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.5-9.5 8-9 10-11 8.5-9.57.8-8.5 6.5-7.5 7.8-8.5 6.5-7.5175-250 7.3-8.3 6.5-7.5 9-10 8.2-9.2350-500 8.5-9.5 7.5-8.5Note: To calculate header pipe length multiply exhaust port diameter by the appropriatemultiplying factor.Single stage refers to a single stage diffuser.72 Multi-stage refers to a multi-stage diffuser.The ExhaustFig.4.1 Basic two stroke expansion chamber.73The Exhaust4.1 Basic two chamber.73Two Stroke Performance Tuningpipe.All dimensions in mm.pipe.Fig.4.2 Expansion chambers with parallel-wall header pipes.necessary to work out the length based on a standard port Also, keep in mindthat the calculated length of the header is the measurement from the piston to the endof the header Therefore, when you fabricate the remember to subtract thelength of the exhaust port.Over the past ten years the majority of manufactures have realised the benefits tobe gained from a header and have gone over to this design.In the firstinstance it increases chamber volume, which effectively broadens the power range.Secondly, its diverging walls reduce flow resistance both in and out of the cylinder.Theburnt gases flow out easily with a minimum of turbulence and any fuel mixture whichhas spilled into the exhaust is rammed back into the engine more efficiently.Thirdly,and this is the most important reason for the justification of tapered headers, theshallow taper allows the exhaust gas to expand and cool more gently.This results in lessloss of kinetic pulse energy than if the gases were to expand rapidly and abruptly onpassing from a straight header into the diffuser section of the expansion WithTABLE 4.2 Standard exhaust port diameterCylinder size (cc) Port inside diameter (mm)62-80 30-32100 34-37125 37-40175 42-4625044-48350-50074 45-50The Exhaustmore pulse energy available, a stronger evacuation wave to scavenge the cylinder, and astronger wave to ram spilled mixture back into the cylinder, is produced.The taper of the header is normally between 1.15° and 1.5°.However, somemanufacturers have used tapers as shallow as 0.8° and as steep as 2.3° in certaincircumstances.If the diffuser taper is very shallow (i.e., 2.8° to 3.25°) then a 0.8°header taper is at times in order.Conversely, if the exhaust port and flange is very long(i.e., 75mm to 100mm) a steeper 1.7° to 2.3° header may be necessary.(FIGURE 4.3).To work out the physical size of a diverging header pipe of a particular length andwe use this formula:-where D2 = header pipe major inside diameter= header pipe minor inside diameterLH = header pipe length minus the length of the exhaust port and flangeCot H = cotangent of header pipe's angle of taperWe will assume our road racer has a cylinder volume of and an exhaust portand flange 70mm long by 40mm id.The total length of the header will be (40 x 6.5) - 70= 190mm.The taper of the pipe will be 1.5°, which has a cotangent of 38.19 (fromTABLE 4.3)TABLE 4.3 Useful CotangentsAngle Cotangent Angle Cotangent Angle Cotangent0.8 71.62 6 9.5144 11 5.14461 57.29 6.5 8.7769 1.1.5 4.91521.25 45.83 6.75 8.4526 12 4.70461.5 38.19 7 8.1443 12.5 4.51071.75 32.73 7.25 7.8712 13 4.33152 28.64 7.5 7.5958 13.5 4.16532.5 22.90 7.75 7.3498 14 4.01083 19.08 8 7.1154 15 3.73213.5 16.35 8.5 6.6912 16 3.48744 14.30 9 6.3138 17 3.27094.5 12.71 9.5 5.9758 18 3.07775 11.43 10 5.6713 19 2.90425.5 10.39 10.5 5.3955 20 2.747575Two Stroke Performance Tuningpipe.All dimensions in mm.Fig.4.3 Expansion chambers with tapered header.The next stage of the expansion chamber, the diffuser, is, as we mentioneda wave inverter.The duration and intensity of the inverted return wave is determinedby the diffuser taper.A shallow taper returns a wave of long duration and lowintensity.This has the effect of cutting maximum power but, beneficially, it boostsmid-range power by allowing the expansion chamber to stay with the engineover a larger band.(FIGURE 4.4).Conversely, a steeply-tapered diffuser reflects a pulse of high intensity and shortduration.Maximum power will be increased but at the expense of narrowing the powerband.In applications such as road racing this may be acceptable if the machine has aclose ratio 6-speed gearbox and the rider has the necessary riding skill to ride a bikewith a narrow power range and a sudden rush ofIn TABLE 4.4, you can see the sort of angles that I recommend forSome people build expansion chambers with larger diffuser tapers, but I tend to valuegood mid-range power and a easily managed power band much more than all-outpower.The length of the diffuser is determined by the diameter to which it expands,which should normally be times the exhaust port diameter.If you have room on thebike, and you wish to experiment, you may be able to spread the power range bymaking the diffuser taper to 2.7 - 2.9 times the port diameter.This move will at timessuppress maximum power, but the improvement lower down the scale usuallycompensates.On many bikes, it is difficult to find room for a large diameter exhaust,so you may have to be satisfied with a less than perfect chamber.Most bikes havetrouble catering for an exhaust in excess of diameter, so this could restrict thediffuser outlet to something closer to 2.2 times the port diameter in bikes over 175cc.Going back to our previous example, we calculate the diffuser size based on a 7°76 taper (i.e., 14° divergence) and 2.5 times the port diameter (i.e., 40mm x 2.5 = 100mm).The ExhaustThe formulaToday, instead of relying on a single taper diffuser, we are using multi-stagediffusers.Generally, a two or three section diffuser is utilised, although some tuners.4 Effect of diffuser on intensity & duration of return wave.77Two Stroke Performance TuningTABLE 4.4 Diffuser tapersDiffuser angle (degrees)Cylinder size (cc) Road race Motocross & Endurosingle stage two stage three stage single stage two stage50-80 6.5 to 7 4.5 & 7 3 to 3.5 3&5100-125 6.5 to 7.5 4.5 & 7.5 4 to 4.8 3.3 & 6175 6.5 to 7.5 4.5 & 7 4.5 & 7 & 10 3.5 to 4,5 3.5 & 6250 7 to 7.5 4.5 & 7 4.5 10 4 to 4.5 4&7350-500 4 to 5 3.5 & 6and manufacturers are turning to the use of four stage diffusers.For motocross andenduro bikes, I usually work with a two-stage diffuser as relatively shallow tapers areinvolved.Road race engines and 125 motocross engines with near road race portingrequire a three stage diffuser, due to the need for steeper angles of taper to pick uppeak power.A multi-section diffuser allows the exhaust gas to expand and cool moregradually, which means there is less loss of kinetic pulse energy than if the gases wereallowed to expand more rapidly in a single taper diffuser [ Pobierz caÅ‚ość w formacie PDF ]