Rear exhaust butterfly delete

[Dogged]

Ok, so I was in my local dealership with my bike on the computer and the tech says, hey you got a thrown code. I asked him if it was the rear servo code and he said yes, how did I know. Well....I'm the one who disabled it. He looked at me puzzled and asked why would I ever do something like that. I explained that it was just for noise control and I wanted it louder. He told me that it was also for backpressure for midrange power. I told him that I was pretty sure it did not have any effect on the power or tuning of the bike, but I could not prove that statement. Has anyone actually confirmed 100% that deleting the rear butterfly valve does not affect the power or is it just conjecture / heresay on all of our parts. We both agreed to research it and meet back later to determine who is correct. Any help would be appreciated, thanks.

[viperclaw]

The only way to "prove" it would be to dyno run it with it working and with it removed or the cables removed.

Pretty sure it is for noise emissions. The front balance pipe valve is for the power or runability.

[altaic]

If he can call 2-5k midrange power, then maybe there's a discussion. The rear butterfly opens fully @ 5krpm. IMHO, the concept of performance with bikes does not start until at least 50% redline or ~7krpm. At the track, you're looking at the gear ratios from redline, at least 11k minimum. Even street fun starts after 5k. So I call nonsense.

Edit: So far as I can tell the rear valve is either for noise (not sure why) or for cruising fuel economy (very plausible IMHO).

[markjenn]

The idea that exhausts can be tuned to deliver more mid-range is certainly proven and putting an active device in the exhaust to vary this tuning is nothing new. I owned a 1998 R1 and the exhaust EXUP system as Yamaha called it was pitched as adding big mid-range to the bike. And dyno tests confirmed that disabling it reduced mid-range power.

Now on the S1000RR, the valve is downstream of the cat where I doubt it would have much tuning effect and BMW's press seems to indicate that the primary reason for it is to reduce noise in normal riding to get past noise standards while allowing the bike to bellow at full power.

In Mythbusters terminology, I'd call what the tech is saying as "Plausible", but you'd need to do back-to-back dyno tests to move this to the "Confirmed" category.

Personally, I don't understand why anyone would want to make the stock exhaust system more noisy while trolling around as the BMW design would seem to me to be the best of both worlds - quiet when you want it quiet and loud when you want it loud. But that's probably another thread.

- Mark