i see many modified cars a day but it seems like its only the WRX's backfire beautifully with a fireball after it. How come this happens on many WRX's. i have a pretty modified Eclipse and mine has never done that even with running rich and having no cat. Can anyone explain this?

BOV
intake
no cat

Not a backfire......

bov, when you have a atmospheric bov and it opens, the MAF dumps more fuel and it doesnt burn so the exhaust makes that noise.
wrx is a touchy car.

The WRX, a touchy car? HA ...I have had mine for more than 2 years, some modding and no problems. Except burning my clutch and flywheel...oops..lol Anyways...I beleive its called an afterfire..this is only what I have heard. My guess to do this you need a full 3 inch exhaust, intake, and an upgraded BOV.

-Chris