Wednesday, August 13, 2008

exhaust flow in an automobile-methods on how to Improve efficiency and power

After the above discussion of the components in an automotive exhaust system, it is obvious that the principle of the engine as a pump is not being utilized to the fullest. Air is not allowed to flow too freely because of restrictions in the form of the catalytic converter, the resonator, and the muffler. However, these components are necessary by regulations to maintain safe exhaust gas emissions and minimal sound levels (noise suppression). Also, in part, it takes time and money to design an excellent performing and free flowing exhaust system; something that car manufacturers just can’t afford to waste resources on. This is where aftermarket companies come in to create cost effective options for performance minded car owners. Of course, a free flowing exhaust would be expected to make more noise than a normal one. But a good manufactured system has a deep throaty tone, while yielding increases in horse power and also passing emission tests. I will now go through some of the modifications of the exhaust system that would "unleash" some horsepower and efficiency, while still being street-legal.

A header is a different type of manifold; it is made of separate equal-length cylindrical tubes with smooth curves in it for improving the flow of exhaust.

Each time a power stroke occurs and an exhaust valve opens, a positive pressure occurs in the exhaust manifold. A negative pressure occurs in the exhaust manifolds between the positive pressure pulses, especially at lower engine speeds. Some exhaust headers are tuned so the exhaust pulses enter the exhaust manifold between the exhaust pulses from other cylinders, preventing interference between the exhaust pulses. If the exhaust pressure pulses interfere with each other, the exhaust flow is slowed, causing a decrease in volumetric efficiency (and thus decrease in horsepower). Proper exhaust manifold/header tuning actually creates a vacuum, which helps to draw exhaust out of the cylinders and improve volumetric efficiency, resulting in an increase in horsepower.

Dual Exhaust Systems

For engines with the "V" type configurations, it would be more efficient to use a dual exhaust system than the "Y" pipe. In other words, two pipes (instead of one) connect the exhaust manifold/header to two catalytic converters, two resonators, and two mufflers. Thereby each manifold will have their own resonators, catalytic converters, exhaust pipes, mufflers, and tailpipes. The advantage of a dual exhaust system is that the engine exhausts air and gases more freely, thereby lowering the back pressure, which is inherent in an exhaust system. With a dual exhaust system, a sizable increase in engine horsepower can be obtained because the "breathing" capacity of the engine is improved, leaving less exhaust gases in the engine at the end of each exhaust stroke. This, in turn, leaves more room for an extra intake of the air-fuel mixture. The disadvantage of a dual exhaust system is that it would be costly due to the additional components. No doubt the addition of another exhaust system adds more weight to the car, but the increase in horsepower is substantial enough to outweigh the horsepower losses through additional weight.

Removing the Resonator

The resonator does not function also as emissions control device, so removing it and putting a straight pipe connecting the catalytic converter and the exhaust pipe will not cause the car to fail emissions test. Instead, some horsepower can be realized and not to mention the loudness of the exhaust. However, with a tuned muffler, the sound can be toned down to a deep throaty sound that is not irritable.

Upgrading to Larger Pipe Diameter

The factory exhaust pipes have diameters around 1.5" to 2" (some 2.25" for newer larger engine cars). Increasing the diameter of the piping will also increase the average path/cross-sectional area that the air can pass with a minute decrease in velocity. As mentioned before, if the diameter (and hence cross-sectional area) of the pipe is increased too much, the velocity of the air flow will decrease more than the area increases, so flow would be adversely affected and power would be lost.

So, depending on the size of the engine, the optimal size pipe to upgrade to varies from 2" to 2.5". On average, a naturally aspirated 2.5 liter engine would suffice with 2.25" exhaust piping from the catalytic converter back to the muffler inlet.

Mandrel Bent Versus Crush Bent Piping

Another way to upgrade the exhaust piping from the catalytic converter back to the muffler is to have the exhaust piping mandrel (heat) bent instead of the conventional crush bending. As the name suggests, mandrel bends are achieved through the heating of the piping before bending whereas crush bent just literally mean that the piping is bent entirely by force. However, the main difference between mandrel bent and crush bent piping is the ease of flow. Mandrel piping keeps the pipe at a constant cross-sectional area throughout a bend which makes exhaust flow easier. On the other hand, crush bending deforms the pipe at the bend(s), which can restrict the exhaust flow. The disadvantage of mandrel bending is that it is relatively expensive, because of the costs involved in operating a mandrel bending heat machine. A popular alternative is to get piping with larger diameter and then have it crush bent. This way, it kind of evens out the differences in air flow ease, especially if that particular exhaust pipe configuration has a lot of bends and 90 degree bends.

Straight Through Versus Reverse Flow Mufflers

Having a optimally free flowing exhaust all the way from the manifold would not do much good if the restrictive stock muffler is still used. The inlet and outlet diameters of the pipe in the muffler should also be as large as feasible, so as to allow free flow of exhaust gases. A straight through muffler would be preferred to a reverse flow muffler mainly because the process of air re-direction in the reverse flow muffler is too restrictive. A straight through muffler design would allow exhaust gases to be expedited out as efficiently as possible, although the muffling abilities would not be as efficient as that of the reverse flow design. Therefore it will be inevitable that the exhaust will sound louder than before, but as mentioned before a couple of times, an aftermarket straight through muffler uses noise suppressing material that tones down the sound to that of one that’s deep and throaty and not irritating. However, as will be discussed in the next section, a new generation of mufflers may be able to tackle this.

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