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The top of the piston is flat, not domed like a naturally aspired engine. The side of the cylinder has a hole in it for fuel injestion. Course on the other side is exhaust but this port is higher up the cylinder so it gets exposed first. With a flat topped piston, something has to get the inlet fuel mixture to stay in the combustion chamber so that when the piston comes back up there is something in there to go bang. Thats where the exhause pressure pulse comes in. It's basically a tuned resonance chamber where the sound/exhause impulse oscillates causing a temporary blockage. This blockage is what is used in the next charging cycle to keep the fuel in the cylinder. Works great and much much improved fuel efficiency over predecessors. Originally, they had to do it in 120 degrees. That's why OMC came out with a 3 cyl initially. Took them a long time to figure out how to get 180 or 90 degree engines (50 hp and 100 hp series 2 cyl and 4 cyl) to do the same thing. Mercury did the same thing with their newly released (at the time) 3 cyl mid range engines (60 hp) and 6 cyl 115 hp. The two little circles or squares (depending on year model) need to be outta da wadda to start. Mark
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