RPI,
That 185 KTAS claim at 8-10K and 2300-2400 RPM is very believable for a Sport Wing F1 Rocket, especially at 8K, with the fuel flow up there at 13-14 (which sounds ROP).
For a good comparison, can you provide a bit more info please. Is the IO-540 C4B5 a stock 260HP 540, or does it have higher compression pistons? What is the advertised HP if it is not a stock motor? What propeller is installed? What was the temp and DA at 10K on that flight? If you have a TAS readout from that flight, that would help too.
I rough calculated a standard day TAS at 10K for your flight, and it pencils out to 172 KTAS. I'm betting it was hotter than standard though, this time of year. Even so, that would seem on the low side...what was your fuel flow that day? Whether you were ROP or LOP will tell part of the story too. May have been very hot the day you tested, so how you were leaned could be a factor too.
F1 Sport Wing pilots will have the best data, but I've flown side by side a number of them, with both me and them testing various props (and the propeller can make a big difference). I have a Super Six that has an IO-540 C4B5, and had a Sport Wing until a couple years ago. However, I have 10:1 compression, and the original dyno in 1998 showed 322 HP at the engine builder's shop. Here are some round numbers from the first several years of ownership, before a lot of aero mods were done, but with a couple notes on propellers. All are 10K average speeds from various test flights and cross countries, and all are WOT and 2300 RPM round numbers. Back then, I'd say my plane was a little faster than most HRIIs, and some F1 Sport Wings, but a little slower than most F1 EVOs...just for a comparison point.
C4B5 with 80" 2-blade Hartzell D-twist that had been trimmed to 79" 189 KTAS, 11gph LOP. Add a knot or two going ROP @13gph
C4B5 with 80" 2-blade Hartzell BA 193 KTAS, 11gph LOP, same + a knot or two going ROP @13gph
In both cases, if I went further LOP, it would slow about 5 knots...so the above two data points were just slightly LOP, but up high, so safe for the motor (and it stayed cool).
That prop change basically added 3-5 knots. At the same time, F1Boss changed from an MT 3-blade to the same 80" Hartzell, and he gained 12-14 knots...no kiddin'. So what prop it has is an important factor.
I recently did a pre-buy and then some training with the new owner of an HRII with a stock IO-540 and D-twist propeller, and it cruised in the mid to high 170's KTAS up at 10K, similar power settings to what you were using. However, it needed a bit of aerodynamic TLC...as the fit and finish, and the paint...needed a little work. That is the other factor to consider...how clean is the airframe build?
Hopefully some F1 Sport flyers will add data, but hopefully that gives you some data to start with. Good luck with the search and the purchase!
Cheers,
Bob