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Exhaust article 1: Back pressure |
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Written by Devin Durham
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Thursday, 22 February 2007 |
Exhaust is one of the more important aspects of a performance car yet at the same time it is one of the most overlooked and misunderstood components. This article, which will be the first of several, will attempt to explain the basics of exhaust and why you should be paying attention to what is happening inside those loud pipes.
Until electric engines find themselves on the dragstrip, every engine you will find there burns fuel in order to develop motive power, and more importantly they rely on the expansion of that burnt fuel to push some form of piston. It is important to word things in that way, not because you are likely to see an engine that uses heat to drive a piston instead of fuel expansion (like a steam engine although that would be pretty cool) but because it implies that there is burned-out used up fuel and heat as a byproduct of that engine's function.
Engines make noxious dangerous gases and heat when they run. Venting that heat away from the motor is important to the life and safety of the engine, and venting those gases away is important to the life and safety of the driver. Hence we have our two major functions of exhaust: removing waste heat and removing waste gases.
A secondary function is to redirect and de-energize the sound of the engine and while that doesn't seem to be a major issue (especially since most of us rather like a loud exhaust) it comes into play because of the amount of exhaust restrictions that are caused by sound cancellation devices such as mufflers and resonators that are factory installed and often required by law.
The single most important concept: Back pressure is never good.
A typical 4 stroke motor uses nearly a third of it's total power output doing nothing else than pushing exhaust gases out of its cylinders and through the exhaust system. Dealing with exhaust is a major power drain on your engine and anything that can be done to make that job easier will translate directly into more power that can be used to push you down the strip. To make this very clear: the easier it is to get exhaust out of and away from your engine, the faster you will go. Period. Many people will claim that you need back pressure to maintain your torque and this will often come from someone with years of experience putting exhaust systems onto cars and paying close attention to the result. One thing that begins to stand out with enough experience is that you can't just stick a 5" exhaust from the engine to the back of your car and expect to see a massive performance gain; in truth what will happen is that you will lose performance instead, making it seem like a smaller more constricted pipe is a better choice. Logic follows that some form of restriction in the exhaust is needed to keep up the engine torque, and obviously there has to be some truth to that since the results are easy to demonstrate. But it isn't happening for the reasons that most people think.
Heat and Density
The density of air changes radically when it is heated or cooled, and also the temperature of air changes substantially when the volume of air is increased or decreased. There are engines based on this principal, most notably the Stirling motor that uses nothing but heat to expand air to drive a piston, when that piston is driven out the volume of the air increases which instantly cools the air and increases the density, this density increase causes a vacuum which pulls the piston back for the next power stroke, and so on. The Stirling motor is one of the most efficient motors ever made and it is entirely a closed system: it uses the same air in the same cylinder to both propel the piston outward and to bring it back in.
Hot air is less dense, takes up more volume and is easy to move. Cold air is much more dense, uses less volume and is more difficult to move. In your engine you want the least work possible moving your exhaust, so you want to be moving hot, low density gases.
(Completely off topic but still interesting: "gasses" and "gases" are both correct spellings. Neat, huh?)
One sure fire way to decrease the temperature of your exhaust, thereby making it more dense and harder to move is to increase the volume that gas is contained in. To put this into different words, when your exhaust gas moves from a smaller pipe to a larger one, the larger pipe has more volume than the smaller and the exhaust gases WILL expand to take up that additional space. That's the definition of a gas: "The state of matter distinguished from the solid and liquid states by relatively low density and viscosity, relatively great expansion and contraction with changes in pressure and temperature, the ability to diffuse readily, and the spontaneous tendency to become distributed uniformly throughout any container." (http://www.thefreedictionary.com/gasses)
When this expansion occurs, the gas cools. When it cools, it becomes more dense. When it gets more dense, it becomes harder to push through the pipe. When it becomes harder to push through the pipe, you are robbing your engine of power. It's that simple.
This is why you will often see heat tape applied to the exhaust pipes of professional racers. They want to keep the exhaust gas as hot as possible as long as it is inside the exhaust system. And this is why there is a limit to how large you want your exhaust pipes to be; if they get too large they allow the exhaust gas to cool and become dense. The by-product of a smaller pipe is that there is greater back pressure, but back pressure not why the smaller exhaust is working better. It's because of the physics of gases. (Or gasses. Take your pick.)
That's the first reason why a larger pipe isn't always a good thing, and why back pressure is misunderstood. The second reason is exhaust pulses, or "scavenging." Pulses are also directly affected by temperature and density and volume. The article on headers will go more in depth on that topic, as it deserves some singular attention.
So why any pipes at all?
Let's assume for the purposes of discussion that you don't have a concern about the toxicity of your engine's exhaust gases. Let's say that the engine is mounted behind the driver's compartment and the exhaust valves are open to the air. (Not too many street cars are going to fit this bill, but whatever). The problem that you are going to run into is the main function of exhaust: removing heat. Typical engine exhaust runs at around 1,200 degrees and goes even higher than that at full throttle. Your exhaust valves will burn, and assuming you could keep the car running eventually they would actually melt.
So some pipe is necessary, at least enough to move the waste heat of the engine far enough away from the engine itself to keep from damaging it. In the real world this entire discussion is pointless however, since by the time your engine expired from waste heat, the chances are great that you will have expired from waste carbon monoxide.
Too big, too small
Just because there is clearly such a thing as an exhaust that is too big, too small is just as easy to do. As a matter of fact most factory exhausts are on the order of .5" too small in diameter. One thing that stands out clearly is that the concepts and theories behind performance exhaust are complicated. It takes men with enlarged foreheads and pocket protectors who are very comfortable with gas physics to come up with high quality exhaust systems, and hopefully now you know that there's more to it than just welding a huge pipe onto your exhaust manifold.
The second exhaust article, "Headers" is available HERE.
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Last Updated ( Friday, 06 April 2007 )
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