Are you trying to do decel (negative Timing) to get it to gurggle and bang?
Easiest way to do that is write some code on all the overall angle timing lookups and force it to what you want when the conditions are set. Subaru you just set the left hand column and off you go. another thing might be to hook in the cruise control so you can manage when it does it so you dont get pulled over... Lastly if its banging really loud then its still charging the coil fully.. If you share where your at (your rom) we can suggest. I could be wrong to, i am a lot as i learn... but i just worked on a project like this. Guess it depends on what ME your running on.
I have timing retard working as desired.
The original goal was to get timing retard without altering KFZWOP.
But after trying everything I could think of,
I gave up and altered KFZWOP and it worked like it should.
Now I'm chasing control of the fuel on overrun,
since from what I can gather from things I've read on different forums
ideally the fuel mixture should be on the lean side to get the classic crackles
and a fat mixture will result in more aggressive and explosive overrun.
And from what I can see, when the car is in overrun the mixture is fat
and the injection time is locked to 1.36ms.
So I'm searching for the module that controls fuel on overrun,
thought I had it for a second but i guess not