I've been trying to search google and forums for a new blow off valve for lfs, can some one please link me to a download for one? also which file do i overwrite to install it?

Here are some, and u can find more by using my pre-search link.

To install them, put the files in ur LFS/Data/Engine folder, go ingame and load the track (just go ingame, so u can drive and stuff). Do SHIFT+A, ull see a bunch of stuff in there, look for "Blow Off Valve" or similar sound name, u can adjust volumes and stuff, there is L and S letters in the very right side, L means Load and S means Save. Press L and load the sound file. You're done.

Edit: Dont forget the engine sound afterwards, the save button for the whole sound settings is in the upper side.

Cool, Thanks