Thanks, it worked but only for those engines with two numbers in the key, so jet engines remained the same, anyway i just realized that when it comes to jet engines there are a lot of exceptions in the key values, so i think it's just easier to make the fuel more efficient. Yesterday i tried making fuel tanks contain 3x fuel and divided by 3 oxidier and liquidfuel density so i had 3x fuel at the same weight, but the generated delta v didn't change, what am i missing here?
EDIT
Ok, the problem was jet engine isps wouldn't change but it was because their node is called ModuleEngineFX instead of just ModuleEngine, so i just added * to the name and now they multiply too