[Moderators Note: The benefits of a VR6 aftermarket header vs. the cost have always been a problem. Fact is a VR6 simply doesn't need a full header to perform well. This guide will show you how to modify your stock VR6 manifold for increased performance. Except for the modifications made in this guide, the VR6 exhaust manifold is actually made quite well.]

Thanks to Matthew Townsend for the write up!

HOW TO PORT YOUR MANIFOLD:

Step 1.
Take off the exhaust manifold.

Step 2.
Port the exhaust manifold to gasket size but don't touch the exhaust port itself. Aim for a .040" - .100" lip on the head side. You can use something like a Dewalt DW887 die grinder with a dimmer switch to control the speed for the heavy grinding and a Dremel for final touch up. Get a set of carbide burrs for the meaty stuff and a set of rolls for clean up. Standard Abrasives makes great porting kits with all the rolls you'll need. Flow won't be impacted by the increase in diameter, but reversion will be assisted by the step down. The idea is to keep the exhaust gas flowing in one direction avoid mixing burnt and fresh mixture in the chamber, compromising detonation.

Step 3.
Get the down pipe, weld the outside and grind down the welded internal lip. Refer to pics below.

Further option 1:
Once everything is ported, have the manifolds media blasted and prepped for a coating of POR 15 high temp manifold paint in satin silver as it's rated for 1400 degrees and resists salt. Feel free to coat both inside and outside.

Further option 2:
Wrap the manifolds with header wrap.

4.
Reassemble, re-using the existing gasket.

Should take two to three hours, excluding the manifold painting and header wrapping.

Some pics, (thanks to Jettafanatic):

Down-pipe before: note large protruding weld:





Down-pipe after: Note much bigger / smoother bore and no internal "lip" from the weld to cause turbulence:





DP ports of the exhaust manifold after / before:





Before / after head ports of Exhaust manifold: see how much more even / smooth and matched they can be: