tuned pipe set up
 

running a tuned pipe (macs)..on a 46 what a good prop to try frist

Wow
I need way more information. What engine is it, what airframe is it in, and what are you trying to do with it. Sport flying and all out speed take way different props on the same engine, and the drag profile of the plane dictates prop size for a speed plane. Altitude and fuel play a pretty big part in the equation too. Give us a little more to go on and we can probably get you pretty close.

Scott

MACS has a header length chart to get you in the ball park from there you can cut down step at a time till the rpm's don't change there is some wiggle room with the silicon pipe coupler.

Ref:
http://macspro.com/tuning.asp

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.