RE: PowerHouse RC Performance Race Engines
That is correct about the improved performance to a certain point with widening the exhaust port. Rule of thumb is the exhaust port shouldn't go beyond 70% of the piston diameter. This pertains more to ringed engines but you also need to have a nice rounded corner with a slight taper on the top of the port to give the ring a chance to tuck in before the port closes otherwise the ring could catch the top of the port and break. The flatter the port, the more aggressive start the the blowdown timing which results in more "pop" on the bottom and the opposite for a more tapered port ceiling. As far as the short circuiting is concerned, there are ways around that if the ports are border line say if the exhaust boosters hang over the front transfer port too far. It's called backporting the window which will redirect and create a surge of charge that act as a wall to keep the fresh charge and expelling charge from mixing too much during blowdown. A little bit a charge will always exit the cylinder but depending how well the pipe is scavaging will determine how much will return to the chamber to ignite on the next cycle.