I just got the new Granatelli plenum. Its a C&L copy, they cut off the top and made a screw on plate. The plate has "shark fins" on the inside that help the air flow better through a 90* angle. So today I went to Advance auto (just like autozone) and I bought a set of 10 die grinder stones for about 8 dollars. I hooked it to my regular drill. It definately smoothed out the plenum where it was rough and especially in areas where it felt really bumpy from cast slag. It would take a while to actually port it to the max, but it definately smoothed out. The only downside was that it was hard to polish it on the inside all the way bc the bit on the stones were short. The removable top plate on this plenum made it ALOT easier.