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Dirt Bike Mods & Maintenance
Two stroke VS four stroke. Facts & figures.
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[QUOTE="Rich Rohrich, post: 892161, member: 16241"] A four-stroke fires and heats the piston crown every 720 degrees of crank rotation while a two-stroke does it every 360 degrees. That additional 360 degrees of rotation allows the piston crown and rings a lot of time to cool and helps extend their life considerably. It's incredibly rare to see a four-stroke engine with a hole burned into the center of a piston crown. I wish I could say the same for my two-strokes. The fact that the rings are not forced to bounce in and out of port windows on a four-stroke is also good for longer term sealing and overall reliability. Cranks tend to last longer on four-strokes for a number of reasons : - oil flow to the bearings on a four-stroke is constant and under pressure which ensures delivery. Crank and rod bearings on a two-stroke only seal oil diluted by the fuel that is splashed on them during rotation. The fact that some pinheads ride MX and mix at 66:1 ratios only makes things worse. - heat to the upper rod bearing surface. Firing every 360 degrees on a two-stroke sends a lot of heat through the piston crown and the upper rod bearing. Oil vaporizing out of the bearing during these times is pretty commonand doesn't help bearing life. - piston speeds - If a CR250 produces max power at 7500 rpm it's mean piston speed is 3543 fpm, if a CRF450 produces max power at 9000 rpm it's mean piston speed is 3662. Not a lot of difference, especially when you consider most people don't spend much time above 7500 rpm on a CRF450 because it works best by using the torque below the peak. RPM trends on a CRF450 on an SX type track look more like this : [url]http://dirtrider.net/teamdrn/showphoto.php/photo/4438/size/big/sort/1/cat/500[/url] As for transmissions, they seem to last about the same in both types of engines. They are built a little bigger in four-strokes but only enough to withstand the additional torque loading. Good four-stroke riders use the engine torque rather than the clutch,and it's been my experience that the only guys who wear four-stroke clutches faster than two-strokes are the same guys who were abusing clutches before the CRF and YZF ever existed. ;) Some people just abuse clutches regardless of what they ride. Riding style seems to play a much bigger factor in clutch wear than engine type ever will. [/QUOTE]
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Dirt Bike Mods & Maintenance
Two stroke VS four stroke. Facts & figures.
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