Hi all,
A few members in another thread and I have been in discussion about why the S55 sounds the way it does, no matter what the current aftermarket solutions offer: the same variation of the S55 exhaust note, only variances in volume and rasp control, but the same mono-toned/one-dimensional sounding exhaust note that sounds like two inline 3's.
Inline 6's naturally have evenly spaced exhaust pulse, so in order to take advantage of this and the resulting smooth and fuller noise, you simply need to create equality within both banks on a twin turbo set-up to capture this smooth and fuller sounding noise. If you deviate from this, you skew the sound more towards having 2 independent inline 3's competing for noise (or current S55 exhaust notes).
The reason behind this is that there is a bit of a variance between the front and rear bank downpipes, almost a foot and a half inch, and enough where the unequal-lengths create unevenly spaced pulses when they merge at any form of crossover system (stock, single mid-pipes, double x-pipes, aftermarket X-pipe or rear-section solutions), no matter what, sound waves move at the same rate exiting the system so these crossover sections are simply creating a higher-pitched inline 3 sounding exhaust because they never merge equally spaced exhaust pulses, instead they simply allow both banks to crossover in unequally spaced pulses because they don't fix the variance in exhaust lengths between both banks.
It never sounds smooth, it never sounds exciting, and most of all, it never sounds like a 6 cylinder sports car exhaust note like a B58/N55/N54 or to an extent, an RB26/2JZ inline-6 noise. In fact, it basically sounds like a truck-ish V6 exhaust note where they have un-equal length downpipes to create that off-set and the resultant noise.
The solution to this is to create equal-length exhaust pipes
before they merge, just like in any other exhaust system on other platforms. A few of us have ventured into this, adding length to the passenger side rear section and creating a new crossover section to create an exhaust note that's never been akin to the "S55 sound" (aside from single-turbo S55's which naturally capture this equally-spaced exhaust pulses).
Here is an example of exhaust note revs where there is a stock, then a popular single mid-pipe, and then an equal-length exhaust set-up, all of them on stock mufflers and stock downpipes:
Another one in 2nd and 3rd gears:
And cruising around 5,000rpm with a microphone near the exhaust:
Here is one from another member, the 1st part of the video is unequal-length exhausts pipes, and the 2nd part is equal-length exhaust. You can tell the difference in pitch and tone where the equal-length exhaust there is a higher pitch noise, like it enjoys being revved like a sports car should
There are others who have ventured into this where they utilized an aftermarket muffler to where our S55's start to sound like RB26/2JZ-like in tone. Anyways, let's discuss set-ups.
This is essentially what was done on my set-up, drawing is terrible I know, but the red represents the new pipes to create the near equal-length piping before it merges at a crossover:
[img]
https://f80.bimmerpost.com/forums/pi...ictureid=72215[/img]
The U-bend that changed the sound altogether:
[img]
https://f80.bimmerpost.com/forums/pi...ictureid=72216[/img]
I'll continually update this post once more development occurs or if I have new videos that capture the sound better.
This thread was to get the discussion continuing since the "starters" including myself had to take a custom approach as no after solutions are currently available. Post your equal-length set-ups, and let's keep it to that topic only.