Go the the in-game home page, go to mods, click on fine tuner, activate it, go into the level, and grab a jet engine or whatever part you want to mod.
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If you have a get engine, open overload, click the menu at the top and click “jetEngine”, and under that menu, click powerMultiplier and increase the number next to that.
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If you have a prop, open overload, open the menu at the top and click” PropEngineAdvanced”, then at the top of the thing there should be a thing that says power, and adjust the number next to it. Note that the number it shows is twice as high as the number of horsepower the plane has.