I have been running YS 45 for more than 6 years , they are very powerful engines , not sure about the HP but with a 10x7 APC , macs quiet tuned pipe 7.5 cc and 10% nitro ( yes 10 % ) I'm getting 15,500 RPM on the ground , they run well with 10x8 APC (shaved ) and 20% nitro , but I haven't tach it with that prop.

If my mind is correct the length of the header is something about 4 1/5 " or 5 ".

UPSSS !!!!! sorry I missed the "RF" part , I was talking from side exhaust version . Probably not much difference between both but I just wanted to point it out .