An exhaust system sounds like a really simple concept, right? All it does is take old, used up engine gasses and get them out of the combustion chamber. Technically, all you need is a short pipe to get an oxygen sensor reading, and then nothing else. On a car with a Carburetor, you could get away with no exhaust at all. But in the real world things are never quite that simple.
Most of us have to deal with a few laws that the Government has put up. The biggest is emissions. That exhaust gas is poisonous, turning the sky black. If you breathe it long enough, you will not only suffocate, but kill off brain cells. Unfiltered exhaust breaks down the ozone layer, creates smog (which is why the Cali sky is brown on hot days), and displaces the oxygen in our air, killing all sorts of plants and animals. So, if for one second, you think you are too important to run without devices like catalytic converters, GET OFF YOUR HIGH HORSE!!! I have dealt with emissions problems before, they are not fun, or cheap, but there is a reason for them. And I will come down HARD on you, on this site or in real life, if you think that 2 horsepower is more important then the ONLY known planet that can support life in this universe.
The other law that concerns exhaust systems is noise. As much as I enjoy race cars, I hate it when a loud POS wakes me up at 2 in the morning. But as we will discover here, a quiet exhaust is not always a slow exhaust.
With a few exceptions, an exhaust system has four main parts: a header (aka a collector, or exhaust manifold), a catalytic converter, piping, and a resonator (also called a muffler when it stick out from under you bumper. Some exhausts have more than one). So lets break down what each part does and maybe break a few myths along the way.
Collectors: Simply, a collector takes the exhaust gas from each cylinder and combines it into one exhaust flow. Because of weight and size restrictions, this is the most practical way of getting exhaust gas out of a car. A header is one of the best upgrades that can be made to the exhaust system. Nearly all headers are designed for cold emissions and cost, so a good set of tubular pipes can net gains of 5% or more on nearly any car on the market. Aftermarket headers also save lots of weight because they are usually made of aluminum, not cast iron.
This is a 4-1 header. You can see in the picture, there is room for improvement in the collector area where it would bolt to the catalytic converter. A full-race header will have a much narrower collector, with very smooth transitions.
The most gains from headers are sought using something called exhaust scavenging. A header is very similar to an organ pipe. This is why they are sometimes called 'tuned pipes.' At a certain RPM, the air reaches a harmonic frequency with the header and the air actually moves even faster. This creates a vacuum and actually sucks exhaust gas out of the engine. It also sucks clean air into the combustion chamber. What RPM this happens at differs between engines and header design. That kind of math is currently over my head so I won't force it onto you guys. However there are a few ways you can tell where your header will make its most power.
-Pipe diameter. Pretty simple, with larger pipes, you will actually lose power at low RPM. Your gains will all be on the top end. More on why this happens later (hint: it's NOT backpressure).
-Tube length. Equal length tubes will give you the most gains, but only on the top end. If torque is your aim, unequal length runners that are shorter will help that.
-Merging. Headers for four cylinders usually come in 3 flavors: 4-1, 4-2-1, and 4-3-1. The terminology just tells you how many pipes and where they meet. 4-1 headers all converge into one pipe, 4-2-1 converges to 2 then 1, and 4-3-1 merges from 4 to 3 to 1. Generally, a 4-2-1 header will give you the most torque, a 4-1 will give you the best top end power, and a 4-3-1 will give you farily even gains, maybe a bit more torque than power. However 4-3-1 headers will also give you the smallest gains. Also important is how the exhaust flow is merged. A good collector will have as much smooth and straight piping as possible. Also the the collector will have a smooth merge, so airflow is not disrupted.
This is how a good collector should look. Note the perfect transitions. These will not disrupt airflow in any way. Cheap collectors will simply slip a pipe over the ends of the header tubes. Some companies even fill the voids with weld which looks terrible and is very bad for flow and quality.
As a side note, this same principle applies to V8s. V6s are almost always 3-1, since that's the easiest way to merge the pipes.
Catalytic Converters: Keep your stock cat. End of story. Why? Since the pellet type cat converter is now a museum piece, catalytic converters actually flow amazingly well. On some cars, removing the cat can actually LOSE power. The power gains will be minimal. On a fully Toyota built racing engine, a cat converter would remove about 7 horsepower. So for your street car, that's more like 2. The gains are negligible, and as I have said, you are ruining the world we live in. High flow cats are a waste of money. The flow gains are laughably small, and more than likely, you will not pass emissions with them. I can tell you for a fact, with a turbocharged car, the power loss from replacing a gutted high flow cat with a stock cat is nothing. You can't feel it. Going poo before you hit the dragstrip will make you faster. In fact sometimes gutting a cat will make you lose power because the added volume slows down the exhaust flow and can cause it to stack up on itself, robbing power and slowing throttle response.
