Jul 20, 2004 10:04 PM
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(Updated Jul 07, 2006 05:42 PM)
Changing the power bandof your dirt bike engine is simple when you know the basics. A myriad ofdifferent aftermarket accessories is available for you to custom tune your biketo better suit your needs. The most common mistake is to choose the wrongcombination of engine components, making the engine run worse than stock. Usethis as a guide to inform yourself on how changes in engine components canalter the powerband of bike's engine. Use the guide at the end of the chapterto map out your strategy for changing engine components to create the perfectpower band.
TWO-STROKE PRINCIPLES
Although a two-strokeengine has less moving parts than a four-stroke engine, a two-stroke is acomplex engine because it relies on gas dynamics. There are different phasestaking place in the crankcase and in the cylinder bore at the same time. Thatis how a two-stroke engine completes a power cycle in only 360 degrees ofcrankshaft rotation compared to a four-stroke engine which requires 720 degreesof crankshaft rotation to complete one power cycle. These four drawings give anexplanation of how a two-stroke engine works.
1) Starting with thepiston at top dead center (TDC 0 degrees) ignition has occurred and the gassesin the combustion chamber are expanding and pushing down the piston. Thispressurizes the crankcase causing the reed valve to close. At about 90 degreesafter TDC the exhaust port opens ending the power stroke. A pressure wave ofhot expanding gasses flows down the exhaust pipe. The blow-down phase hasstarted and will end when the transfer ports open. The pressure in the cylindermust blow-down to below the pressure in the crankcase in order for the unburnedmixture gasses to flow out the transfer ports during the scavenging phase.
2) Now the transfer portsare uncovered at about 120 degrees after TDC. The scavenging phase has begun.Meaning that the unburned mixture gasses are flowing out of the transfers andmerging together to form a loop. The gasses travel up the back side of thecylinder and loops around in the cylinder head to scavenge out the burntmixture gasses from the previous power stroke. It is critical that the burntgasses are scavenged from the combustion chamber, in order to make room for asmuch unburned gasses as possible. That is the key to making more power in atwo-stroke engine. The more unburned gasses you can squeeze into the combustionchamber, the more the engine will produce. Now the loop of unburned mixturegasses have traveled into the exhaust pipe's header section. The gasses aren'tlost because a compression pressure wave has reflected from the end of theexhaust pipe, to pack the unburned gasses back into the cylinder before thepiston closes off the port. This is the unique super-charging effect oftwo-stroke engines. The main advantage of two-stroke engines is that they cancombust more volume of fuel/air mixture than the swept volume of the engine.Example: A 125cc four-stroke engine combusts about 110cc of F/A gasses but a125cc two-stroke engine combusts about 180cc of F/A gasses.
3) Now the crankshaft hasrotated past bottom dead center (BDC 180 degrees) and the piston is on theupstroke. The compression wave reflected from the exhaust pipe is packing theunburned gasses back in through the exhaust port as the piston closes off theport the start the compression phase. In the crankcase the pressure is belowatmospheric producing a vacuum and a fresh charge of unburned mixture gasses isflowing through the reed valve into the crankcase.
4) The unburned mixturegasses are compresses and just before the piston reaches TDC, the ignitionsystem discharges a spark causing the gasses to ignite and start the processall over again.
CYLINDER PORTING
The cylinder ports aredesigned to produce a certain power characteristic over a fairly narrow rpmband. Porting or tuning is a metal machining process performed to the cylinderports (exhaust & transfers) that alters the timing, area size, and anglesof the ports in order to adjust the power band to better suit the rider'sdemands. For example, a veteran trail rider riding an RM250 in the Rockymountain region of the USA will need to adjust the powerband for more low end power because of the steep hill climbs and the lower airdensity of higher altitudes. The only way to determine what changes will beneeded to the engine is by measuring and calculating the stock engine'sspecifications. The most critical measurement is termed port-time-area. Thisterm is a calculation of a port's size area and timing in relation to thedisplacement of the engine and the rpm. Experienced tuners know what theport-time-area values of the exhaust and transfer ports should be for an engineused for a particular purpose. In general, if a tuner wants to adjust theengine's power band for more low to mid range he would do the following things.Turn down the cylinder base on a lathe to increase the effective stroke(distance from TDC to exhaust port opening). This also retards the exhaust porttiming and shortens the duration and increases the compression ratio. Next thetransfer ports should be narrowed and re-angled with epoxy to reduce theport-time-area for an rpm peak of 7,000 rpm.
