exhaust port restriction
 

my valkyrie exhaust ports are round, with the top one fourth filled in. what is the purpose of this restriction ?

The "top"??? It's common for ports to be D shaped with a flat floor, the flat being on the short side radius. These tend to flow a tad better and keep velocity high, due to later onset of turbulence and more organised flow of gases following a flat SSR curve.

you are right, the flat spot is on the bottom, on the short side radius. with the heads off, I got turned around:o thanks for the info. it sounds like I better not touch the ports

Well they might benefit from more of a hump on the bottom for a smoother turn out of the valve and some compensation out of the roof. But unless you know someone who can build them up with weld, then there's little that can be done, apart from fabbing your own port plates. But that's getting towards high magick, so if you're just cleaning them up, just take off any seams around the valve boss etc, make sure there's no sharp edges around the valve exit and leave the rest alone.