Exhaust Tuning

Exhaust Tuning

Once a Mystery, Then a Tuner's Secret, Careful Selection of Exhaust Lengths Has Many Benefits.

Ch. Christophe

Mention exhaust length tuning to most people involved with motorcycles and the discussion will center around two-stroke motors. Certainly proper scavenging of a two-stroke cylinder can he aided by the shock wave phenomenon in a pipe, but the same sort of shock waves can help scavenge a four-stroke motor and result in a fatter power-band, if not greater power.

For four-strokes of 750 to lOOOcc the power output per displacement is seldom greater than 100 horsepower per liter. Specially tuned racing machines based on stock engines may get as high as 145 hp/ liter, but this is far less than the 360 to 400 hp/liter achieved by racing two-strokes of various classes. Yet shock wave phenomenon is used on four-strokes with much lower specific power than the twostrokes are delivering. It's just that the four-strokes use the shock waves to even out fluctuations in the power curve while two-strokes use the shock waves to boost maximum horsepower.

Tuning the length of exhaust systems to benefit from the natural shock waves in the exhaust doesn't require a separate exhaust system for each cylinder, as might be assumed. Even 4-into-l and other collector exhausts use the technique. All it requires are four pipes of equal length, the number of mufflers being of little significance. How an exhaust system comes together, being either an interference or independent type, is another story.

To get an idea of what shock wave tuning can do for a four-stroke, look at fig. I. The lower curve shows the dynamometer results of a four-stroke motor run without regard to exhaust length. At various points in the power curve the power produced jumps or drops as various counteracting conditions operate in the combustion chamber. Surprisingly, the benefits of dilution waves, or shock waves, are more important on mildly tuned, or less powerful machines. With a motorcycle putting out 60 bhp or more, power peaks are not especially noticeable because peak power isn't needed for usual riding. On a smaller motorcycle w here the motor will be operated near peak output, the small deviations become much more apparent.

To understand how shock waves can work to help scavenge an engine, it is necessary to consider what occurs inside the combustion chamber during valve overlap: when the exhaust valve has not yet closed and the intake valve has begun to open.

Imagine a four-stroke engine's cycle. First the piston sucks in the incoming mixture, then the second stroke compresses the mixture, then as the mixture burns with the piston at the top of its travel a third stroke begins so the power can be transmitted to the crankshaft. Finally a fourth stroke, an exhaust stroke, pushes the burned exhaust gases out the exhaust port.

When the exhaust valve first opens a pressure wave immediately travels through the gases in the exhaust system at a speed of 18,500 inches per second, or the speed of sound. When the shock wave gets to the pipe exit, a negative shock wave is produced and travels back through the outcoming exhaust gases.

Now think w hat would happen if everything were timed so the negative shock wave got back to the combustion chamber just before the exhaust valve closed and when the intake valve was just opening. If it is timed just right, the shock wave, or dilution wave, will run right through the head, all the way to the carburetor where it would help pull the incoming mixture into the combustion chamber. That is how exhaust length tuning works. (See fig. 2.)

In practice it’s not so easy. When the first engine to use pressure wave phenomenon was made in 1892. engine speeds of 200 rpm were normal. Designers Crossley and Atkinson found that an exhaust length of 65 ft. was needed to take advantage of the dilution wave. With a size like that it's no wonder most automotive and aeronautical engineers ignored pressure waves in engine design until the 1920s when engine speeds picked up enough to make the use of pressure waves practical.

The long exhaust pipe length necessary for low engine speeds is now a simple matter of calculation, although w hen the Crossley engine was made there must have been considerable experimentation. The shock wave begins when the exhaust valve first opens and returns in the amount of time it takes to reach the end of the exhaust pipe. But the faster an engine spins, the less time it takes from exhaust valve opening until intake valve opening when the shock wave is most useful. Therefore the length of the exhaust system determines the engine speed in which the dilution waves help.

The equation used to determine exhaust length is Bellilove's equation:

Applying this equation to a practical application can help explain how exhaust tuning works. A common cam timing holds the exhaust valve open for 298° of the engine’s revolution. The exhaust valve opens at 78° before the piston reaches bottom dead center, stavs open for the 180c exhaust stroke plus another 40° into the intake stroke.

Plugging this common example into the equation gives:

That would be the necessary exhaust length to benefit from dilution waves at 6000 rpm. Other lengths, of course, would be necessary for other engine speeds. In fig. 4 the exhaust length is shown tor this engine from 2000 rpm until 14.000 rpm using exhaust timing of 228°, 234 ', 270 and 298°. The reason: a figure of 298° is the maximum exhaust cycle, a figure taken from the time the valve opens until it is absolutely closed. In fact, the first 10e and last 10° of valve opening allows little room for gas flow and so the exhaust cycle is really shorter and the exhaust length becomes shorter. Most tuners, however, work with the longest figures in calculating exhaust lengths because it’s easier to cut ofi exhaust pipes than to weld on extensions.

Calculating the maximum length of time both the intake and exhaust valves are open shows that exhaust length tuning can work for a range of engine speeds. Running the numbers through the formula shows that a pipe on our theoretical engine of 25.2 in. can have an effect through 2000 rpm of the engine's operating range.

Operating with a straight pipe of the correct length can measurably increase power through such a narrow range. And a range of 2000 rpm is fine for racing machinery in some applications, but on the street a broader range is better. Help comes from the rest of the exhaust system. The negative wave begins forming when the cylindrical exhaust pipe changes diameter, or when it ends. Put a megaphone on the exhaust pipe, however, and things change. The megaphone, a long expanding cone shape, provides a succession of expanding

diameters, each one creating its own negative shock wave like a series of steps.

For our example, consider adding a 27.5 in. megaphone on the end of the 25.2 in. exhaust pipe. The first negative shock wave would benefit the engine at speeds as high as 9000 rpm. As the exhaust wave traveled down the megaphone a series of reverse shock waves would be sent back, each one sucking exhaust gases out and the incoming mixture into the cylinder. This effect

would continue until the last shock wave was formed at the end of the megaphone, helping produce power down to 3450 rpm. (See fig. 5.)

While this illustration has been simplified (such things as curves in the exhaust pipe or intrusions at junctions interfering with the shock waves as much as the gas flow ), it explains how a shock wave can be used to improve gas flow throughout an engine.