Port Your Stock Turbo
Denis Kefallinos kefallinos@aol.com
Looking to extend the performance of your stock GN/TR/TTA turbo? Then porting may be the solution! Porting can be accomplised very easily since the turbo can be readily removed, worked on and reinstalled. The procedure outlined below can be accomplished in a day with dedicated effort - daily drivers need not despair!
You Will Need
Emery cloth (coarse, medium and fine grit)
  1. Degreaser/cleanser
  2. A copper, triangular header/turbo gasket from ATR (for those of you with stock headers and turbos) *optional*
  3. Grinding stones mounted on either 1/8" or 1/4" shanks (alternatively you can use carbide burs)

and you must have either one of these:

  • DREMEL and Dremel bits: high speed steel bit (cylindrical and pointed) x 2, sanding wheels
  • If possible, an Air or electric Powered Die Grinder (makes short work of things), carbide cutters, sanding cones and flap wheels on long shanks.

and it would be real cool if you had access to one of these, like I did:

  • A milling machine
  • A 15/16" end mill bit

Since the turbo will need to be entirely disassembled, please refer to the discussion by Chris Fasulo on turbo rebuilding for disassembly and reassembly procedures. We will assume the the turbo charger is disassembled into 4 main components: compressor housing, center housing rotating assembly, turbine housing, and elbow. No work is to be performed on the chra so put that in a safe place for now.

Some of the grinding being performed here is on aluminum. Aluminum is rather soft, and tends to gum up grinding stones in a hurry rendering them useless. If using carbide bits, you can lessen this effect by using "Grinder's Grease" or candle wax on the bit cutting surfaces.

Its preferable to use either a coarse sanding wheel or flap wheel, or a carbide or high-speed steel cutter when grinding aluminum. Steel and cast iron respond well to grinding, as well as sandpaper.

Compressor Inlet Bell and Outlet
  1. Clean, degrease and dry thoroughly.

  2. Inspect the housing. If you have the stock inlet bell, you should see a 0.1" to 0.15" lip all the way around where the bell meets the compressor housing. The entrance to the compressor here should appear smaller than the bell. If you have a gasket in between, leave it in place and just grind it as well. The end result will be a perfectly sized gasket.
    Using the flap wheel/cutter, ENTER FROM THE BELL MOUTH SIDE, and proceed to grind/sand away the lip until the junction between bell and compressor is smooth and even. STAY AWAY from the polished surface on the compressor which is adjacent to the compressor blades. When you are done, go over the area with medium and fine emery cloth to smooth things out.

  3. Port the outlet of the compressor (where the compressor meets the intercooler) to a wall thickness of about 0.150". The idea is to enlarge the outlet of the turbo so you can subsequently enlarge the inlet to the intercooler where the two parts meet. If you have a flap wheel or sanding wheel, smooth the interior of the compressor exit as far in as your tool will go. At this point, you may want to match the intercooler inlet to the now enlarged compressor outlet. To do this, make sure the intercooler is clean and dry, stuff some rags down the neck of the intercooler, and grind away.

  4. When done, clean and degrease thoroughly with water, soap and degreaser. If you have compressed air, blow it out, and let the part dry.
Turbine Housing
If you've never seen the inside of the turbine housing and elbow before, you will note that the surface is about as rough as 50 grit sandpaper! This causes frictional loss which in turn robs power. STAY AWAY from the large diameter hole where the turbine blades exhaust to the elbow.
  1. Clean, degrease and dry thoroughly.

  2. The inlet to the turbine housing is between 0.100" and 0.125" smaller than the outlet of the stock headers (..on the radius, about a 1/4" on the diameter). A good indicator of the size difference is the copper turbo gasket you can buy from ATR. Otherwise, make a paper gasket that is the exact size of the outlet of the headers, then place this on the turbo and the interference (amount of material to be removed) will become apparent. Using a grinding stone attachement on your dremel/die grinder, grind away the interfering material on the turbo by gently sloping the entrance to the turbine. The idea here is to match the outlet of the headers with the inlet of the turbo.

  3. Using the flap wheel or other soft-abrasive like emery cloth (not the grinding stones), smooth out the surface of the turbine inlet as far as your tool can go. Cast iron smooths out very nicely and you should be pleased with the results.

  4. *Optional Procedure* - Wastegate hole enlargement
    If you are experiencing problems with boost creep, this procedure should cure the problem. The size of the wastegate hole in the turbine housing is about 0.830". The size of the wastegate puck is about 1.12". It is possible to enlarge the wastegate hole to about 0.938" with a 15/16" end mill, and still have plenty of sealing surface left around the periphery of the puck.

    It is doubtful this procedure will improve performance, but it will increase the area of the wastegate hole by 25.5%. This will reduce the overall movement required by the wastegate for a given boost level, and substantially increase the ability for the turbo to control boost at high rpm. At the time of this writing, it is unclear whether this procedure has any positive/negative effects on dynamic response for boost control (overshoot/undershoot).

    This procedure is best completed on an end mill, with some attention to how it is done. By the marks left on the surface around the wastegate hole, it was clear the puck on my turbo was not perfectly centered with the hole - but it didnt matter since the puck was so much larger than the hole. When enlarging the hole, make an effort to center the new enlarged hole with where the puck sits. On my turbo, we had to offset the new hole by about 0.075" to be sure it was centered with the puck. If you do this on an end mill, it will take about half an hour including setup time. You *could try* to attempt this with a dremel/die grinder but the results will not be as professional.
Wastegate Elbow
  1. Clean, degrease and dry thoroughly.

  2. Do the same on the inside surface of the wastegate elbow as you did on the turbine housing - get rid of all the rough stuff. Sanding wheels/flap wheels work best here. Get into every nook and cranny, and after sanding it all out, you will be amazed at how smooth the surface becomes. STAY AWAY from the edges where the elbow mates with the turbine, as well as the sealing surface of the gasket where the downpipe bolts up.

You are done! Clean, degrease and dry thoroughly all parts one last time, blow them out if you have an air gun, and reassemble per instructions. Happy cruising! If you have any questions, give me a call.


Last updated: