Give F1 a try...
This is the ultimate racing genre, and anyone who saw qualification today from Hungary will know you will be hard pressed to find more excitement in any kind of sport.
If you follow F1 for a while you can notice all the differences in aerodynamics and understanding both tyre and engine development is a truly amazing and exciting part of watching F1.
I watch most kinds of racing and I can guarantee you Nascar of Champcar cannot deliver the same level of quality racing !
Back to your question.
Before some F1 races there is Porsche CUP. This is Porsche ONLY and if you ever go to a F1 grand prix weekend, dont miss this ... there is F1 engine sound (or was - back when it was V12 engines and no RPM limits) and then there is Porsche Cup race engine sounds...
Then there is DTM - a very high quality racing league in Germany with megastars like Pedro De La Rosa (McLaren), Tom Kristensen (Our danish Le Mans pride), Mika Häkkinen, Gary Paffet (McLaren)... they race Mercedes and Audi cars in 2007 and 2006 spec cars... very entertaining !!
BTCC is cars like SEAT and BMW ... I think it lacks great drivers! In denmark we have DTC and we have guys like Jan Magnussen (Former F1 driver) ...
More Porsche can be found in Le Mans races ...but seing a Porsche being overtaken by a AUDI every minute is not much fun