So I figured out with the editor tool how to edit the base LFS cars, but not mod cars. I want to tweak the horse power and such of them.

It's actually the same for all cars; load the car and click the 'edit engine' button. Be sure that you're in the editor, not in the modeller.

But since you did find it for base cars, is there anything different for modded cars for you?

Quote from Van Sterberkt :

It's actually the same for all cars; load the car and click the 'edit engine' button. Be sure that you're in the editor, not in the modeller.

But since you did find it for base cars, is there anything different for modded cars for you?

The problem is, when I export it, I take the .eng file and override the .eng file for the mod cars and it doesn't change anything.

Make sure you only edit mods that allow it, or you could find yourself locked out of the LFS Editor.