Reduce backfire on LSA

Hi everyone

I have a 2013 ZL1 with auto transmission. I love the sound it makes on decelerating or downshifting. The "brabbels" and sometimes there are also some pops. However I have an issue if I rev it when the car is stationary. If I rev it above 3000rpm and let of the gas, it makes quite a loud bang. It really sounds loud and I am concerned about my cats.
Or is this normal for the ZL1/LSA? The other concern are the cops. In Switzerland we have very strict rules and fines are astronomical for "too loud" exhausts. My car is allowed to have 96db on stationary sound test revving to 3750rpm and letting of the gas. Now with that single loud bang I reach 115db and this would screw me if I ever have the cops checking my car.
Anything I could do to reduce this "bang" when stationary or is this just how that car/engine behaves and not much I could to to program that out?
What I can say is that this bang just happens if the engine is hot. It also happens if I turn it off when hot and start it again - it starts and there is this single loud bang.

Car is bone stock but the previous owner had it tuned because of 200cpi cats and CAI. According him everything (incl software) was put back to stock but maybe it is not. I am currently waiting for my mpvi2 to compare my software to a stock ZL1 and see if there are still some differences.

Sorry for language - I am from Switzerland.

Br
Zeee

I suspect they disabled DFCO, and possibly retarded timing from the low load part of the spark tables. You'll have to connect to it with HP Tuners in order to determine for sure.

Thanks. I will check that once my device arrived.
I found several stock files of zl1s but all manual cars. Can i use these to compare my car or are engine/spark settings different for manual/auto cars?

Update: I received my device and I found that my SW was modified quite heavily. Not in DFCO but nearly everywhere else -.-
I think it still had the tune on it and it was not reverted to stock. I used a stock file from the repository and put everything back to match the stock file. However the backfire is still the same so I think it is just the behavior of the engine.

Maybe your car has an exhaust leak?

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Originally Posted by Zeee

Update: I received my device and I found that my SW was modified quite heavily. Not in DFCO but nearly everywhere else -.-
I think it still had the tune on it and it was not reverted to stock. I used a stock file from the repository and put everything back to match the stock file. However the backfire is still the same so I think it is just the behavior of the engine.

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This is just general advice... but if the car was modified and then had custom tuning done, especially if these mods include a pulley change, you probably should not be using the OEM programming vs what was on the car.

Quote:

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Originally Posted by Frost

This is just general advice... but if the car was modified and then had custom tuning done, especially if these mods include a pulley change, you probably should not be using the OEM programming vs what was on the car.

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Thank you for the advice. The car only had 200cpi cats and a CAI. This was done by the first owner and it got a "tune" for these mods. My guess is that this was just a handheld based tune/calibration. Maybe even a remote tune. Some examples of changes I found in my software compared to the stock file from the repository:
- Several O2 DTCs disabled
- MAF and timing
- trq. management completely removed
- shift pressures increased

I am happy that especially the trans did not blow up so far with all the torque management removed :( I guess based on these mods (which are not in the car anymore, all is stock) it is ok that I reverted to stock settings?

Depends on the quality of the tune, but even with those basic mods you might have cost yourself a bit of power and you will need to keep an eye on fuelling to make sure all is ok. Making the assumption of a decent tune, you only had to put the torque management and shift pressure back to stock . The 6L90 does not need much modifying in the tune and in stock form can handle a sizeable chunk of power so long as the fuel and air model is right. If your mods are only as you listed then the stock injectors should still be in the car so your fuel model should still be ok, therefore so long as the MAF is tuned correctly the torque model will be very close so the trans will select the right pressure and spark timing to make a nice quality shift. The only thing I would touch on your trans would be to cut the shift times down and increase the shift inertia profiles in the higher torque areas.

Just to make everything clear:
The car is at the moment 100% stock (from hardware side). The modifications (CAI and cats) the first owner put on it were removed long time ago. But he was not putting back the stock software once he removed the stuff.

Regarding 6L90 tuning: Are there any references for good settings which are also safe for longterm use? After I put back all the stock settings, shifting was much better. Befor with the "tune" the shifting was very harsh and clunky. Now everything is smooth and sometimes I even think it is shifting quicker then with the tune.

That makes more sense now with those parts removed. Stock is good.

It will be shifting better with the torque management back in there. Those tables I mentioned above will get you started without doing any damage as you not are mucking around with pressures, volumes etc. Try bringing your wide open shift times down to 0.2 and increase the wide open areas of the inertia tables up a couple of points. You gotta start somewhere. You're stock so all you're doing is adjusting for shift feel.

Thanks for the feedback. Will drive a bit with the new transmission feel and then maybe try around with the settings you mentioned. Currently it feels quite good compare to before.