A Thunder Tiger Pro 40 is not the only engine allowed in APRA. There are other engines on the list, Super Tiger GS 40, etc. The TT Pro 40 is a AMA 424 legal engine, along with others.......Read page 123 prargraph 16.3.3 424 Special provisions. Looks like APRA rules, down to the letter.......In fact, read the AMA rule book.........The way an engine makes it on the APRA engine list, is by extensive testing. I know because Jim Allen & I were the APRA engine committee, at one time, and Jim & I tested lots of engines, and not just one of each, 5-10 of each as I remember. We tested them with a APC 9-6, & a unload prop. If the engine turned both of these props, at to high of an RPM, they were not allowed, plus they had to retail, for $100 or less. These two props are in Jim Allens possesion, and stored in a neat little box, for consistancy. Things may have changed since I was involved in the testing, but that was the basis. Nobody wanted any particular engine to monopolise, but the TT Pro 40 was the engine of choice on the list.....This too is old news, but every three, or four years, the new guys start asking these same questions. This the third, or fourth time we have told this same story, but I don't mind.....What are use old farts for ?