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OS 25 FX prop


Old 05-13-2007, 02:41 PM
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Default OS 25 FX prop

Running a 25 FX with a macs tuned pipe header is cut 3" Installed on a SIG Kobra looking for a good prop?
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Old 06-02-2007, 05:46 AM
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Default RE: OS 25 FX prop


If you had not yet determined what prop you intend to use, how is it that your header is already cut 3" from stock???

Are you trying to harness the carriage in front of the horses, so to speak???

The OS.25FX is a standard engine, intended for 'normal' RPM for its size. I.e. not over 17,000 with any prop.

Its intake, bypass and exhaust duration numbers, do not support good power when running at higher RPM.

I would not recommend that you rework the ports with a Dremel, since this will break the continuity of the nickel coating on the sleeve of this ABN engine. All you will get is a peeled sleeve...

The shortened header may already force you to inefficiently use an inefficient prop-size.

The way to go with a tuned-pipe setup, is first to decide on a prop. An 8x6, or a 9x5 are OK for this engine.

Then, you go to gradually cutting the header shorter (1/4" at a time), until:

1. No RPM is gained on the last cut.

2. The engine can be throttled down to idle at the running needle setting and it returns instantly to max RPM, when the throttle is advanced (i.e. you mustn't play with the needle to make it come back on-the-pipe).

The second item must be checked every cut! Failure of the engine to get back on-the-pipe, means the system is too short.
Too short a pipe could destroy your engine!
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