The easiest way to find new engine sounds is on other cars.
I'll explain myself;
You've got a car with an engine sound you like. (since you're american, try my Dragster_Knutsel for a nice V8 sound, and no I'm not prejudice, just a wild guess
)
You unzip the skin file of the car, and the one of the car that you want to give that engine.
Then you place the .ogg files with the word engine in their name together with the files from the car your changing.
You rezip the files (dont zip the directory, only the files) and place the zip in the skins directory, change the name to not overwrite the existing model.
I'll look into the subject a little further if you want, it's possible to create your own sounds too, but that's something i've never looked into but i'm quite curious to know myself, so this would be the occasion for me to find out.