There is a guy selling a mint condition Dinan x-pipe in the parts for sale section.
Dinan has extensive R&D invested in their x-pipe design. I know this based on a conversation with their chief engineer. This particular x-pipe design made the best power and torque of the half dozen different prototypes they built in-house. (two resonators, no resonators, primary cat delete, secondary cat delete, etc.)
The 2.75" primary exhaust piping diameter, and the four resonators were properly sized to increase horsepower & torque while also eliminating DRONE in the low-to-midrange rpm band.
Due to the high energy created during the combustion process inside the engine (when exhaust gases first enter the exhaust pipes), they will contain a variety of sound frequencies. Most of these frequencies are not
very acoustically pleasing to the ear. Both OEM and aftermarket exhaust manufacturers use resonators on vehicles, as a way to 'deaden' or reduce these unpleasant exhaust frequencies. The exhaust gases leaving our high revving M3's will have a lot of high-pitched frequencies, better known as RASP.
Strategically placed resonators with the correct size, length, and internal sound deadening material (ceramic in this case), can actually be used to 'tune' the exhaust note (like a musical instrument), by canceling out undesirable sound frequencies. (leaving behind only those which are most pleasant)
BTW: The noise suppression characteristics of the 2 resonators Dinan uses before the 'X' is different than the noise suppression characteristics of the 2 resonators after the 'X'. (by design)
Now that's what I call Engineering...
This design concept is much easier for me to explain, than it is to actually build.
This requires a lot of R&D (trial and error), which is why Dinan worked on their x-pipe for over a year.
An aftermarket performance company like Diann has there work cut out for them. They have to come up with an exhaust design that gives the least resistance to the exhaust gases flow, while also removing unwanted frequencies that will lead to droning. That's not easy!
In fact, it's a lot tougher than you might think.
You have to find just the right combination of pipe diameter, wall thickness, the physical size of the resonator itself, sizing of the internal perforated holes inside the resonator, the sound deadening material that's encased inside the resonator, etc.
You may think it's just a piece of stainless steel pipe, with a couple of resonators welded in...but you'd be wrong.
In this x-pipe design, Dinan has manged to strike a nearly perfect balance between allowing the exhaust gases to pass through (with almost no restriction), while also removing the unwanted sound frequencies contained in the exhaust gas stream. (that leads to droning)
I'm telling you right now, you're about to make a big mistake if you pass on the Dinan catless mid pipe brother...
Keep in mind, that Dinan has performance software was tailor made to match up with their catless mid pipe.
The Gintani rear muffler section will be loud enough by itself. (not to mention that it drones)
Adding an x-pipe that drones as well will only compound that problem.
You're going to be kicking yourself later, if you remain on the path you are on now.
You might want to take a step back and re-think your decision here.
It's your money, so you can buy whatever you want.
I just wanted you to know the score.
An informed buyer
tends to make smart, well thought out purchase decisions. (on average)
It's funny how often that philosophy seems to pay off.