I built my RV-14A with a Superior XP-400. As discussed, Van's exhaust for the IO-390 will not fit on the XP-400 due to Superior's cold air sump. I contacted Vetterman who provided their 2-to-1 by 2 exhaust (each side has 2 pipes joining to one 2" pipe - separate units for each side). I understand this "trombone" design has been used on RV-7s - it adds a loop in one exhaust pipe on each side before they join together to spread out the exhaust pulses. With this design, the 2" pipes are too large to fit in the tunnel, so the pipes must exit the cowl similar to other Van's models - in the case of the RV-14, this is forward and below the cross hinge on the nose gear.
Here is the right side:
... the left side:
Here is the view from below:
View from the side:
The parabolic cutouts are the only cowl mod I had to make:
These are two views with the cowl installed:
The plane has only flown 13 hours so far, but the exhaust is working great. I was delighted to require such minor cowl mods. I was concerned about clearance between the pipes, the nose gear and the cowl - there is about 5/16" - with engine shake at start up but I've looked at the video of my first start (the most shake I ever expect to have, but that's another story) and while the engine may behave like a wet dog on occasion, the pipes barely move.
Others have expressed dissatisfaction with having built a tunnel that isn't used. I initially felt that way, but now think of it as a cooling tunnel. During my first flight at 25 squared (75% power), the CHTs maxed out at 334 - on a day in the 70s. I now think I will not have any problems in the heat of summer and cowl louvers will be unnecessary.
I did experience some "drumming" on the floor boards, especially at high power settings like climb, but some carpet has helped quite a bit. Observers on the ground have commented the engine is a bit noisier than other RV's. The power is AWESOME!