We used WaveOsScope to clock the Predator II and GP Viper with OS .25FX's, and the Club 40 SRM2 with the TT Pro .40BB. The Q25's were in the 91-94 mph range, and the Club 40 was in the 93-97 range, all of this measured on multiple passes. The two classes are very comparable on speed. For some reason, the Q25's look faster, even though the measurement says they are not.

On my Q25 I used a long Hayes mount, similar to what Don did, but added a Higley Heavy Hub and bolted some lead onto the mount, tucked in behind the engine. My tank is up in the front of the airplane, butted up to the firewall, and the battery on the CG, Rx at the back of the radio compartment. I have it set up so I can swap engines quickly, TT .40 vs OS .25 FX or vice versa, without the CG changing. just four bolts and the throttle linkage. I don't even need to take the wing off to do it. My Predator II ends up 2 oz lighter overall with the 25, vs. with the .40.