RE: World Models .40 size P-51 speed question
The 10X8 should be your prop of choice. When tuned properly it will have a little more pull to combat the poor aerodinamics of the mustang. I run a 50sx on a 9x9 but the 10x8 will pull about the same. Like someone said you probally don't have your header cut to the proper length to gaine the tuned pipe advantage or your low or high needle isn't letting the proper amount of fuel get to the engine. The tuned pipe will require more fuel so open the slow needle up a little and retune. I think I get a little better than that with a Tower muffler on the 9x9.