[MARRAT]

Dear members,
Some time ago I have installed two oval cutout valves in the exhaust of my 550i 2011 (prior to the rear mufflers in the region of the rear axles) in order obviously to improve its original too quiet exhaust sound. I checked myself that the whole installation work was very well done (including all cutting and welding work in the stainless steel tubes) and the outcome in terms of sound is very good when the valves are opened. However, I have checked and confirmed on the streets (using chronograph and other supporting devices) that surprisingly the car's acceleration performance is quite worse when the valves are opened when compared to the situation when the valves are closed. Any reason for that? Does it have anything to do with backup pressure? It is really intriguing to me. I actually thought it would be the opposite outcome: performance would increase with the valves opened. Any idea why this is happening? Any similar experience with other members (incl. the ones who did muffler delete or even the so-called “replacing of perf pipe w/unperf pipe in the mufflers”?

 

[MARRAT]

I was informed by the QTP technical representative that performance gains or loss are completely dependent on each vehicle. He mentioned that in the particular case of a Chevi Camaro, power is lost all of the way open as well. In this particular case, the car needs to retain some of that back pressure. He says However, through tuning they found that 60% open is the sweet spot for the application in the particular case of that vehicle. The only way to get the most of them or many modifications for that matter is to work with a local tuner shop to find out what***8217;s going to work best for your car. They say. It***8217;s just a matter of finding your cars ***8220;Sweet spot***8221;. Trial and error as they say. That sounds bit weird, doesn't it? Well, I will do further investigations***8230; The issue here is whether all potential modifications in exhaust system of F10***8217;s (including the use of cutouts and exhaust system modifications) may end up in loss of power***8230;. that most of people has never realized. In the particular case of the 550i F10, it is so much power that it is indeed not easy to confirm loss of power after modifiying the exhaust system. Well, comments, experiences and contributions about this subject is more than welcome. Marco

 

[ndinh]

Thanks for the info Marco. I wasn't aware of the potential power loss with the QTP cutout valves but it does make sense because I've heard of guys feeling power losses when they do muffler deletes. I installed aftermarket mufflers on my car and I can feel a little power loss even though it's louder and gives you a false sense that you're going faster.

 

[MARRAT]

Thanks for your comment. Yes, I do want to trigger this discussion: do muffler delete, installation of cutout valves and other exhaust related measures/modifications negatively affect performance? How to mitigate/address that? Why does that happen? In the particular case of the installed cutout valves in my 550i F10, it is just a matter of closing the valves for having performance back at the level prior of their installation. I do believe that in the particular case of performance exhaust systens such as Eisenmann Exhaust and others, these system are designed and tested in a way that their installation does not negatively affect aspects such as back pressure and others (?) in a way that performance is reduced by the engine ECU. Inputs/contributions from the ones who did muffler delete would be appreciated.

 

[CALWATERBOY DUE]

....surprisingly the car's acceleration performance is quite worse when the valves are opened when compared to the situation when the valves are closed. Any reason for that? Does it have anything to do with backup pressure? It is really intriguing to me. I actually thought it would be the opposite outcome: performance would increase with the valves opened.

A highly tuned engine has little tolerance for outside-the-envelope operation - yep, has everything to do with back pressure. Lowers torque, thus thrust.

A happy conclusion may be reached by removing cut-outs; doing [this] - intended parameters restored.