The jett-stream "SS" muffler are not compatible with a 13x4 prop. The engine has to be able to turn up over 15,500 (closer to 17K rpm) for the SS jett-stream to become effective and work properly.

The standard Jett-Stream .50 size muffler may work ok. The webra has some good power, and the timing is pretty good for broad-range performance - if you can get it up over 14,500 with the 13x4 it will work fairly well. Typically for the 46-50 size engines max out at the 12.25x3.75 and 12x4 props.

For reference, I run my P-51, Jett SJ-46 - turbojett combination with a 12x4 turning just at 14,000 rpm ground peak.