Originally Posted by nicknaz
Other than switching the downpipe, do you think there are power gains from the exhaust?
Or to ask it another way, if someone wanted the "stage 2" power with OEM sound, this is possible by changing the downpipe and keeping the OEM cat back exhaust, or are there bottlenecks in the airflow with the OEM exhaust?
I do not have any experience with switching downpipes. What I did was simply replacing the secondary cats by installing Miltek decatted mid pipes.
Like OP I did this mod only to get better sound. Nothing else. I am not aiming for more power or faster turbo spool up. I just want my turbos to last as long as they would on a normal 35is. Anyway, removing the cats has had a much bigger impact on the sound than any ECU mods I have done. I don't think one can replace the other.
As for power gains, here is how I think it works: The ECU is programmed to target at certain boost psi, which, unless you change something very significant, translates more or less directly into a certain amount of available torque and power. If you get more or less power than what was intended by the ECU, it is because the ECU is not doing its job well enough. You can change the targets with software like ESS or by fooling the OEM software with piggies, but that we all know. This is of course different from what we see in NA engines, where the engine depends on the outside air, and hence looses hp with altitude, "bad" weather or poorly designed intakes as it cannot control it with turbos. Also, as the N54 is forced induction, I don't think it cares much about the exhaust as long as it can reach its psi targets with whatever lag the system has. Opening up the exhaust, especially near the turbos, would reduce lag, but "hopefully" not increase power, as the ECU should control this. And even with the OEM exhaust the turbos have no problem reaching even very high psi levels. So it is not restrictive in this regard. Other things give up much sooner than those. Such as the fueling system for example (like the low pressure pump).
So I should think that even if you removed the downpipes, the ECU should control the turbos enough that you do not see any significant power gains. Any such power gains would have been a result of "unwanted" spikes in boost psi, as the turbos spool up faster than "predicted" by the ECU, and hence get a little out of "control". I wouldn't know if I am right that this could happen, so any engineers are welcome to correct me if I am wrong.
I think installing a better intercooler is a much better way to get the same, or slightly more power, as the ECU realizes it can put less work on the turbos and still reach the targets it is programmed to reach. What is not to like about less pressure drop (flow resistance) AND colder intake air at the same time. The M4 is an even better example of how it should be done.
Back to your original question. I know that other people disagree with me, but I still think that the ESS ecu mod does not increase the sound volume of the exhaust overall. The nature of the sound is different, but that is a different discussion.
But as for volume, I think ESS makes the car slightly more muted in everyday driving, probably because the engine has more torque down low, and can "relax" more. A relaxed engine is a less noisy engine. As for WOT, probably ESS is slightly louder, but where I drive, it is hard to tell the difference.
Overall removing the cats has a much bigger impact on the sound volume than an ECU tune if you ask me. Ie I feel certain that if I went back to the OEM software, I would still have a much greater sounding car than I ever had with our without ESS before installing the miltek decatted midpipes. Hope this makes sense.
Now the next question is what the nature of the sound is with decatted mids vs decatted downpipes vs muffler delete (all 4 cats still installed). Some say that the sound is deeper with decatted downpipes, but it is probably hard to find someone who has tried all three options on the same car. And the mufflers on a Z4 are not the same as on a 1 or 3-series. But I can honestly say that I personally do not want my car any louder than it is now.