nope, the engine sounds are generated on a bunch of number values in addition to the wav files embedded in the .eng files...
if you want a car to sound like a 6 cylinder, the only way to do that is to make the car actually have 6 cylinders by using tweak.
edit: well, that's not entirely true... you can sit there fudging around with sounds until it sounds right, but it's just a lot easier to give the engine the right amount of cylinders to begin with.