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.This is not to imply that the first exhaust you fabricate will be perfect.The Japanese manufacturers, and in particular Yamaha and know as much asanyone about exhaust but you will find they are constantly updating theexpansion chambers on both their works and production racers, proving they are stilllooking for the best design.Understanding exhaust design begins with an appreciation of the behaviour ofsonic waves travelling through a pipe.These waves travel at a speed determined by thetemperature and of the outflowing exhaust gas.This speed always equals thespeed of sound, which averages around 1675ft.per second in hot exhaust gas.Sonic waves have the strange property of being reflected back along the pipe theyare travelling through, regardless of whether the pipe has an open or closed Evenmore peculiar is another fundamental law of acoustics which causes a pressure wave toinvert its sign on reaching the open end of a pipe.A positive pressure wave, on reachingthe pipe's open end, goes back up the pipe as a negative wave, and vice versa.Reflection a pipe's closed end does not change sign, a positive wave stayspositive.The earliest exhausts were a piece of straight pipe, but these were not able to take70 full advantage of waves to exhaust gases out of the cylinder.In this type of The Exhaustsystem a positive pressure wave charged down the pipe immediately the exhaust portopened.On reaching the end of the pipe it was reflected as a negative wave, but withreduced intensity because much of its wave energy was lost to the surroundingatmosphere.However, some energy did and if, when the negative wave reachedthe exhaust port, the port was still open, it would assist in a small way in evacuating thecylinder.This being the case, the wave would turn round and travel back down theexhaust still negative, then, on reaching the open end of the pipe, be reflected up againas a positive pressure wave.If the exhaust was of the correct length, the positive waveshould have arrived back at the exhaust port just before it closed, forcing any fuel/airmixture that had spilled into the exhaust back into the cylinder to be burned.In theory it sounds good, but in practice the straight pipe exhaust never workedtoo well, primarily because so much kinetic energy was lost each time the sonic wavereached the open end of the exhaust pipe.A two-stroke engine requires strong pressurepulses to work efficiently, so engineers added a megaphone to the end of the straightpipe.A more correctly called a diffuser, is in effect a relatively efficientenergy inverter.In a diffuser the walls diverge causing the sonic wave to react just asthough it had reached the open end of the exhaust.However, the reflected wave retainsmost of its energy and can create a vacuum as low as 6 Obviously a pulse wave ofthis magnitude can be very effective in drawing exhaust gas out of the and inpulling the fresh charge from the crankcase up through the transfer ports.The problem with this system is that much of the time the strong negative pulsewave will arrive at the wrong moment, and draw a considerable amount of fuel/airmixture into the exhaust.The exhaust port will close before the reflected positive wavearrives, to force the mixture back into the cylinder.The next step was to add a reverse cone with a small outlet to the diffuser, and thisproved to be the real break-through in two-stroke exhaust design.This type of exhaustis referred to as an expansion chamber.The addition of the reverse cone with a smallbleed-off hole acts as a closed pipe, giving the exhaust a double pulse action.When thepositive wave reaches the diffuser, part of the wave is inverted and reflected as anegative wave to evacuate the cylinder.However, part of this wave continues on to bereflected by the reverse cone.Because of the pressure buildup caused by the small bleedhole, the reverse cone acts like a closed pipe, reflecting the wave with the same positivesign.This strong positive pulse arrives just before the exhaust port closes, forcing anyescaped mixture back into the cylinder, increasing power output and reducing fuelconsumption.On paper that theory, too, sounds very simple, but there is much more involvedwhen we actually set about designing a Obviously the expansion chamber mustbe of the correct length to ensure the pulse waves are reflected to arrive at the exhaustport at the proper time.The formula we use to determine the tuned length of the exhausttuned length in mmED = exhaust duration in degrees= engine speed exhaust is tuned to work best Two Stroke Performance TuningAssuming we had an engine with an exhaust duration of 196� and producingmaximum power at the tuned length would be:The length of 758mm is measured from the piston face to the assumed reflectionpoint of the reverse cone.This point is half way along the cone but, because the top iscut from the cone, the point must be calculated mathematically.(FIGUREThe first part of the expansion the header pipe, may be either a straightpipe with parallel walls, or a tapered pipe with diverging A tapered header pipe isto be preferred, as it will improve the power and power range; however, it is muchmore difficult to fabricate than the straight pipe.For this reason, some designs utilise aparallel-wall header.Most go-kart headers, and also the Honda CR250R expansionchamber for example, have a non-diverging header pipe.(FIGURE 4.2).The actual length required for the header pipe can only be determined accuratelyby testing.Over the years, I have devised and tried all kinds of formulae to calculatethe length of the header, but I've never found one that works too well.It has been myexperience that it is much quicker to make an educated guess and work from there.In TABLE 4.1, I have set out what I consider to be a good starting point inworking out the header pipe length.For example, if you are building a chamber for aroad race 125 with an exhaust port inside diameter of 38mm and you intended tofabricate a multi-stage diffuser, then the header would be between 247mm and 285mmlong.if you were to use a single stage diffuser, the header would be a littlelonger at 296 to 323mm.Usually, you will find the shorter length in both instances tobe very close to what is required for best performance.Lengthening the header has theeffect of increasing mid-range power at the expense of a drop in maximumIt important to note that the above rule for calculating the header length canwork only if the exhaust port is of a standard diameter for that particular size ofengine.If the engine has a port size outside that shown in TABLE 4.2, then it will be4
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