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Here's the Excel file screenshot. All the light blue boxes is where you enter the needed info. The info for Aprilia 143cc is already there as a good example. The left side lets you see the peak power RPM if you know the transfer/boost port area. The right side lets you evaluate the blowdown to see if it is more than half the exhaust pulse time at high RPM (which it should be). After measuring the exhaust port area then change the exhaust port number at K9 till the correct total port area is displayed at K11. The normal range is 1 for a single old-fashioned exhaust port, to 1.5 for one with two auxiliary ports like the Aprilia has. The pie chart shows three time zones. The first zone in red is the time between exhaust port opening and transfer port opening. The second zone is transfers opening to BDC. The third is BDC to end of transfer time when the crankcase pressure goes to zero. The higher the RPM, the later in degrees this happens.  To know the area of the ports just take the cylinder off the bike and insert printer paper in the clean barrel and hold the paper firmly in place while you press the paper along the edges of each port. Then take the paper out and use a pen or pencil to draw along each of the impressed port edges on the paper so its size is clearly seen. Then draw lines across them spaced 1mm apart. Measure the length of each space between the lines and write each number down. Do that for all the spaces in each port and then total the numbers. Then divide the total by 100 to get square centimeters.  How is this calculator helpful? The peak power RPM is usually the RPM at which you upshift. If you race on an asphalt track and you know the maximum speed you want in a certain gear then you can change the ports to match what you want. Also if your bike won't reach the RPM you want then this will give you an idea of the current design and how to change it. But you'll need to work with the port duration calculator at this site so you'll know how much higher the transfers/boost ports need to be for a certain area and duration open. Keep in mind that raising the ports increases both the open duration degrees and the port area. If you find out the engine design is adequate then make sure the intake flow area is adequate. On my Suzuki 100cc street bike it has a half reed valve which limits its peak power RPM from 90000 (which this calculator predicted) down to 8300. It would be a major modification to install a full reed valve and so I need to design the expansion chamber to work in harmony with that 8300 peak power RPM. If I didn't have this calculator then I'd mistakenly think I need to keep raising the ports. |