There are about 3 options: 1 the Thunderchine, 2 a lockwood (U-bend) and 3 the M40.
Option 1 is not realy an option. the first revision is incredibly unstable and has VERY tight build tollerences. A lockwood will give you the most thrust (~55lbs) but they're mighty inefficient. The M40 has about 42 pounds of thrust and is a lot more fuel economic, the build is a bit more challenging though.
For a lockwood (U-bend) engine, use these plans: Lockwood
The M40 can be found here
There are more plans out there for U-bend engines but they're not as good as this one. Resist the urge to go for an easy, low cone build
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