For those boneheads who gut cats, you have now wasted a very expensive piece of equipment. The precious metals inside cat converters (the metals that make them work) are worth a lot of money as scrap. Somewhere in the neighborhood of $100. Bits of a core are worth nothing. Smooth move knucklehead.
Piping: Length is kind of set, since you have to get the exhaust out the back of the car. Side pipes are pretty impractical, since they heat the floorboards and smell. Most of the gains from a cat-back exhaust are from the diameter of the pipe and the shape. A larger diameter flows more air, and hence, gives you more power. The shape of an exhaust pipe is also crucial. Quite simply, a pipe with many bends needs energy to make the exhaust gas flow in a new direction. A straight pipe only needs to move this column of exhaust air out of the pipe.
When bends are necessary, the good exhausts use mandrel bends. OEM and cheap aftermarket exhausts use crush bends. The pipe is put into a bender and crushed to shape. A mandrel bend is done the same way, except an object called a mandrel is inserted into the pipe. This stretches the metal a bit and makes a smooth bend. The pipe also does not change diameter, which would restrict airflow.
For any NA Toyota, a 2.5" exhaust is about all you will ever need. Above this, you will start losing power. Why? Well some people will say that the backpressure is gone, so the scavenging is not as good. WRONG!!! Backpressure is exactly what it sounds like. Exhaust gas is like a big column of stinky, toxic air. The force it exerts is a pressure. Backpressure is the force opposing the exhaust coming out. And if you have ever tried to push something, you know it takes work. The engine is doing this work, and backpressure is taking away power that could be used to make you go faster.
The TRUE reason why large exhaust pipes lose torque is totally different. When exhaust is leaving your engine and going through your exhaust pipe, it has some speed associated with it. Since this speed has a direction, 9th grade Physics calls this a velocity. Now I have called exhaust gas a column of air already. So let's think about this for a second. When you shift gears and your engine drops its RPM, the exhaust gas slows down, since all the pressure on it has ceased temporarily. When you let off the clutch, you need to get that column of evil air moving again. A smaller exhaust has less air to move than a larger exhaust. So the power loss on the engine is not as great and it does not last as long. This where the extra torque comes in. Your engine is making the same amount of torque with both exhausts (probably more with the larger pipe), but it will read as less, because more of the engine's power is being used to move that larger column of air instead of turning the crankshaft.
For turbo cars, there is almost no limit to how big an exhaust you can go. Because the turbo needs to spool up, the torque loss isn't important. And once the turbo does get moving, that air shoots out quick.
Mufflers/Resonators: Engines are loud. Just stop by a track and see the race cars with straight pipes and you'll see what I mean. A resonator muffles the noise so you can actually hold a conversation while driving your car. Older mufflers used fiberglass to muffle engine noise. These are called glasspacks. Modern technology uses harmonics to block noise. Two mufflers in the same pipe will allow the vibrations to bounce off each other and cancel themselves out. The bonus is because this is sound energy, the air is not restricted and can still flow freely. In the early days of compact car tuning, big, loud mufflers were common, because a loud exhaust was associated with high flow. Ricers will use a big muffler on a stock exhaust. They get tickets. Smart tuners will take advantage of the new crop of quiet high-flow exhausts.
So the moral of this story? When planning your exhaust, there is only one thing you need to think about
: exhaust velocity. The higher it is, the more power you will make. There are many ways to get that high velocity, and many of them are listed here. I hope this clears up some misconceptions over exhausts and how they work. As always, feel free to comment and ask questions.
DISCLAIMER: The contents of this article are not necessarily the official views of, nor endorsed by, the owner(s) of Toyotanation.com.
originally posted by 8695Beaters (Hondaclub.com)
http://www.hondaclub.com/forum/artic...st-system.html
95 Camry
An Exhausting look at the Exhaust
AUTO to MANUAL Swap Guide (3rd Gen) - 
camry
There are many misconceptions of how an exhaust system works, and I figured this might clear some of that up 
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Piece of Crap
Linear Mode
