Mike_k

Most likely it's to cut down on low RPM drone. Most BMWs have the same valving. I have the vacuum solenoid in my 550i bypasses so the valve is always open.

CampioniShop

SuperSPrint's X pipe is 650 + S/H and can be purchase separately. The resonated X pipe is 999 + S/H.

MP4.2+6.0

Agreed that the valves are designed primarily to quiet the car/reduce droning at low RPM's. That's what the Audi W12 shop training documentation says. But, the old classic tuner's caveat on exhausts for normally aspirated engines that I'm factoring in was that a "cost" of opening them up for higher flows with less back pressure at high RPM's was some loss of torque low down where the back pressure can actually be a good thing with typical intake and exhaust valve overlap. Hard to know exactly how that gets interpreted now in an N.A. configuration on a modern Audi motor with variable valve timing, multivalve motor, multipath intake runners and the like.

Audi's W12 documentation does says the secondary flap only opens when all of these conditions are met: >3mph; engine load>50%; engine speed >2500 rpm. Given other similarities in the S8 and W12 intake and exhaust set ups, fair to guess similar parameters likely apply to the S8. Thus in the case of high load but low engine speed the flap isn't open--also fits the old rule of thumb to pick up a bit of torque at lower RPM's where it is weaker in general. In the real world with an automatic that would likely be holding the gear in tiptronic mode while getting on the gas below the full throttle point. The Vette option for the valved exhaust set up on the base motor shows it adding 6hp and 4 ft pounds overall in a motor with HP and torque specs in the same league as W12's and S8's, so overall differences here will presumably be pretty subtle rather than butt dyno obvious. And notice, they still opt to do it with valving even in an optional package rather than leaving it open all the time. The Vette specs also fit with the other general rule of thumb that in an NA motor with a well thought out exhaust to start, hard to squeeze out much more real power. Tuned turbos and especially full exhaust (as Mike alluded to earlier in the thread), different result.