Wayne,

Dave Phillips will be able to help you with this - the exhaust length.

As for plugs I'd suggest you stick with the NR plugs - C6S is the standard plug for this engine. There is no OS #5 that I'm aware of but a #8 should also work. 10% nitro is right and a 50/50 synthetic/castor mix is a good idea with most European engines (i.e., castor in the mix). Some (e.g., MVVS) perform well with castor only and 0-5% nitro but that's not the case with the NR Speed/13.

What header are you using - the 1" or 2.5" rise? You should be able to run the engine on the stock 1" rise (40401), NR standard pipe (50400) and the 11x7 and it will give you in the neighborhood of 16K... Behold the power of this engine!

David

PS Do post some snaps of your Super Arrow!