I'm having trouble running with the big dogs at my age so I thought I would try the Rossi .12 rear exhaust in my clowns.  So far it gets almost the same number of laps as a  good OS .15la but is 3 to 4 mph slower.  I'm still getting some dark in the exhaust oil after about 40 minutes total run time.  How long is breakin with this engine and how much more can I expect in speed and laps when it is?  One thing it does better than the OS is restart hot.  One or two hits and it is running even if you flood it a little.

Later
Richard

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.