Exhaust systems do more than just get air into and out of aircraft engines. Exhaust system design, in terms of exhaust pipe length, diameter and tuning, affect the efficiency and performance of the engine, for better or worse.
DAR Mel Asberry clears up airspace restrictions for homebuilts during flight testing and discusses how the original builder of a homebuilt design remains the builder unless the airworthiness certificate is surrendered, which has implications for reselling an aircraft.