• I don’t know what power to weight ratio you are looking for but if you have an idea you will already be able to work out what % weight loss you are looking for. As these engines are designed for flight they pretty much are made of the lightest materials available within a reasonable cost range. I would be surprised if you can save more than 20% of the original weight without running into reliability issues.

  • Have you considered using an alternative engine, such as the 30cc gas-powered 4-stroke Saito? It is less than half the weight of the Fuji, and a lot more compact. It may cost a bit more.

  • Most of the weight reduction has been done by the manufacturer, there is little left to reduce the weight on. An alloy muffler could be used and may save 50 grams or so. If you need a lighter engine, you should probably stick to a 2stroke, you will get get more power from a lighter engine.

    Is there a particular reason you are wanting to use the 4 stroke engine over a 2stroke?

  • I don't have any experience swapping out engine parts made of different materials - but I'd imagine that it would be difficult to find an appropriate match. 

    The materials not only have to match geometrically, but also chemically - piston engines operate within tight tolerances and under varying temperature ranges, meaning that you'll have to pay attention to the co-efficient of thermal expansion for what ever material you're going to use, unless you want to sieze your engine when it gets hot.

    Further, lighter metals may not be able to take the stress of rapid explosion cycling without being prohibitably expensive for most DIY projects...unless you have a Ti-1100 hook up you want to share with the rest of us ;)

    Other than that, the other main concern would be finding parts of similar geometric shape (unless you have a fabricator that does custom engine parts out of exotic materials).  I don't know anything about Fuji-Imvac engines, but you'd have to match their parts, or part-systems exactly to get the same kind of performance.  In essense, I'd be like finding another diamond with cut, colour, carat, clarity, and certification to match the one on your wife's ring.

    But then again I haven't done it before, but thinking about everything I'd have to do to get it to work...I'd stick with the 2-stroke as alot of people here have.

    Aside: You must be building one hell of a big plane, even the 25cc is pushing 2kg and that puts you out of the US amature flight catagory, I hope you have deep pockets lined with big friends.

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