to clear something up when you hear a pop out of the tail pipe it is called an after fire in which unburned fuel (Hydro Carbons) gets pushed out the exhaust with oxygen and due to high exhaust temps it ignites in the exhaust which causes an after fire. A true backfire come from the intake when the air fuel mixture ignites in the intake manifold and comes out the intake. Backfiring isnt necisarily bad. Most older vehicles do it when you are cruising or give it lots of gas and let go of the throttle. New cars do it to if you take off the resinator and muffler. So in my experienc ei wouldnt say it is bad but obviesly doesnt sound pleasant. Theres my 2 pennies!