Sure, just change the engine.wav sound in the car's data folder to whatever you want. The sound needs to be 44.1khz 16-bit and can be stereo or mono. The engine.wav sound gets played at normal pitch when the engine is at 7700 RPM, and pitched down or up at higher or lower RPMs.
If you want more control, there's a way to specify multiple sounds at different RPMs using a .aud file... see the data/cars/TL2/TL2.aud file for an example. The file works in a similar way to the ISI-engine game (GTR2, GTL, rFactor) AUD files. See this link:
http://www.nogripracing.com/forum/showth...p?&t=21694