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1994-2002 [Subscribe to Daily Digest] |
Search the NG900 and 9-3 buletin boards for "airbox" and "throttle body transistion casting". I have also enclosed some of my own write-ups. Several people, especially Dean, have made very good postings regarding the modifications that can be made.
MODIFYING THE STOCK NG900 TURBO AIRBOX FOR A 3” PVC INLET TUBE
PREMISE
Enlarging the stock NG900 Turbo airbox inlet tube is easy and allows for far more air to enter the airbox. First off, the stock NG900 Turbo 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 pipe. This changes the inlet tube area from approx. 3 sq. inches (20 sq. cm) to approx. 7 sq. inches (46 sq. cm). The PVC pipe is labeled 3” PVC Sewer/Drain and has a 3.25” outside diameter with a .080” wall thickness. This pipe is available in two types, a solid PVC and a newer style cellular core PVC with more impact resistance. The PVC pipe should be available at most home improvement or plumbing supply stores. 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 wrapped my PVC intake with fiberglass for more strength and impact resistance. Aluminum, Fiberglass, or Polypropylene (like a leaf blower tube), would also be good choices for the intake tubing. 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.
STEP BY STEP INSTRUCTIONS
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 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 (trumpet shape) 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 Turbo. 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 turbo intercooler inlet tube be removed, and that the abrupt edges in the throttle body transition housing (where the turbo intercooler outlet connects) in front of the throttle body be smoothed.
SMOOTHING/PORTING THE THROTTLE BODY TRANSISTION CASTING ON THE NG900
PREMISE
As first pointed out by Dean on this 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. The following is the procedure I used to remove the intake restriction caused by the throttle body transition casting.
STEP BY STEP INSTRUCTIONS
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 (http://www.eastwoodco.com). It consists of a couple of 1/8" dia. spindles that accept 'cartridge rolls' of sand paper. I used a 3/8" electric drill to drive the spindle. I started with 80 grit and finished with 240 grit. 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 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.
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.
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 ‘silencer’ from the inlet tube to further reduce any restrictions to airflow. The ‘silencer’ is a black plastic perforated piece wrapped with insulation, installed inside the intercooler inlet tube directly after the turbocharger on all 1994-1998 Saab 900 turbos. The ‘silencer’ appears to be designed to act as a muffler for 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.
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 (http://www.thermotec.com). 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. I don't know that the insulation will make a big difference but it looks 'fast'. As it is cool and rainy here in Seattle now, I will have to wait until summer to see if there is any real difference in the temperature of the intake charge.
Insulating the intercooler tubes took about a 1-½ hours. 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.
CONCLUSION
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 w
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