RE: tuning a pipe
First you get a baseline reading with open exhaust. Be prepared to start out with the NV opened up alot. If the venturi is too big, then you would need to use a bladder feed or pressure tap to get adequate fuel draw. The open exhaust reading is your benchmark, the reading through the stock muffler is meaningless, unless it is a very well designed muffler. If you know what the practical RPM limits of your engine are [usually stated by the manufacturer], then you can approximate which prop you want to tune the engine for. I would say 90% of your .45 sized applications would do well with a 9x7 or 9x8 prop. MACs says to chop 1/4" off the header at a time until no improvement is noticed. The extra boost of power will also require you to shim the head or open up the chamber slightly if you are blowing plugs every run. No way is the header 9 inches long or longer on a .40-.50 sized setup.