Port Timing Basics
Air/gasoline enters the engine from the carburetor at a ratio close to 15:1. It enters via the intake port (as the piston rises) into the crankcase and then is transferred to the combustion area via the transfer ports (as the piston descends and the piston top uncovers the port opening to the cylinder). After it is burned the exhaust gas exits via the exhaust port. For higher rpm there needs to be more degrees of opening for each port because with more revolutions per minute there are more “cycles” dividing up the same minute which means each cycle of crank rotation takes a smaller amount of time so that more degrees opening are needed for the same amount of minimum time needed for movement of gases from one area to another.
For a piston port intake there needs to be around 120 degrees of port open duration for a peak rpm around 6000. Higher rpm needs more port duration. For a reed valve intake there needs to be holes in the piston that allows passage of fuel mixture from the carb to the crankcase starting at BDC (bottom dead center). The total volume of the holes should be at least 20% more than the open flow volume of the valve.
You can refer to the following listing for a generic setting of port timing according to peak rpm.
But there are factors which let you vary from these settings. Less duration favors low rpm power whereas more duration favors high rpm power. More crankcase compression ratio means there will be more pressure when the transfers open and so the entrance of fuel mixture will be more rapid, needing less degrees to make the complete transfer. The durations in this chart are for around a 1.5:1 ratio. More ratio means more pressure which is advantageous for high rpm power but a disadvantage to low rpm power. Why? At higher pressure the mixture enters the cylinder too fast at lower rpm and loops around to exit partially through the exhaust port before the ascending piston closes it off. Another factor is the angle of the transfer ports roofs. That affects the angle of mixture entry. A steeper angle gets the mixture up to the spark plug faster which is good for high rpm power but bad for low rpm power. A good example is two cylinders I have for the same engine. One had transfers with only 114 degrees and 45 degree roofs. The other had 120 degrees and 15 degree roofs. The one with the lesser duration achieved 8000 rpm, whereas the other only achieved 7500 rpm. But the one with 15 degrees had more power at low and mid range rpm. One way to cheat the system is design for high rpm power (long duration and steep roofs) but allow a bleed off of crank pressure by a narrow boost port that opens much earlier than the main part of the transfers. This does not ruin high rpm power because anything other than the main opening of the transfers is hardly noticed at top rpm. So its like not even there at high rpm, but at low rpm it effectively releases much crankcase pressure before the main part of the transfers open so that the mixture enters at a slower speed to be less likely to loop around and exit the exhaust port.
What really maters is the degrees between exhaust opening and transfers opening. This is called “blowdown” degrees. If the exhaust port opens at 90 degrees ATDC, and the transfers open at 115 degrees ATDC, then the blowdown is 115-90=25 degrees.
The exhaust port duration has no significance apart from the duration of the transfers because that relationship sets the amount of time available for the exhaust gases to leave the cylinder before the low pressure intake mixture is transferred into the cylinder. The pressure of the intake mixture is only around 5 psi and needs to have almost all the exhaust gas out before the transfers open so that the cylinder pressure is no higher than 5 psi. Anything higher than 5 psi will delay the entry of the intake mixture. The exhaust port shape is important too. The more squarish the top is, the better the top rpm power is.
Too square though is dangerous because there is more possibility that the ring will snag on the top middle of the port, especially if there isn’t a sufficient bevel there. Take note of how wide the flat transfer ports tops are. That tells you how wide you can make the top of the exhaust port level without risk of snagging a ring. Here is a chart showing blowdown needed at various rpm:
What can change the needed blowdown is the width of the exhaust port and the squareness of its top. More of both of these allows less amount of blowdown. To figure out the needed exhaust port duration for a given transfer duration and blowdown all that is needed is to multiply the blowdown by two and add it to the transfer duration to get this chart: