Webra 55 with a Jett ss tuned muffler and 10% fuel. 10/8 apc 15500, 9/10 apc 16500, 9/9 apc 17500, 10/7 apc 17000. Might even run better with less nitro but all I had at the time was 10%.

Good numbers MyWay. I tried my Webra 50 with both 5% and 15%. The best rpm's were with 15% with no problems. Maybe Webra shims the head on engines destined for the U.S.?

Good numbers. Right where they should be

You will be ok with 10% on the webra. Runs well with it.

How about numbers with 13x4?

Uhhh? do I need a new tack? Webra 55 with Macs 8.5cc Double Quiet pipe 9x9APC 19,800 RPM on Wildcat 15%

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.