Pop & Bangs — exhaust pops and crackle on the overrun

Pop & Bangs is a software function that makes the exhaust pop, crackle and burble when you lift off the throttle. On the overrun the ECU delays the fuel cut and retards the ignition hard, so part of the mixture finishes burning in the exhaust. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

Where the pops on the overrun come from

On stock software the exhaust is quiet when you lift off. As soon as the driver releases the pedal at speed, a fuel-injected engine stops injecting almost immediately: the car is engine braking and there is nothing left to burn in the cylinders or the exhaust.

Pop & Bangs changes two things. The ECU delays the fuel cut and keeps injecting for a while after the lift-off. And it retards the ignition hard, so the mixture does not have time to burn in the cylinder and finishes burning in the exhaust manifold and silencer. A soft burble and loud bangs differ in the amount of fuel, the ignition angle and the length of the window.

It goes by many names: burble, crackle, overrun pops, and combustion in the exhaust is called afterfire. Faults can make similar noises: a rich mixture, misfires or an air leak into the exhaust near the manifold. The difference is that a faulty engine pops at random, while here the ECU does it by set rules.

What can be set in the file

The pops do not happen on every lift-off, only inside a set window. BMW tuning platforms such as MHD expose these settings, and the idea is the same on most ECUs:

The road speed threshold is worth a separate thought. Pops while revving at a standstill are what neighbours and passers-by hear most, so many owners keep them for driving only. MHD, for example, only allows burbles in neutral or park when the minimum speed is set to zero.

Only the overrun logic changes: the fuel cut delay, the ignition angle on the overrun and the conditions under which this works. The rest of the calibration stays as the factory set it, unless Stage 1 is ordered too, and checksums are recalculated. Put your wishes for mode, rpm and loudness in the order comment, and we will tell you what the software of that particular ECU allows.

Burble, crackle or bangs

All three are the same setting at different strengths. The more fuel is left on the overrun and the later the ignition, the louder the sound and the more heat goes into the exhaust.

What you risk: catalyst, turbo, exhaust

Pops mean combustion where it should not happen. Part of the fuel burns in the exhaust and inside the catalyst itself, and its temperature rises. How seriously regulators take this shows in the OBD II rules: in California the ECU must flash the Check Engine light when misfires threaten to overheat the catalyst. Pops work on the same principle, just on command.

• Catalyst. Heat ages it faster, and severe overheating can melt the substrate or make it break up.
• Manifold catalyst. Workshops report that on Hyundai and Kia Gamma 1.4/1.6 engines overheated ceramic breaks up, and the dust can be drawn back into the cylinders.
• Turbo and exhaust valves run in hotter exhaust gas. According to Viezu, constant aggressive pops shorten the life of turbos and catalysts.

So it makes sense to keep the pops soft and tie them to sport mode rather than to every lift-off in town.

How to tell the catalyst is already suffering

The catalyst efficiency fault comes late: the ECU logs P0420 (bank 1) and P0430 (bank 2) only once the catalyst has clearly lost efficiency. Workshops report that other signs show up earlier:

• rattling or ringing from the exhaust, especially on a cold engine;
• flat spots and a dull throttle response;
• a chemical smell from the exhaust and higher fuel consumption;
• on engines with a manifold catalyst, rising oil consumption.

If you see these signs, flash a file without pops and check the exhaust: back pressure, catalyst temperature, O2 sensor signals and, on engines with a manifold catalyst, the cylinders with a borescope.

What to check before flashing

• No misfires: plugs, coils and injectors in good order, no misfire or mixture faults.
• A sealed exhaust. A crack near the manifold or a burnt gasket causes pops and air leaks on its own, so tuning the sound on such an exhaust is pointless.
• An intact catalyst. If it is already breaking up, pops will speed that up.
• If the exhaust has flaps, the sound depends heavily on their position. They can be kept open with the Exhaust Flaps Always Open Fix option in the same file.

Stage 1 is often added to the same order: pops and extra power are done in one file.

How to order the file

1. Read the original file via OBD or on the bench with the tool you use. We need an unmodified stock file.
2. Upload it to the portal and select POP AND BANGS. Add your wishes in the comment: mode, rpm, loudness.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file and check the car with a warm engine: pops should only appear inside the set window, with no misfire or mixture faults.

What to put in the order comment: in which mode you want the pops (sport only or always), from which rpm, whether they should work at a standstill, soft or loud. And a few words about the car: sports exhaust or not, catalyst in place or not, manual or automatic.

A log is the easiest way to check the result. Inside the pop window the injection time does not drop to zero on the overrun, and the ignition angle swings sharply towards retard. Outside the window the ECU cuts fuel as on stock software. If you hear pops where there should be none, look for an air leak into the exhaust or misfires.

Supported ECUs are listed below. If yours is not there, send the file with a request and we will check whether there is a solution for that software.

Pop & Bangs makes a car louder. In the UK it is illegal to modify the exhaust system to make a vehicle noisier, and the police can act on excessive noise on the road. In Germany a change that makes a car noisier voids its operating permit, and unnecessary noise is banned by the traffic rules. Similar rules apply in many countries. Use the function where the law allows it, for example on track and at closed events. Responsibility for using the file lies with the vehicle owner.