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bob27s -> RE: Jett muffler for a OS 91FX -More power?? (3/15/2006 9:53:23 PM)
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Our testing and experince shows you can expect a gain of 700-1000 rpm or more, depending on your prop choice.
The 12x8 prop is a decent choice. With a stock OS muffler, the engine will tend to turn that around 11,500 - 11,800 or so. With the Jett stream it will turn up closer to 12,500 - 12,900 rpm.
The key with the Jett-Stream on this engine is to prop the engine to run over 12.2K peak rpm on the ground. Hitting 12,500 is ideal. You want it to unload up over 13,000 in the air if at all possible.
If your OS 91 is new, wait on breaking it in. Break it in using the jett-stream. Break in prop of a 12x6 is a good idea. Let it get up to 13K rpm, while being slightly rich. Get a few runs on it like that, then switch to the 12x8 for your final couple of runs. You can find your peak rpm with that prop, and identify the proper ground mixture setting - ideally 500-600 rpm down from peak rpm to start with.
Bob
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