It'll probably do 353 KTAS @ 17500 but it would be at full power and that's only rated for 5 minutes. The Walter is normally aspirated. It has no turbo or super chargers. It develops 750 SHP @ sea level standard day. It continues to lose horse power as it climbs. I don't know the exact figures, but I believe it is producing about half the 750 in the mid twenties. The gas mileage is best for the given mach at about FL250. Rick S. knows a bit about this subject, maybe he can chime in. I think one of the super charger setups that Rick can install on the Legacy will out perform the Walter at FL250. The Legacy guys generally are not interested in going that high. They get great performance at the lower levels. The Walter needs to go up there for range purposes.

Regards. Dave.