your exhaust system where the gasses further away from the headers are forming a high-pressure zone. As you move back to the headers, the pressure drops.
That high-pressure zone essentially pulls the gasses from the system outward. If you were to weld a wide diameter pipe straight to your headers, you would negatively affect these different pressure zones, possibly causing the gasses to flow back into the engine.
Controlling Restriction Points
With all that said, there are ways to find the perfect pipe diameter and header formula to optimize the flow of gasses. It all comes down to controlling various restriction points in the system. One such point of restriction are the cats. However, removing the catalytic converter releases all of the nasty gasses into the atmosphere and we don’t want that.
One of the main solutions to upgrading the exhaust is to get a cat-back system. A cat-back system represents the complete aftermarket setup that is mounted right after the catalytic converter.
These systems are created according to the car model so that there are no issues with fitting or routing. One thing most such aftermarket setups have in common is an optimized piping diameter that allows for a much better flow of exhaust gasses through the system.
But wait, things get even more interesting. As it turns out, every engine has its sweet spot in the rev range that is usually up there around 5+k RPM depending on the engine. When car manufacturers develop stock exhausts, they are looking at what kind of driving the car will be doing the most over the course of its service life. That is why most stock exhausts are optimized for the low RPM range.
Mounting an aftermarket cat-back system might disturb this balance as these are generally optimized to work best around that sweet spot in the RPM range. In other words, cat-backs offer the best results in track cars and performance vehicles.
NA vs Forced Induction
Another thing to keep in mind is how aftermarket exhausts behave in different types of engines. Naturally aspirated engines will see a slight improvement in horsepower as well as throttle response. However, forced induction engines with turbochargers are a whole different ballgame. This type of engine can greatly benefit from a properly designed aftermarket exhaust setup.
At the end of the day, it is clear that aftermarket exhausts can do much more than just make your car sound louder and more aggressive. A properly designed and installed system will give you a performance boost in the higher RPM range as well as improve the overall throttle response. The rumbling noise is just an added bonus that makes the whole investment that much better.