Opel Astra (touring car) V8 Coupe - just in general, and try shifting to higher gears too. Try slow revving as well. My favorite race car next to 787B.
Mercedes CLK and CL
787B - love it in low gears when going around turns especially, and the F1-esque sound.
Audi S4 - with or sport or stock muffler, forget which but it gives it a nice roar.
Z06 - fully tuned, looooooove this
Viper GTS - fully tuned accept for sport or stock muffler, forget which again, gives it a great sound.
350Z concept
Mustang - ditto Viper. Sounds extra great on replays too.
GT40
Esperante
Cobra - stock or sport muffler.
Skyline R34
ZZ-II
Speed 12
Zonda Race Car
Jaguar XKR - fully tuned, awesome, want to get the race car which sounds like the same, but love the look fo it.
Wish I could of got the CLK-DTM, as that would be cool to have too, love the wya it looks, but passed on it way way back when I first got the game... can i get it again?