2 Stroke Engine Port Timing

by Michael Forrest

   An engine "port" is an open window in the cylinder wall for the intake or exhaust of fuel mixture (air/gas). There is one or more forward-located exhaust ports, two or more side transfer ports, and one rear intake port. The intake and transfer ports allow fuel mixture to enter the cylinder, and the exhaust port allows the burnt fuel mixture to exit the cylinder into the exhaust pipe.

   The main determinant of peak power RPM is the duration and size of the ports. Click here to read more.

   A powerbands RPM range is determined primarily by the port heights and sizes, and secondarily by the design of the exhaust pipe. There are pipes for top RPM powerbands, and ones for mid RPM powerbands. The pipe should match the type of powerband that the engine porting is designed for. A high RPM pipe can be changed to a mid RPM pipe by lengthening the header which means it will take longer for the exhaust wave to reach that rear cone, and longer for the wave to return to the cylinder.

   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) with a thinner base gasket or 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. Then put the head back on with the correct torque and slowly kick over the engine with the spark plug cap off of the spark plug. Then remove the head again and remove the solder in order to measure its "squished" thickness. Measure the thinnest part with a micrometer. 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:
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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