2 Stroke Engine Port Timing

by 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 two main things that determine peak power RPM are the 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 cylinder ports. The engine porting of a race bike should match the pipe in that the portings power emphasis should be around 1000 RPM before the end of the pipe powerband. The pipe powerband is usually the last 20% of the engines RPM during which the pipes return baffle wave prevents some intake loss out the exhaust port (thus increasing burnable intake charge which benefits power) and the pipes return diffuser wave increases transfer of fue/air into the upper cylinder and more air/fuel from the carb thru the reed valve. If the port durations are long then the upper part of that range will have extra power, and if the port durations are short then the lower part of that range will have extra power (which is better for street and trail bikes).

   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 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). 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.

Porting for the following models, according to the specs of Eric Gorr, can be read about in his book "Motocross & Off-Road Performance Handbook ":
1999 - 2004 Yamaha YZ 250 for more top end power.
1992 to 1998 Yamaha YZ 250 for more top end power. 
1994 to 1996 Yamaha YZ 125 for more top end power. 
1990 to 1999 Suzuki RM 125 for more top end power. 
1992 to 1994 Honda CR 250 for more power throughout. 
1998 to 1999 Honda CR 125 for more top end power. 
1997 to 2004 Kawasaki KX 250 for smoother broader powerband with more on top.

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