Under the "soundconfig" section of the engine's .jbeam, there's a line called "sample_name". Change it from V8_1, V6, I4, etc. to V10. Then, press Ctrl+R to reload the car and hear the engine roar! Notes: Never edit game files. Make a new engine (use the search bar, there's plenty of tutorials. Especially the one by @fufsgfen), and put it in a mod. Edit the engine of the car you want it in, and select the engine you created.
it doesnt matter editing the game files, just dont share them with anyone else, and remember to delete your files after an update.