What are the possible causes of a backfire or pop sound from the tailpipe that only occurs when the throttle is suddenly closed from WOT, as in shifting gears during a drag race?

I thought there might be an exhaust leak in the header, but after installing new header pipe gaskets and tightening all the pipe connections, the problem still exists.

Of course the headpipe connects to the turbine inlet, and the tailpipe connects to the turbine outlet. There are copper gaskets here.

I have a throttle body with butterfly and throttle position sensor. The ECU runs in a lookup table mode. For each throttle postion, the rate/amount of fuel delivery is pre-programmed/mapped to the motor. Corrections to fuel delivery are made based on charge air temperature and pressure.

I'm wondering if the fuel map values associated with high RPM and low throttle may be too lean causing the pop when releasing the throttle.

Although I do have some igniton timing issues to resolve such as pre-ignition, I don't believe the noise is a pinging sound from detonation in the cylinder. The sound comes from the tail pipe.

 

Backfire in the exhaust is usually too rich. My racing background is in cars, but still 4 stroke. Backfire through the intake is usually too lean. Since you are running DFI, fatten it up some and see what happens. It should get worse. The trick will be to find where in the rpm range the excess fuel is getting into the exhaust. Alcohol is much different than gas as you have seen. Since you have to flow so much, it may be dumping a bunch into the cylinder. When the throttle shuts it instantly goes way rich and some gets into the exhaust. Then you get the bang. I work on a blown alcohol funny car for fun. I'll keep watching this post. Does it do it all of the time? Just at certain RPM? Just keep the throttle open and it won't backfire!! j/k

 

I had the same thing happen,only after running the hill 5 or 6 times,I found the header pipe was glowing red and the pipe was making a popping noise after releasing the throttle,checked the plugs and it turned out I was running to lean.I bumped both main jets from 160's up to 165's and it stopped getting hot and the popping went away,

 

Doesn't popping on acceleration indicate a rich condition and popping on deceleration a lean condition?? At least thats what I have been told.