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 higher the top edge of the exhaust port is in the cylinder, the higher the powerband is. That is to say that the "band of power" will be higher in rpm's (rotations per minute). That's because when the expelled exhaust reaches the rear cone of the exhaust pipe some of it bounces back as a pressure wave that, when it reaches the cylinder, can keep the newly entered charge of fuel mixture from wastefully exiting from the cylinder into the exhaust pipe. The time it takes for the returning pressure wave to reach the cylinder is usually around .003 seconds, but the time from the exhausts first entrance into the pipe till the new fuel mixture tries to escape thru the exhaust port (due to the upward piston movement trying to compress the mixture there) is dependent on the engine rpm. At high rpm's, that time period is short, and at low rpm's it is long. The rpm range centered around when the two time periods coincide is the range of the powerband, or peak efficiency of the engine, when there is little or no loss of fuel mixture out thru the exhaust port.
So when the exhaust port is high in the cylinder, the pressure wave cycle begins at an earlier degree of crank movement (starting from the pistons topmost position), let's say at 80 degrees past TDC (top dead center). The new fuel charge will have completely entered the cylinder at around 180 degrees, when the piston is at its lowest position and the crank is halfway thru its 360 degree cycle. At around 230 degrees the piston seals off the intake ports from the combustion chamber (the area between the piston top and the cylinder head) and starts trying to compress the fuel mixture and some will exit the exhaust port unless the pressure wave blocks it. If the pressure wave comes back sooner or later than when the piston has halfway closed off the exhaust port (say around 260 degrees) then some of the fuel mixture will escape out the exhaust.
A typical pressure wave return time is .003 seconds. With the exhaust port beginning to open at 80 degrees, this wave would return exactly when needed at 10,000 rpm because then the time it takes for the piston to move from 80 degrees to 260 degrees is .003 seconds. If the exhaust port was lowered to open at 100 degrees then the center of the powerband would lower to around 9,000. This is essentially what a power valve in the exhaust port does to broaden the powerband by 1000 rpm.
A power valve is just a piece of metal that blocks or unblocks a higher section of the exhaust port according to the engine rpm. At low rpm's it blocks that upper section of the exhaust port until the upper target rpm is reached when it will move to unblock that area. This essentially changes the exhaust port timing. Looking at the graph, you can see the difference in the powerband between an continually opened power valve (A) and a continually closed valve (B). So if the valve opens fully at X rpm's then the resultant powerband can be represented in the second graph. Ideally, the transfer/intake ports should also have upper area valves, but that is a technological hurdle that hasn't been accomplished yet. These ports upper edges also should be higher for a high rpm powerband, and lower for a mid rpm powerband.
A powerbands rpm range is determined primarily by the port heights, 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 or widening the fat part of it so that the length and area till the rear cone is increased 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 trails 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. The ideal head to piston clearance is 1 - 2 millimeters (2mm for >250cc engines) which is what you need after all machining is done. 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. It should be equal or slightly more than what it was before the machining work was done. Also you should double check the compression with the cylinder head on. An 80 - 200cc engine 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.