i have a 1990 5.0 recently installed headers and off road h-pipe (no cats) with super 44 mufflers. when i really step on it i will get a loud backfire almost every time i shift at high rpms only really happens when i floor it but its kinda annoying and i fear its harmful to the car. anyone know whats causing this backfire? im guessing its a lack of back pressure but not sure, anyone ever had this problem and know a cheap way to fix it?

 

Well if its thru the exhaust its an afterfire and if its back thru the intake its a backfire and ur prolly hitting the Rev limiter!

 

also keep in mind at WOT with most all EFI cars they are designed to just dump fuel to avoid leaning out, it would have something to do with this happening

 

Check fuel pressure if backfire is in intake.

 

run a koer test and look for codes related to the thermactor air system......is the tube that connects to the back of the heads still in place?? if so the check valve on the end could be bad......mine was and it'd back fire so loud at night it'd shoot two blue flames out the tailpipes..looked cool from behind, mine did it on a high rpm decel...the valve is called an anti-backfire gulp valve

 

it must be called an after fire then cause its loud as hell and seams to be coming from the exhaust. it is harmful to my car in any way

 

im not hitting the rev limiter i only get to around 4.5k rpms before it pops

 

I have seen the valve mjr is talking about cause probelms too.