RE: O.S. 46 FX on a pipe
First of all..a 6 pitch prop won't get you much more than 100 mph in most applications. If your plane is kind of heavy, draggy and you fly off a short field then a 10x6 might be the best all around choice.
If you want speed from a .45 sport engine, then a 9x7 or 9x8 prop is more like it. Plan on dead stick landings if you have a short field.
If that engine is broke in [no metal grit in the exhaust spray], then set it on the bench with open exhaust, wide open and get a tach reading with a 9x7 prop. This will be your base line target rpm.
Now run it with the pipe and get a quick reading before the engine gets too hot, but make sure you get a good reading.
Use a 32 tooth or finer hacksaw to remove a 1/4" slice off the header and get another reading.
hopefully there will be a rpm gain. This means you are moving in the right direction. If not, then put the slice back into the silicone coupling and leave a small gap between the sections of tubing. Run it again to verify whether or not the system needs to be shortened or if it is either already short enough [or possibly too short] for the prop that you have decided to use.
No matter what, every sport engine I've ever converted to full wave pipe needs to run at a higher rpm than it can with open exhaust or it is a waste of time in a speed application. I have spent more time over the years typing out how to tune pipes than I have spent actually doing the work, so don't be afraid to get your hands dirty and get after it. It's not brain surgery, it just takes a willingness to put in the time to do it.
A spray can of brake cleaner is handy to clean out the aluminum saw dust after each trial cut on the header. I just patiently go 1/4" at a time until I see no improvement with the target prop.