Since I learned how easy it was to disconnect this two vacuum-driven butterfly valves on the tailpipes, I've wondered what would be the consequences of disconnecting them and using some wire to hold the valves open?

Yes, I know it will be louder. I'm fine with that. That's kinda the point. But would it affect emmissions? Fuel economy? Would I get a check engine light?

I might try it for a while this weekend, just to see if I like the sound. I could probably disconnect both sides in less than five minutes. Total.

 

I'm not sure if what I've done is any different than physically disconnecting the system by holding the valves open, but I've pulled the fuse for the active exhaust (fuse 19) after reading about that on here not long after I bought the car(going on 2 years). I have had no noticeable difference in fuel economy, nor did it cause the check engine light to come on, same thing goes for emissions, no noticeable difference. The only noticeable difference is the sound the exhaust makes in the lower RPM range, sounds much better as now you can actually hear it.

 

Yep, what Rhemler said; simply remove fuse 19 and your GTG.

 

The valves have a spring on them and the default position is open with vacuum removed (fuse 19 removed).

I took the fuse out when I first got my '10 XKR but then put it back in again and use the Dynamic mode switch to control the sound because they are permanently open in Dynamic mode which is what I run my car in 99.9% of the time but it gives me the option to make the car quieter first thing in the morning as I leave my house on the way to work.