RE: MVVS 26 IFS SP
George,
The 'benchmark' prop sizes for the MVVS 26 cc engine is 18x8.
With this size prop the engine would spin at about 8,200-8,300 RPM, on the tuned-pipe.
If you need to use a smaller diameter prop, a 16x10 - 16x12 is what you need; not a 16x8! An 18x10 sizes will also work well.
As to the tuned-pipe; regard it as a 'pre-tuned' unit... I know of no one that cut and adjusted the length of the steel header...
This pipe is of cone-and-disk design, with an effective RPM range of 6,500-9,500 RPM.
I can tell you that it also works wonders on OS1.40RX, OS1.20AX and OS1.60FX engines, using suitable headers...