Justin,

I've been really considering doing a MTW75K mini-pipe on this Cap. I'm not looking for more power as I am at 13.75 pounds on a DA50 but I wanted to drop some noise. I also own the GP Ultimate which is basically the same front end as the Cap is. I added a smoke tank to the Ultimate and here is what I did, I think using the same setup and adding some 1/16th balsa and it would work.

Remove the stock tank, trim the former where the tank goes all the way to the top of it. Add a piece of very light ply between where the battery/servo tray is to the former you just cutout. Change the tank to a 16 ounce dubro and mount it to that light ply plate. The tank will now be over the CG and higher up in the fuselage. You now have room for a pipe that is about 2.5 inches in diameter give or take. Cut out a hole in the bottom of the fuselage right below the engine box and you'll probably need to cut some of the bottom of the engine box off as well and also some off the bottom of the former that you cut for the tank, just enough to get the pipe to fit and use a 0mm drop flex header. Once you get everything cut out to fit, line the whole area with 1/16th balsa sheeting to make a pipe tunnel. The little section on the bottom between the firewall and the wing saddle can then be slotted for airflow. If you are using a heavy pitts muffler, you may only gain 1 to 3 ounces between the pitts muffler and the header/mini-pipe, be a bit quieter and get another 100 or 200 rpms.

I have not done this yet but after seeing the mini-pipe that Comp-arf sells for the 2.3 Extra it might just work. It's fairly short and just under 2.5 inches in diameter. Oh yea, you'll probably have to cut off a bit of the exhaust tube on the pipe since this plane really sits low to the ground or add some ply under the landing gear to raise it up