RE: MVVS .61 rpms
The MVVS came out with low noise and RPM in mind.
It does have the the 'Long Stroke' era bore and stroke dimensions, of 23 and 24 mm respectively.
In two independent engine reviews, this engine (RCM - Clarence Lee review) spun a 14x6 prop, more than 2,000 RPM faster than an OS.61FX!
Its advantage grew smaller with lighter props, until they were about equal with an 11x6.
Curt (RaceCity) has one with the MVVS #3250 pipe and it spins a 13x8, or 9 APC at over 10,700 RPM...
This is a very respectable achievement, for an engine intended for low RPM, with a 15 mm O/d crankshaft.