NAMBA October 2020 Web.pub

Exhaust Port The exhaust port is much simpler. It opens around 30 degrees before the transfer ports to relieve cylinder pressure and create the important pulse that drives the tuned pipe pressure waves. The top area of the port needs to be as wide as possible. However, the upper edge of the piston tends to catch on wide ports. This can be helped by dividers that create two, or even three port windows. Arching the top edge and/or adding a small radius also helps. Divided ports can actually be as wide as the cylinder bore and extend over the transfers. PROPWASH 10 October 2020 Quickdraw Divided Exhaust CMB 35 Triple Exhaust ports The area of the exhaust port after the transfers open is much less important. Smaller is better since it reduces flow into the exhaust from the transfers and reduces exhaust duct volume. It doesn’t need to reach the top of the piston at bottom dead center. Some people think that an exhaust port bottom edge 10 degrees after the transfer opening is enough. The duct from the port to the pipe should be shaped as was explained in part 4. The MB40 exhaust port and duct are close to ideal, see the pictures in the transfer port section. Restricted Exhaust Port Intakes All high-power nitro engines have some form of rotary in- take valve into the crankcase. The crankshaft rotary valve is by far the most popular. Low cost is a factor, but I believe low friction, especially in smaller sizes is the most important. Its flow limitation is crankshaft diameter, limited by bearing size. The intake valve typically opens around 150 degrees before top dead center as the piston rises and closes around 60 to 70 de- grees after top dead center as the piston descends. This gives an open duration of 210 to220 degrees. Another point is that the transfer ports open around 110 to 120 degrees before top dead center. That means the intake is open for 30 to 40 degrees at the same time the transfer is open with a typical rotary valve. This allows the pipe to “suck” mixture into the crankcase as well as into the cylinder from the crankcase. MB 40 Crankshaft Rotary Valve