Having just one sample though, with no layering, makes the car sound really wimpy ;)
Not always - try grabbing the Ferrari sample used in Grand Prix Legends - it's a single looping wav but the overall effect is awesome.
You can get sound samples like these from any site that does GPL / GTR etc.. mods, for example Blackhole Motorsports - have a search in the mods section for various games and download some of the audio mod packs. You can't really use them commercially, but at least you'll have some great samples to play with.
Also, try the Xpand Rally demo, which has some great engine SFX wavs.