Originally Posted by Mandingo

I basically copied Megan370z (thanks ). I used an air dremel and various sized sanding wheels and stones to carefully (CAREFULLY!!!) remove the rough parts from molding process and open it up a little. Then I used various grits of sandpaper to finish everything, all the while maintaining the stock shape (just getting the sharp edges out and opening it up a tiny bit). I also matched the upper and lower manifolds with the dremel and sandpaper.

The HR upper manifold (or M370) is a mess inside. The molded halves weren't meeting up correctly inside the runners. Turbulence was almost certainly reduced after I smoothed everything out, but I have no dynos to prove anything.

One thing to look out for when installing just the upper manifold:
There is room for the upper manifold to move around in relation to the lower manifold due sloppy tolerances in the bolts and holes that line up the two manifolds. This could cause a less than optimal flow. So if you're anal about it like I am, your best bet is to remove the upper and lower manifolds and match them. Then make a note of how they need to be attached to avoid a kink in the flow. There is probably an ~2 mm movement possible between the upper an lower manifolds on my Z. It might not matter, but I feel better knowing that everything now functions like one continuous runner with no kinks or sharp angles.