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04-24-2009 | 02:41 AM

#37

DarZeelon

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From: Rosh-HaAyin, ISRAEL

RE: os 61sf

Quote:

ORIGINAL: anuthabubba

...Its long stroke and timing require a longer/greater internal volume pipe than a 'normal short' stroke .61 uses. What pipe are you using? It should probably run a 12~10 at nearly what you are getting now. Try adjusting/tuning the header length.

Terry,

The 'long-stroke' is just a buzz word...

These engines all have a stroke of 24 mm and a bore of 23, with 'pre long-stroke' engines having a 24 (or 23.85) mm bore and a 22 mm stroke.
So, they're all virtually 'square'... And 'real world' definitions are 'over-square', for an engine that has a larger bore diameter and 'under-square', for what was termed 'long-stroke'...

And in F3D engines; 'long-stroke' engines have a slightly greater stroke than the norm, but they are still 'over-square'...

What did change for the F3A 'long, when manufacturers went to the slightly under-square design, was the port timing.

Earlier, over-square F3A engines, such as the YS .60, Rossi, Enya XF(#), ST and the OS.61FSR/V, were designed to spin an 11x7 to 11x7¾ props, at 14K and more, with a rather short tuned-pipe (NOT low-volume).

Due to noise constraints, later 'long-stroke' engines (OS.61SF, YS.61, MVVS .61...), through more conservative timing numbers and a longer tuned-pipe (NOT high-volume), targeted the 10-12K range, spinning larger props (up to 13x9), to sound a bit less 'buzzy' to the average ear...

The SF will not like to go to 15K; not because it has a 'long stroke', but because its port timing makes it breath better at 10-12K.

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