What can be done to free up air flow in the TPI plenums?
The stock plenum has about 52mm openings, but there are some serious blockages about 1/4″ inside. This brings the open area down considerably. There are 2 cast ‘walls’ just inside that need to be knocked out before any “opening” is done. These apparenly shrowd the EGR holes. Start by removing the plenum and then carefully grind out the walls. This should result in a noticable performance increase by itself. Then you can go even further by putting on a 58 mm throttle body, but this will require boring out of the plenum air inlets. This is a large amount of metal to take off and requires a lot of time and patients. It is doable though with a carbide bit and an air grinder. You will need to radius blend the opennings (you can’t just bore them, there isn’t enough metal all around). The result is much better flow. If you’re not familiar with using the tools mentioned, then you’re better off having it done professionally. TPIS will do it for $85, but you’ll have to send them the plemum.
