My first piped engine was a Webra .50 that was mounted to a Midwest Aerosport .20. Basically, an "Ugly Stick with about 400+ sq inch wing area and about a 12% thick airfoil. I ran the header and pipe full length with 9 x 8 props. In those days, I was happy with the performance...but I had a feeling that there was much more to learn about tuning the exhaust system's length and working to find the best propellor.
Fast forward 20+ years after tuning in to this forum and that same Webra .50 got rebuilt, had the exhaust timing and intake timing increased. The engine was put on a .40 sized delta. By this point in time I had formed some sort of an idea about a "Target Prop" I wanted to run. Thanks to education gained at this forum I decided on trying a 8 x 8 prop. The header pipe needed to be shortened quite a lot to hit resonance. It pulled the model pretty well. I don't remember all the details, but I kept experimenting with less diameter but the same pitch. I had to keep shortening the header until there wasn't any more to take, so I had to shorten the pipe. I finally arrived at 7.5 x 8 prop for what ran best on that plane. Any less diameter than that and the engine could not increase rpm enough to pull the plane any faster. I was satisfied that it was close to it's max potential. By this time, I had 3 different ways to monitor the plane's speed...which beats guessing.
So..this is how I went about my business. It took many years of flubbing around without any help or knowledge before hooking up with the internet and RCU and finally learning how to put a half decent speed project together.