Dansk vs. stock muffler - which really flows more cfm?
 

Anybody actually test the volumetric efficiency between Dansk (one in-one out) and a stock Carrera muffler? Reason I ask is I have both and It seems the motor runs better esp. top end with the stock muffler. Could it be the Dansk is really corked-up compaired to stock?

Don't know but an experienced p-car mechanic told me that the factory muffler flows pretty well.

I think that you might need to broaden your question a little bit beyond just "volumetric efficiency".
Here's a couple of threads where I did posted some "research" (such as it is) about 911 exhaust design and tuning. Here's another one in a related vein.

I guess the aftermarket performance muffler market doesn't get it. At least a few post their ratings in CFM for comparison. (I assumming that's at STP, and some sort of testing per ASTM). Has anyone done a comparison between the Dansk and stock Carrera muffler for flow efficiency?