Article taken from gmtips.com

Porting the Plenum

Porting of the plenum is basically a free mod that will free up a few horsepower. This modification will not add “dynamic” horsepower that will bury you in the seat springs…..but moreso it will improve the response of your throttle. First some theory. The airflow entering the plenum is forced to make hard right angle turns to enter into the runners. There are square block areas to the left and right of the runner passages that can have excess material reduced to improve airflow.

For some reason GM put restrictive “blocks” in front of the EGR passages and to the left and right of the throttle body openings. You can grind down the front blocks and you can radius the side areas for improved airflow.

The white dotted line represents what is meant by radiusing. If you went directly from point A to point B you would cut through the outer wall of the plenum. But by curving your grind (radiusing) you are able to grind away excess metal without cutting through.

Be extra careful and go slow when making your radius grinds.

You will need a dremel tool and the above bits:
Drum Sanding Roll
Tapered Metal Cutting Bit
Drum Metal Cutting Bit
Tapered Sanding Roll

Basically take your time and grind away the areas above. The sides must be radiused to prevent breaking the outer wall. Use caution here and go slowly.

This shows the block like obstructions on the inside of the plenum. It is a little hard to get to but it can be worked down.

This is a front on view of the same restrictions.

This is an angled view of the restrictions with metal ground away.

And another view of the inner post blocks ground down.