NG 900/9-3 TB Transition Cast Porting

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Before air enters the throttle body it must first pass through a cast aluminum pipe which connects the intercooler exit hose to the throttle body. The problem occurs where this pipe meets up with the rubber connetion, there is a lip which poses a rather large restriction to the incoming air. This modification requires grinding and sanding to smooth out this transition to allow for smother airflow.

Throttle Body Cast External

Cost and Difficulty

Cost: Free Time: 45 min Difficulty: Easy


As first pointed out by Dean on The Saabnet BB, the throttle body transition casting has a pronounced 3/16" lip where it meets the intercooler outlet tube and a small lip again where casting slips into the throttle body. These mismatched transitions are an easy place to reduce the restriction in the intake system by smoothing or porting the throttle body transition casting. The idea is to make the transition casting ID match the inlet tube ID on the one end and the throttle body ID on the other end, and to smooth out any rough spots in between.

With the throttle body transition cleaned-up etc., my seat of the pants performance evaluation shows that the car has a much smoother accelerator response and a smoother idle. Turbo spool-up time seems to be faster as well.


  1. First I removed the throttle body transition casting from the engine. Be sure to cover the throttle body with a cloth or plastic bag to prevent anything from getting into the intake. Remove the O-ring and sensor prior to porting the throttle body transition casting. I used an engine porting kit, Item #46056, about $30 from the Eastwood Company ( It consists of a couple of 1/8" dia. spindles that accept 'cartridge rolls' of sandpaper. I used a 3/8" electric drill to drive the spindle. I started with 80 grit sandpaper and finished with 240 grit sandpaper. I don't know how many 'cartridge rolls' came with the kit. I borrowed the kit from a neighbor and he had purchased extra 'cartridge rolls'. Others have used 1" to 1.5" diameter drum sanders or dremel tools and had good results. The job took about 1-2 hours with a couple breaks along the way. A vise, or Stanley Workmate, to hold the casting is a big help. In the end, the throttle body transition casting made a smooth, step less transition from the intercooler outlet tube and to the throttle body.
  2. Thoroughly wash and dry the throttle body transition casting using soap and water prior to re-installing it. Lubricate the o-ring with a little silicone grease (dielectric grease), and then reinstall the o-ring and the sensor. This would be a good time to clean the throttle body and apply some good lubricant to the accelerator cables and linkage.
  3. Next I removed the intercooler inlet and outlet tubes. I needed the car on ramps to reach the hose clamps on the intercooler inlet tube. I wanted to remove the insulation from the inlet tube to further reduce the airflow restrictions. This insulation is sometimes called the ‘silencer’. The ‘silencer’ is a black perforated plastic piece wrapped with fiberglass insulation. It is installed inside the intercooler inlet tube directly after the turbocharger on all 1994-1998 Saab 900 turbos. According to some sources, the insulation is designed to reduce the amount of heat the intake air absorbs from the turbocharger and exhaust manifold. It may also be intended to reduce the ‘whine’ noise from the turbocharger. Remove the ‘silencer’ by grabbing it with a pair of needle-nose pliers and pulling it out of the intercooler tube. The ‘silencer’ is destroyed in the process. Mine tore into a few pieces as it came out.
  4. I then wrapped the intercooler tubes, both the inlet (lower) tube and the outlet (upper) tube, with Thermo-Tec Thermo Shield high temperature reflective insulation ( A foil faced fiberglass duct or pipe insulation could also be used for this application. I cleaned the tubing first with lacquer thinner inside and out. Insulating the tubes will reduce the amount of heat the intake air absorbs from the turbocharger, exhaust manifold, and engine compartment. Insulating the intercooler tubes took about a 1-½ hours.
  5. You probably don’t want to use silicone grease on the rubber coupler or the ends of the intercooler tubing, as it might allow the tubing to slip out of the coupling when under pressure. If the fit is too tight, then a little soapy water on the ends of the intercooler tubes will help to slip them back into the rubber fittings. Be sure the tubing is properly seated and the clamps are tight, as the turbo pressure will be trying to blow the tubing out of the joints. If one does pop off, be very careful putting it back as the turbo and exhaust get very hot very quickly.
Throttle Body Cast Porting Before
Throttle Body Cast Porting After