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Thread: Dirt bike engine for mimimax f 103

  1. #1

    Dirt bike engine for mimimax f 103

    I have a lot of experience with dirt bike engines and am curious what you opinion would be . I have a 201 KTM 125 engine that I can bump up to 200cc with ease. Would that be a safe option for something like a minimax ultralight?

  2. #2
    Dana's Avatar
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    Jul 2011
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    790
    It can be done, but... With a bike engine, you'll have to design and build a reduction drive, or build a prop adapter and use elements of the bike gearbox. Either way is not a simple thing. Also most of these engines are tuned to produce max HP in a narrow powerband, not what you want for a propeller drive.

    Some years ago, I built a paramotor using a KX100 engine (about 25HP at 11,000 rpm). It was a lot of work. I used the original 3.4:1 first stage reduction and made a propeller shaft and bearing assembly to mount where the bike clutch would have been. It worked, but it was very peaky. If I kept the prop pitch flat enough for the engine to get up into the powerband, it would overspeed at full throttle, up to 12K, and only slightly more prop pitch meant it'd never get past 8000 or so, at which rpm it developed very little power.

    But if you have a lot of experience with dirt bike engines then a 2-stroke Rotax should be easy to deal with.

  3. #3

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    The Minimax was more or less designed around a Rotax 277 engine which has over 2x the displacement of your KTM engine. Remember the old quip attributed to Caroll Shelby? "There ain't no substitute for cubic inches!"

    So while you may be able to spin your engine at the speed of heat to get a theoretical hp number, to make the plane go it has to have a propeller of the proper diameter and pitch turning at the proper speed. Otherwise, performance will suffer.

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