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.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.With extra pulse energyavailable, the expansion chamber can do a better job of scavenging exhaust gases outof the cylinder and drawing up the fresh fuel/air charge through the transfer ports.In TABLE 4.4 you will note the diffuser angles which I have found to work wellwhen multi-stage diffusers are utilised.For example, a twin cylinder road race 250would use a diffuser with the first section tapering at the second section at 7° andthird section at Just how long each section should be is a secret most two-stroketuners keep to themselves.However, I will tell you this: motocross and enduro bikesusing a two-stage diffuser will usually require the first section to be 200 to 240mmroad race bikes with a two-stage diffuser will usually require the first stage to be 140 to160mm long with a three-stage diffuser, the first stage will be to 140mmlong.If, after building an expansion chamber with a diffuser like you find that theengine is too peaky, then increase the length of the first stage of the diffuser.This willbroaden the power band.Conversely, if the engine lacks peak power, decrease thelength of the first diffuser section and lengthen the second stage.In FIGURE 4.5 youwill note the diffuser designs which a variety of manufacturers are using.The parallel belly section of the expansion chamber naturally has the samediameter as the diffuser outlet, but we cannot calculate its length until we have arrivedat a suitable size for the rear baffle cone.After the mid-section fills the gap to givethe chamber its correct tuned length.As noted earlier, the baffle cone (or reverse cone) reflects a wave of like sign tostuff the fuel/air charge back into the cylinder.A flat plate could do the same job butthe wave duration would be so short that this would only occur over a very narrowA cone, on the other hand, extends the duration of the pressure pulse, althoughreducing its intensity.This serves to broaden the engine's useful power band.Again, the actual taper of the baffle cone affects the pulse time/intensity factorjust as in the case of the diffuser.A short, sharp baffle will increase maximum power,but the motor will lose out in the mid-range; also it will tend to cut dead at just a couple78 of hundred revs past maximum power rpm.A shallow taper baffle reduces top endTwo Stroke Performance Tuningpower, but the engine will develop more power lower down the range and it will-ev on well past the maximum power engine speed.This broadens the engine's effectiverange and means that the bike's speed down the main straight will be lessby changing wind conditions during a race.If a head wind blows, the enginehave enough power below maximum to fight against it and, if a tail wind comes in,:he shallow baffle will enable the engine to over-rev and pick up some more speed.;FIGURE 4.6).TABLE 4.5 indicates the baffle cone tapers with which I prefer to work.Theseingles have proved to give a good power range without suppressing maximum hpIn our example, we will assume that an baffle taper is used.The formula is:-80Fig.Effect of baffle angle on horsepower outputThe ExhaustYou will note that this calculation gives us the overall length of the cone, withoutallowing for a part of the top to be cut off where the stinger will be attached to bleedpressure from the By halving this figure, 246mm, we can determine the meanreflection point of the baffle and then go back and calculate how long the belly sectionhas to be to give us the tuned length of 758mm which we originally worked out.In an effort to broaden the power band on motocross bikes and smalldisplacement road race bikes, tuners and manufacturers have been experimenting withtwo-stage baffles over the past few years (FIGURE 4.7).Unfortunately the results havenot been as promising as expected.However, small gains have been made by sometuners working with 62cc and 125cc size cylinders.If you wish to do someexperimentation along these lines, start out using a baffle which has the first stagetapering at 4.5° to 5 [ Pobierz caÅ‚ość w formacie PDF ]