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OK, now let's get away from simply computing info about engines. Let's start on something that will truly be helpful. In this section we'll deal with exhaust tuning. The part we will deal with is from the exhaust port out. We will leave dealing with the exhaust valve and port until we discuss cylinder head modifications.

During this discussion, when we speak of Header Length, we mean from the center of the exhaust valve to the end of the header. This does NOT include any muffler or collector. Admittedly, this dimension can be difficult to measure. But the closer you can get to it the better.

First let's discuss what happens in the cylinder during the exhaust stroke. The piston is on an upstroke and pressure is building in the cylinder. At a specific point, the exhaust valve starts to open. All the gases, products of combustion, are under tremendous pressure and the open exhaust valve provides a means of releasing that pressure.

The burnt gases immediately start to exit the combustion chamber and travel through the exhaust port and into the exhaust system. They are under tremendous pressure and even though they travel as a "wave", they are far more powerful than a typical "acoustic" or sound wave.

This high pressure area, or compression wave, then travels down the exhaust pipe and into the collector or muffler (if one is present), or directly out into the atmosphere. When it reaches the end of the collector, an expansion wave (or low pressure wave) is reflected back up the exhaust pipe. The reflected expansion wave is of only slightly less force than the original compression wave.

Please refer to the above graph. If you consider all Compression waves to represent Positive pressures (or up on the "y" axis) and Expansion waves to represent Negative pressures (or down on the "y" axis), and Time to be represented by the "x" axis, then when the curve crosses over the "x" axis at 0 pressure, that would roughly indicate the time that the Compression wave hits the atmosphere at the end of the exhaust pipe or header. As a point of information, when a Compression wave hits an open end, an opposite wave (or an Expansion wave) of slightly less amplitude is reflected back. When a Compression wave hits a closed end, a similar wave (another Compression wave), also of slightly less amplitude, is reflected back.

An interesting thing occurs with these waves. The spent gas particles do not travel at the same speed as the wave itself. However, with an compression wave, the particles travel with the wave. With an expansion wave, the particles travel in the opposite direction of the wave. Therefore, both waves move the particles towards the open end of the exhaust.

The best analogy we have found to show this is the "log in the lake" analogy. If you have a log floating in a lake and something causes a disturbance that creates waves, the waves will move the log in a direction away from the disturbance. But if you watch carefully, the waves will move in that direction faster than the log.

So in this analogy the waves in the lake are analogous to the waves in the exhaust header, either pressure or compression, and the log is analogous to the actual gases in movement in the header. Therefore, you would NOT want the reflected expansion wave pulling any of the particles back along the exhaust pipe.

Here's where things get interesting. If you can time the arrival of the reflected expansion wave back to the exhaust valve, the low pressure will not only help draw spent gases out of the combustion chamber, but they will also help draw a fresh charge of gas and air in! Amazing! This is called Scavenging and is very important.

One final point. During our study of both intake and exhaust dynamics we have found that most authors take one of two sides.

There is one school of thought that only considers the effects of the pressure waves. This would be the compression and expansion waves discussed above.

The other school of thought focuses on the effects of particle movement. This would be the actual combustion by-products that are moving in the exhaust pipe.

When you realize that both things are occuring, you have a particle movement (spent exhaust gases) and extremely high pressure waves present, it would make sense to include both phenomenon. We have not found anyone who takes both of these dynamics into consideration at once.

And if the particle movement (or Kadency theory) is inappropriate, then how do cars and motorcycles "draft" in races?? Perhaps the mass of particles involved doesn't amount to much. This is an area that definitely deserves some looking into and we will.

Note that for this to work, you must compute the length of the exhaust pipe. The amount of time it will take the reflected expansion wave to return to the exhaust valve is figured into the equation. You should also write the result down, rounded off to 2 decimal places, as that number will be needed to compute the suggested header diameter.

For example, our Triumph TR6 will be running at about 6500 RPM. Using stock cams, the exhaust valve opens at 55 degrees before BDC. Using these figures, we come up with a header length of 27.73 inches.

We will now compute the best header pipe diameter. Using the header length computed above, and taking into consideration the cubic centimeters of one cylinder (750 cc's divided by 2 cylinders equals 375 cc's per cylinder), we find that our best inside header diameter is about 1.544 inches. I would tend to take this out to the next size pipe, say 1 5/8 inches or 1 3/4 inches.

We must digress here for a minute. Most of you are aware that it is important to maintain equal header length for best performance. This is so the timing of the returning expansion wave impulse is equal at all cylinders. However, of equal or greater importance is to make the headers of sufficient diameter to insure that restriction is very small. The detrimental effect of restriction is much greater than the detrimental effect of unequal header lengths.

In other words, header diameter controls the volume or amplitude of the the compression wave exiting the exhaust pipe (please refer to the above sine wave graph), while the header length determines the timing or frequency of the returning expansion wave.

It is now time to move on to Collector Length and Diameter. This is much more difficult to compute but if you apply the principles we learned above, you can make some assumptions.

However, before we begin this discussion, it should be pointed out that for our test bike we will not actually be using collectors but individual pipes for each of the two cylinders. Now, hopefully, the results will be the same as if we consider whatever we place on the end of the header pipe to be the collector. Or you may want to assume (as we are going to do) that the megaphone exhaust we are going to use function the same as an individual collector.

Oh well, on to the project.

In the collector, the same factors apply. Collector length and diameter affect performance. However, the ANGLE of the collector also affects the outcome.

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