2 Stroke Engine Port Timingby Michael Forrest An engine "port" is an open window in the cylinder wall for the receiving or expeling of fuel mixture (air/fuel). There is one or more forward-located exhaust ports, two or more side transfer ports, and one rear intake port called a boost port. The intake port allows air/fuel to enter under the piston, the transfer ports allow fuel mixture to enter the cylinder above the piston when it is low, and the exhaust port allows the burnt fuel mixture to exit the cylinder into the exhaust pipe. The main things that determine peak power RPM are 1) pipe design, 2) crankcase compression ratio, 3) degrees of port open duration and area of the ports. Click here to read more about this. A bikes top RPM is determined by the length of the exhaust pipe, but what range of RPM is given power emphasis is determined by the crankcase compression ratio (CCR). The CCR of a race bike should match the pipe in that the peak delivery ratio should be around the middle of the pipe powerband. The pipe powerband is usually the last 25% of the engines RPM during which the pipes return baffle wave increases dynamic compression and the pipes return diffuser wave increases transfer of air/fuel into the upper cylinder and more air/fuel from the carb thru the reed valve. An engine that was designed for trail riding can have its ports top edges raised more with a grinding tool for more of a top end powerband for motocross use, and its CCR increased by filling the holes in the crank wheels with some lightweight heat resistant material. And a motocross engine can have its powerband lowered by lowering the cylinder (and therefore the ports) by machining 1 or 2 millimeters off the base of the cylinder (where it rests on the base gasket) and machining the same amount off of the cylinder head's squish band (which also makes it wider which is better for mid-RPM engines), and the CCR decreased by removing the hole stuffers in the crank wheel holes. Without machining the cylinder head there would be too much of an increase in the compression ratio and squish velocity and therefore more possibility of detonation. The ideal head to piston clearance can be found by using my squish velocity calculator. The clearance can be measured with the help of some plumbers solder before and after the machining work. Just put a length of it across the top of the piston (left to right) after taking off the cylinder head and glue its center to the piston. Then put the head back on with the correct torque and slowly kick over the engine with the spark plug removed. Then remove the head again and remove the solder in order to measure its "squished" thickness. Measure the thinnest part with a micrometer [or digital caliper]. Also you should double check the compression with the cylinder head on. Water cooled 80cc to 200cc engines should have 150-190 psi, while a 250cc engine should have 170-230 psi. Too much compression should be reduced by using a thicker head gasket or by lathing off more metal from the squish band of the cylinder head.
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