RE: MVVS .77
C,
Both engines are optimized for tuned-exhaust usage. Using a normal muffler does not allow them to attain their impressive ability.
The larger .77 is a 'torquer'. Because of its timing numbers and its narrow crankshaft passage, it inclined toward low-mid RPM, with a mini-pipe, or a with full-length tuned-pipe.
Both are offered by MVVS.
It is best propped with a 13x8, a 14x6 or a 13x9 APC, with the tuned-silencer adjusted to support RPM between 10,500 and 11,500 static.
These numbers should be reachable, with the engine in its prime (from 6-10 hours after break-in, onward).
The .49... with the tuned silencer an the header slightly extended, spun a Bolly 11.5x6 at no less than 13,600 RPM.
This RPM may not sound impressive, but it is an actual output of 1.67 HP! For reference, spinning an APC 10x6 at 14,300 RPM (OS.46AX), is just 1.1 HP...
With a normal muffler, you can expect 'normal' .45-.50 performance level, on any prop.