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.There must be 2mm down each side of the inlet port againstwhich the piston will effect a seal.If the rings run into the port at BDC, you will haveto ensure that you do not increase the width so much that the ends of the ring becomeexposed.However, if you decide to run just the top ring, and it is the second ring whichis running into the inlet tract, you won't have to worry about this.Beside reducing frictional losses and bore wear, discarding the second ring canalso have another benefit.With the second ring out of the way it is possible, in manyinstances, to increase the inlet port height.At times this won't work without alsoincreasing the port timing, as the piston skirt will block the top of the port at TDC,unless it is shortened.Actually, the first check that you should make before loweringthe inlet port to increase the port open period is to see that the lower edge of the pistonskirt does not protrude into the top of the port with the crank rotated to TDC.Whenthe skirt is shortened, cut off just the inlet side and be sure to put a good chamfer on50 the skirt so that it encourages lubricant to stay on the cylinder wall.Port and Cylinder ScavengingALL dimensions in mm.Fig.3.21 Yamaha KT-1OOS porting & piston dimensions.A lot of tuners lengthen the inlet timing just by shortening the piston.Sometimesthere is no alternative, as the cylinder may be too weak to stand having metal removed,but, generally, skirt cutting is the easy way out.Even though cutting 3mm off the skirtwill increase the inlet duration to the same figure as lowering the inlet floor by 3mm,you will find that maximum hp will not be as high and the engine will not rev as far pastmaximum hp revs.The simple truth is that the port area, as well as the duration, mustbe increased to flow the amount of air necessary to improve the power output.I havefound, as a general rule, that the piston skirt will have to be shortened by 4mm to givethe same high speed power characteristics as obtained by lowering the port 3mm.However, mid-range power is not as good, due to increased blow-back caused by thelonger duration.For maximum power, the inlet port area should be about 10 to 15%larger than the area of the carburettor bore.When the inlet floor is lowered, the full length of the port floor right back to the 51Port and Cylinder ScavengingRaise carburettor to minimisekink in induction tract.23 Raise carburettor to improve inlet flow.Fig.3Inlet port open.Rotary valve disc.Fig.3.24 Rotary valve induction arrangement.53Two Stroke Performance Tuningto 10° before the transfer port closes (i.e., 120 to 130° before TDC) and to close theinlet port at about 55 to 60° after TDC.This results in an inlet duration of around 180to 190°.For more power at the top end of the power curve, the duration is increased tosomething like 200 to 210°.There will, however, be some loss of low speed power andthe engine won't take a fistfull of throttle at low revs without stumbling.The increasein duration can be obtained in two ways.Either we can have the rotary valve open alittle earlier at 135 to 140° before TDC and close a little later at 65 to 70° after TDC, orwe can leave the valve opening point alone and pick up the extra duration by closing theport at 70 to 80° after TDC.The effect on the power curve will be quite different, eventhough the inlet open period is the same.Opening the valve at, say, 140° before TDCand closing it at 65° after TDC (205° duration) will tend to lift maximum power a little,but the main effect will be to considerably increase power in the upper mid-range.Leaving the opening point at 125° before TDC and shifting the moment of closing to80° after TDC (205° duration) will reduce mid-range power due to increased blowback, but there will be a good power rise right at the top end of the power curve(FIGURE 3.25).In high rpm road racing engines, where mid-range power is of only minor concern,the inlet duration is increased to about 220 to 235°.The rotary valve will open at 135 to150° before TDC and close at 80 to 90° after TDC.The main concern here is that theinlet duration is of sufficient length to ensure complete crankcase filling at the rpmwhere maximum horsepower is desired.If we want peak power at 14,000rpm then the54 Fig.3.25 Effect on power curve of changing rotary valve closing angle.Port and Cylinder Scavengingduration will be around 235°, but if we want peak power at ll,500rpm the durationwill be close to 220°.TABLE 3.4 sets out the rotary valve timing for a number of go-kart and bikeengines.All of the l00cc kart engines have fixed gearing.Before you set about altering the valve timing, check to see that the inlet port is ofthe correct shape and that the valve cover perfectly matches the inlet port in thecrankcase.Any obstruction here will disrupt air flow.You will find in many enginesthat the port in the valve cover does not align with the crankcase port.Grinding theport in the valve cover or the crankcase will affect the inlet timing.In some engines theinlet port opens and closes slowly because the sides of the port are the wrong shape.The port illustrated in FIGURE 3.26 should be reshaped as shown.The port area isincreased and it will open and close more abruptly, generating beneficial pulse waves inthe inlet tract.The actual side profile of the inlet port is very poor in many rotary valve engines.In FIGURE 3.24 you can see a common mistake made by manufacturers which is verydisruptive to air flow.The mixture rushes straight down the inlet port and proceeds tobang right into the crankwheel, losing a good deal of inertia.Some of the mixture willslowly rise up and around the crankwheel into the crankcase and a little of the air willform into a turbulent eddy current.When this kind of situation exists, air flow into theengine is severely restricted at high rpm.To increase air flow, and consequently highspeed hp, there are two options open.Either the inlet port open period can beTABLE 3.4 Rotary valve timingEngine type Capacity (cc) Valve timing Transfer closingArisco C-75 kart 100 155/43 124BM K96-3 kart 100 115/60 123BM FC-52 kart 100 115/60 120Can-Am MX-6 bike 125 140/85 113Can-Am MX-3 bike 250 140/85 125Can-Am MX-6 bike 250 140/85 113Can-Am Qualifier bike 175 137/75 113Can-Am Qualifier bike 250 137/75 116Can-Am Qualifier bike 350 137/75 116DAP T81 kart 100 132/58 117DAP-JM T71 kart 100 120/55 113.5Komet K78 kart 100 132/60 118Komet K78 TT kart 100 132/60 117Morbidelli 125 bike 2x62 150/79 109MZ 125 bike 125 135/70 112Rotax 124 LC kart 125 120/87 113Sirio ST50 kart 100 134/75 116.5Sirio ST504 kart 100 135/65 120Sirio ST52 kart 100 134/75 117.3Zip ZED1 kart 100 140/66 121.5Note: The first valve timing figure refers to the opening point in degrees before TDC and thesecond figure is the closing point after TDC.The transfer closing figure refers to theclosing point in degrees before TDC.55Two Stroke Performance TuningLeave 2mm.minimum for discto seal against.Required port shape.Fig.3.26 Modifying inlet port shape without affecting rotary valve timing.increased, which will reduce mid-range power, or we can reprofile the inlet port andincrease air flow in this way.Top end power will improve and often mid-range powerrises too.What we must do is change the shape of the inlet port, so as to encourage themixture to turn up and over the crankwheel.In effect, the edge of the crankwheel hasto become a part of the inlet tract floor, instead of a barrier at the end of a hole.InFIGURE 3.27 you can see the shape we have to aim for [ Pobierz caÅ‚ość w formacie PDF ]