I, for one, very much like the sounds of the cooper S engine and stock exhaust but the pop and burble isn't quite regular.

I noticed that it always takes a little time for everything to get to temperature before it activates, and the harder the car is driven the better. But still it's not what I'd wish it was in terms on consistency.

Has anyone looked into tuning that sounds? What causes the pop and burble when the button is pressed as opposed to not on? What's the mechanics that effect these sounds? And what could be done to improve it?

 

Normally when you lift off the throttle the ECU immediately drops the duty cycle on the injectors causing the engine to lean out. When the sport button is pressed there is a .5 second delay allowing additional fuel to be injected while off throttle. There is not enough air to completely burn the fuel in the cylinder so it burns in the exhaust system. This causes the burble. Adjusting it requires modifying the tip out transient in the ECU.

 

Im going to throw this out there and I might get flamed for it.

But I believe the pop is not from the gas being burnt in the exhaust but programming of the ECU to spray a shot of fuel and ignite it with the exhaust valve open. I've got the pop driving 3km from cold traveling at a consistent 60kph and sports mode off, not enough to heat up the cat.

Of cause this is only my theory.

I thought the burble came from the headers and twin scroll turbo?

Now commence the flaming

 

Lol Hey Scubby. I was looking for a picture of a flame retardant suit to loan you virtually and came across this instead. Quote should read "Damn, I knew I shouldn't have had that second bowl of chili before the race."

But in all seriousness it's not a bad theory. But the cat really has nothing to do with it. Removing the cat only increases the volume of the burble. In fact the burble is very hard on the cat and can destroy one. As long as the exhaust air is hot enough you can create it. You also don't need fuel injection to create it. A carburated engine if adjusted slightly rich will also do it when down shifting.

There is actually two ways to create the sound in an EFI system. One which I mentioned above, but you can also retard the timing at lift off to create or accentuate the effect. Injecting fuel while the exhaust valve is open would be a very bad idea though due to valve overlap. At the end of the exhaust stroke the intake and exhaust valves are both open at the same time. This serves two purposes the first is EGR or exhaust gas recirculation. This lowers emissions and improves efficiency in moderate engine operation by lowering pumping loses. It is also the primary cause for carbon buildup on our valves. The second function of overlap is to cool the exhaust valve by allowing a cool shot of air to pass over it before closing. Having a still burning flame traveling over the exhaust valve could burn one pretty quick and also cause backfiring into the intake manifold.

 

So essentially the pop and bubble is purely driven by the tune. And the engine management valve timing of the S button is what changes with the S button not pressed as far as pop and burble are concerned.
Further more, that there is no additional mechanical element changing between the S and no S function, like a hole being open on the exhaust or something else of sort.

I was aware that any exhaust can have back fire depending on the condition, but I also thought that was partially controlled by the exhaust on the end of the system. Exhaust not being what creates it but what controls how it comes out. Is that a false assumption.
Anyway, it doesn't sound like it can be easily adjusted to improve the quality and/or frequency of the sound...

 

That's correct. The sport button makes three changes. Steering becomes heavier, throttle response curve is altered and tip out fueling is altered in the ECU. Previously verified the delay in fuel cut using DashCommand, so I know that is occurring. Not sure if BMW altered tip out timing as well. Once Tiggers HPFP issue is resolved and I get him back on the road I can check for changes in timing as well.

 

Tigger, based on what you're saying, I'm wondering if constant sport mode use and resulting p&b may reduce cat life and increase the intake valve carbon problem? I use sport mode most of the time and love the very pronounced p&b, crisper throttle and heavier steering, and in fact have an FES autosport plus waiting to be installed. Now I'm having 2nd thoughts.

 

I wouldn't worry. Could the burble reduce cat life some. Yes. Should you worry about it no. In a turbo application cats should be replaced at 50k miles to reduce back pressure. Excess back pressure will tend to push the turbine wheel in and wear the bearing seal faster. Sport mode will also have no adverse effect on carbon buildup. In fact it will probably reduce it due to more aggressive driving. Under higher load the ECU advances the intake cam thereby reducing valve overlap and EGR.

 

Thanks Tigger, all good to know!