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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.
INTAKE PORT
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. The Maico motocross bikes had around a 160 degree 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 reed valve.
TRANSFER PORTS
I wish I could give you all some really sweet golden rules but I've learned that this stuff can't be guessed at and it's complex enough to need a computer program to figure it out. Without getting into specifics I can say that the height of the transfers first needs to satisfy the need of enough time to let the combustion pressure to go nearly to zero before the transfers open (at top RPM). This distance between the top of the exhaust port and the top of the transfers/boost port is called blowdown. A good program will show if you have too much or too little blowdown. After that then you have transfer port area. This works in conjunction with the CCR (explained below). The more port area, the sooner the complete transfer of intake charge happens but at a slower speed. People are baffled when they create a boost port or widen the transfers and that added transfer area reduced the top RPM.
Crankcase Compression Ratio (CCR)
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. Most common is between 1.4:1 and 1.5:1 ratio but you can increase that ratio by stuffing the crank. You can decrease that ratio by putting a spacer between the cylinder and the reed valve. Small engines have a short Schnurle loop distance from transfers to exhaust port and generally need a low CCR. Large engine have a long Schnurle distance and need a higher CCR. CCR works toether with transfer port area. A good computer program will tell you at what RPM the maximum amount of intake charge is trapped within the cylinder when the rising piston closes the exhaust port. You want that maximum delivery ratio to happen at the RPM at which you want the peak power to happen. My porting calculator gives you all the info you need to make the correct decisions when optimizing an engine. EXHAUST PORTS - refer to my previous page about this
Info on Exhaust Port Shape
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