The exhaust is responsible for carrying and expelling waste gases away from the engine as well as silencing the engine's noise output. Gasses leave the combustion chamber under extreme pressure and enter into the exhaust manifold. After the exhaust manifold joins the gasses from all the cylinders together, the gasses enter the catalytic converter where they undergo a process discussed in our Catalytic Converter (coming soon) article. After the gasses exit the catalytic converter, they enter into the B-pipe where there is usually a resonator to help silence some of the noise caused by the escaping of the pressurized gasses. After that the gasses enter into a specialized muffler where the majority of the silencing occurs and then are expelled out into the air.
The Basics
This here is a simplified picture showing one of the four cylinders inside our engines. The piston in this picture has begun it's intake stroke and air is starting to enter the combustion chamber. So what does this have to do with exhausts? Well look at the exhaust valve, you can see that it is still open even though the piston already completed it's exhaust stroke. This is called valve overlap and is necessary for high revving engines such as Integras. At high RPMs, air is being pushed out the combustion chamber so fast that not all of the air can escape in time after the piston completes it's stroke. The overlapping period as you see here allows the momentum of the air to keep moving it out of the chamber even though the piston already has started creating a vacuum with it's intake stroke.
This works great at high RPMs, but what about the low RPMs? The air is usually not moving fast enough for the overlapping period to be beneficial so if exhaust gasses aren't moved quickly through the exhaust, the vacuum created by the piston's intake stroke can actually suck some exhaust gas back into the combustion chamber. This is how aftermarket exhausts can cause a loss in low end power. Incorrectly sized exhaust pipes can disrupt the airflow, allowing exhaust gasses to be sucked back in during this valve overlap period. Obviously we don't want already burnt, noxious gasses to mix with our fresh air/fuel mixture so it is very important to have a properly tuned exhaust for the right application.
To understand how to properly tune an exhaust you must first understand exactly how the gasses will be flowing through it. Exhaust gasses do not flow in one continuous stream they get shot out in pulses. Each time a valve opens and gas is pushed out of the combustion chamber, it creates a pulse. As you can see from the picture, the head of the exhaust pulse remains at a high pressure after it is shoved out of the combustion chamber. The center of the pulse is closer to the ambient pressure of the exhaust system and the tail turns into a low pressure vacuum.
Now look at these exhaust pulses all lined up as they travel through the exhaust. This would be an exhaust system which was sized perfectly for the car at the right RPM. The pulses, to an extent, will move through the B-pipe to the muffler by themselves. But we want the exhaust gasses to travel as fast as they possibly can. How can we get them moving faster? Well you get them to line up like shown, and the pulses will then pull each other through. As with most naturally occurring phenomenon, opposites will attract. The high pressure heads are pulled into the low pressure, almost vacuum-like tails. So each pulse will follow after the other, effectively leading each other to the muffler and away from the engine.
Pipe Sizing
In a perfect exhaust system with the engine at the right RPM, all the heads would line up perfectly with the tails of the exhaust gas pulses. Of course in real life this can't always be that way but that's why we must choose our pipe diameter carefully. In the lower RPMs, pulses are smaller, and further apart. When you rev up into the high RPMs, pulses get bigger and closer together. So we want to keep the small, spread out exhaust pulses in line in the low RPMs but we also want to accommodate the larger, quicker pulses in the high RPMs. For this reason we have to decide what pipe size will give us the best trade-off of low-end vs. high-end power so we can get the highest total HP increase possible throughout the RPM range.
We see here a restrictive stock exhaust system. In the low RPMs, this tight formation of pulses becomes an advantage as it keeps the them close together so they can pull each other through the exhaust rather than lingering around. However as the RPMs rise, the pulses become closer and closer together, raising backpressure to restrict the piston in pushing out more exhaust gasses.
Here you can see a correctly sized exhaust system. The backpressure is minimized as much as possible while allowing pulses to line up nicely and provides a good trade-off between both low and high-end power. For most N/A applications this pipe size would be 2.25" in diameter. A common mistake sometimes talked about in "tuner" magazines is that you need some backpressure for the system to flow properly. This is a myth created by amateur testing methods. You can see the exhaust here is properly sized and will get the best performance out of any other sizes, bigger or smaller. So because it outperforms the bigger exhaust they just assume that backpressure was the key to success here. As you can see from this picture, they are wrong. The reason this exhaust performed the best was because the pules were perfectly in line and able to draw each other out of the exhaust at the highest velocity possible.
Now we are looking at the opposite end of the spectrum from advice commonly given by "tuner" magazines. "The bigger exhaust you have the better it flows and you have no backpressure anymore so you make tons more power." Yeah right. Funny how these "tuner" magazines will say something on one page and then say something opposite on the next. And yeah I keep putting "tuner" in quotes because I'm being sarcastic. I honestly thing most of those magazines are a joke. Anyway in an exhaust system that is too big, the pulses get disorganized and don't follow each other in line. Some of the pulses will bounce around, causing exhaust gasses to hang around in the exhaust pipe with little forward motion. These gasses are susceptible to being sucked back into the combustion chamber on the piston's intake stroke, diluting the fresh air/fuel mixture and ultimately causing power loss. So while you have minimized backpressure even further with an over-sized pipe, you did so at the price of disorganizing the exhaust pulses.
Forced Induction
Gasses exiting the combustion chamber of an N/A car are already under very high pressure, which is what creates the exhaust pulse in the first place. Turbos and Superchargers can raise that pressure dramatically, causing much bigger pulses. Because of the larger pulse sizes, you can see that having larger exhaust pipe sizes are okay.
Backbox/Mufflers
This is a picture of an OEM muffler. These decrease noise by reflecting pulses into each other which causes "destructive interference". Basically if a sound wave runs into another sound wave of the same frequency and opposite phase, then the sound waves are cancelled out and no noise is heard. Of course no muffler can ever do this perfectly so there is still sound to be heard. The performance of these mufflers as you can guess is less than wonderful. The air is forced into many dead-end chambers, creating backpressure and making it difficult for the air to flow through smoothly. Sounds like the stock muffler sucks, and maybe just changing the muffler will give us good performance gains right? Wrong.
This would be another one for the Magazine Mechanics. "This muffler will reduce backpressure and you'll see at least 15-20HP with this ultimate free flowing design!" By now I hope we can see what a big fat load of crap that is. In the stock exhaust system, the pulses have already been jammed together, and the backpressure has already affected the engine. So you change your muffler to a better flowing one, so what? The amount of backpressure in the exhaust system remains unchanged. You're just putting a fire hose at the end of a garden hose, the flow remains unchanged. So unless your exhaust has reduced backpressure enough so that the muffler becomes a restriction, then changing the muffler to a "free flowing" design will have no effect on real-world performance.