I have been using the side exhaust REX 0.12 in sport clown for a three years now. I am not sure how it differs from a rear exhause. Mine is in the "sport engine" REX line. APC 6.5x5 is a good start. 10 mins break in before putting it into racing with no issues. Over time it has gotten better and better fuel mileage with no change in airspeed so it seems that it can take a while to break in completely. Air speed is just right for our sport event (faster than almost all la 15s), mileage is great, restarts are easy. The single most consistent racing engine I have ever used.