I used audacity to cut new engine sound wich is a V8. I made it 40 milliseconds and i exported it as 16-bit PCM Wave. However when i put it in the engine folder and try to load it ingame it says "file header check failed...."

does anyone know how i can mend this little problem?

Engine sounds are made purely within LFS and don't use any sound samples for the sound itself, apart of turbo, starter etc.

.eng files are not audio, just a bunch of settings that tell LFS how to generate the sound.

If you want to use samples you should check the thread of Car Sound Remixer (CSR)