I am using an ASP Super Series 52 engine with an APC 11-5 prop. The plane has gobs of power and will fly very fast if you want it to with this combo. The muffler points down but the exhaust outlet doesn't clear the bottom of the fuselage. I made a muffler deflector from balsa that I epoxied to fuel proof it and then JB welded it on to the top edge of the exhaust outlet. The fuel hits the deflector and flows against it until it clears the boootm of the fuselage. Because it is outside the the muffler and is open, it doesn't add any back pressure to the muffler or the engine. I actually had to make a big square cutout for the engine. I had to use a needle valve extension and an inline fueler. The wires dangling down from the servos look crappy and I didn't realize that they would be that bad until the plane was completed. Where they exit into the fuselage is also a pain and they get in the way of the servos unless you tie them up. Haven't had time to see how to correct that as yet. Maybe others can offer ideas about that.