engine backfires at 5500 rpms with straight pipe.

does it only do it at wide w.o.t. if so it could be ignition related something called spark "blow out" and maybe its time for new plugs and wires also wat is your spark plug gap?

its a 1.5 jz, and the gap is .32 on NGK-R series sparkies. and ONLY at WOT. I wire brushed the plugs and it helped alot last night. but it still does it a little. I have an insane abount of carbon coming from my exhaust at idle, its not smoke, just enough carbon in 15 minutes to turn the ground black.

your valves might have Carbon buildup on the intake side, resulting in low compression and back fire. or... your injectors are worn and not closing all the way.
thats my guess.