Modifying NG900 Turbo airbox...(long) - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Modifying NG900 Turbo airbox...(long)
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Posted by REM in Seattle [Email] (#4) [Profile/Gallery] (more from REM in Seattle) on Thu, 25 Jan 2001 14:54:02 Share Post by Email
In Reply to: options, Dean, Thu, 25 Jan 2001 09:22:57
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Modifying the stock airbox for a 3” PVC inlet tube

Enlarging the stock NG900 airbox inlet tube is easy and allows for far more air to enter the airbox. First off, the stock NG900 airbox has a 2” (51 mm) inlet tube that goes from behind the front right headlight/bumper to the airbox. There is a 2 1/2” (64 mm) outlet from the airbox to the turbocharger inlet. Therefore, the 2” inlet tube is the first restriction to more airflow.

I chose to replace the 2” inlet tube with a 3” (76 mm) inlet tube made from PVC vent tube available at most home improvement centers. This changes the inlet tube area from approx. 3 sq. inches (20 sq. cm) to approx. 7 sq. inches (46 sq. cm). I chose PVC because it is easy to cut, easy to shape with a heat gun, and is widely available. The use of PVC tubing in this application should not pose any hazard as the airbox inlet tube is far enough away from the engine heat to make melting or softening a non-issue, and if the PVC should break or shatter due to an impact, the engine air filter will keep any PVC fragments from entering the engine intake. I will probably wrap my PVC intake with fiberglass for more strength and impact resistance. Aluminum or fiberglass tubing would also be good choices. For the best results a smooth internal wall and a flared opening (bell mouth) at the front of the inlet tube is needed. These will help minimize losses in the intake flow.

Start by removing the grill, right front headlight assembly, and right turn signal assembly.

Pay special attention to how the current inlet tube sits in relation to the bumper and headlight assembly. You will want your new 3” inlet tube to sit in nearly the same location.

Remove the stock airbox from the engine compartment.

Measure the length of the stock inlet tube from the front of the airbox to the end of the tube.

Cut a piece of 3” PVC vent tube about two inches longer than the measurement for the current inlet tube. Preferably the 3” PVC pipe you have chosen has a bell mouth on one end to accept the next length of PVC vent pipe. This bell mouth works very well as the inlet tube air in take end near the front bumper. Alternately, a bell mouth can be formed on the end of the tube by using a heat gun/hair dryer to soften the PVC tube and a funnel or similar tool to form the softened PVC tube. The bell mouth acts to reduce flow losses in the inlet tube.

Using the 3” inlet tube as a pattern, mark a 3” circle on the airbox over the current inlet tube opening. Be sure that the new opening will be below the lowest point of the air filter element.

Cut the opening for new 3” inlet tube. A utility knife, small saw, and a file or drum sander all work well.

Fit the 3” inlet tube to the airbox and check the airbox and inlet tube assembly for correct fit in the vehicle. Reinstall the headlight assembly to be sure there is enough clearance. Trim the airbox inlet tube opening, the inlet tube, and the ribs in the airbox as needed to get the proper fit.

At this point I stopped to paint the PVC tube black. This is for cosmetic reasons only, to blend in with the rest of the airbox and tubing. Use lacquer thinner to clean the 3”inlet tube prior to painting.

Clean the airbox where the tube will mount with lacquer thinner. Tack the 3” inlet tube to the airbox in a couple places using hot melt glue. The hot melt glue is just to temporarily hold the new inlet tube during the clearance check. Check again to be sure that the inlet tube will clear the headlight assembly. Then completely seal the inlet tube to the airbox both inside and out using Black RTV Adhesive/Sealant. Be sure to keep the water weep hole in the bottom of the airbox clear.

Reinstall the airbox, headlight assembly, turn signal assembly, and grill.

Obviously, this is just one step in improving the airflow to the NG900. A K&N filter element and a low restriction exhaust are additional steps that should be taken. Dean has also suggested that the plastic silencer in the tube intercooler inlet tube be removed, and that the abrupt edges in the throttle body transition housing (where the intercooler outlet connects) in front of the throttle body be smoothed.

_______________________________________ REM in Seattle Current: 2013 Volvo XC60 R-Design, Rebel Blue, 3.0L 6-cyl turbo, Automatic 2024 MINI Cooper S Convertible, 2.0L 4-cyl turbo, 6-speed manual Former: 2005 Saab 9-3 Aero Convertible, Steel Gray metallic, 2.0L 4-cyl turbo, 5-speed 1998 Saab 900SET 5-door, Silver, 2.0L 4-cyl turbo, Automatic 1991 Alfa Romeo Spider convertible, Alfa Red, 2.0L 4-cyl, 5-speed 1984 Saab 900T 3-door, Red, 2.0L 4-cyl turbo, 5-speed


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