ok, I have a 04 RX-8 GT with 78k miles on it and I just change the ign coils, spark plugs, and spark plug wires and even cleaned the MAF. My car idles normally but it still back fires a little. I still hear the low popping sound after I rev the car. What can be the damn problem?

Do you have an aftermarket exhaust? Also you should know that Mazda has the 8's tuned on the rich side from factory.

I have a greddy sp2 exhaust but it use to back fire even before I put on the greddy

If the popping sound is the only think you are experiencing I wouldn't worry about it. Any power loss or other symptoms?

Did you get the car new (know the full history)?

that's normal... get a catless midpipe, and that 'pop' turns into gunshot noises :)

http://www.youtube.com/watch?v=wIQevFl-arY