A perfect exhaust system would have the exhaust from each cylinder scavenge the next cylinder in the firing order. However, unless the exhaust system is designed correctly the pressure pulses in the exhaust system are higher than the intake system and the exhaust gases flow back into the intake. Sort of like a built-in EGR.
One way to overcome this is to have short, stubby "zoomie" type headers like most of the smaller aircraft have. It isn't the best system but it's cheap, fits the airframe and isn't too bad power wise.
Ron
