why not just swap the sounds in the carmodel file, or even swap the audio files. Then just raise the pitch on the engine a bit in a hex editor
That doesn't work properly anymore. I showed this twice before, but listen to what happens on downshifts since patch 1.05 with pitch-shifted engine BDLs:
You also need to alter the external audio for the cars as the V6 sounds will remain on the replay camera with absolutely no hint of whatever engine BDL file you swapped out. That needs to be done in each car's ERP file in the carmodel data.