991 GT3 Center Plenum In-Car Removal and Installation

This document contains information regarding the removal and installation of the 991 GT3 Center Plenum.

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Created by: Anthony Butay                                                                           date: 06/19/2020

Edited by: Eric Rosu                                                                                       date: 07/24/2020

Center Plenum Removal and Installation

1. Remove tail lights, fans and rear bumper to get the car as pictured below. 

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2. Remove plug cover, oil fill cap, and coolant cap. Unplug connector that is attached to carbon support piece.

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3. Remove carbon support piece, the 2 screws near the back of the car have plastic covers that pop off easily.

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4. Loosen airbox clamps, clamp that attaches to throttle body, and plastic tube that goes into intake boot. Remove two 10mm bolts that hard mount the airbox to the chassis. Remove airbox from the car. This will vary slightly if you have a non-RS, the ducts will look different. Same mounting locations and intake boot. Remove air box.

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5. Unlcip the 02 sensor harnesses on both the driver and passenger side. This sit o make sure we can lower the engine without damaging wiring. You do not have to unplug the 02 sensor plug, just release the harness.

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6. Remove oil filler cap and hose/assembly with metal pipe. Disconnect at clamp near TB.plenum 11-1

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7. Remove rear diffuser, rear plastic under panel, cross braces, and silver under panel.

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8. Paint mark rear camber bolt, driver side shown here. After marking where bolt locates, loosen camber bolt. This is to allow the cross member to be installed.

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9. Remove suspension cross brace

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10. Install cross member back into car upside down.


11. Place about one 1/8" of spacer onto top of upside-down cross member. This could be wood or pressboard.

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12. Support engine with screw jack and block of wood onto lower sump. Lift engine up until it is well supported. Then remove both rear motor mount nuts, the driver side is shown below.20190610_162016

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13. Now slowly lower the engine down onto the inverted cross member with the block of wood on it. Remove the screw jack out from underneath the care and make sure there is nothing under the car.
14. We will now move back to the top side of the engine bay. Lower the car down on the lift until it is at a comfortable height to work on. The engine bay should look like the picture below now that the engine is lowered onto the wood block.


15. Remove brake booster line from plenum/crankcase/body connections (mark and watch vacuum line connections). After removing vacuum line connections, remove bracket that is attached to passenger intake manifold, it holds these vacuum lines in place.

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16. Remove throttle body, set bolts aside as we will use these on new center plenum.

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17. Un-clip coolant line from center under throttle body, shown in hand below.

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18. Un-clip high pressure fuel line.

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19. Now we are ready to start loosening the plenum clamps. Remove the front plenum clamp bolts all the way.

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20. This is for an example of what the rear clamp bolts should look like. Spin the rear clamp bolts out just before they come out of the brackets. DO NOT REMOVE THE REAR BOLTS ALL THE WAY.


21. Push tops of clamps aside and push plenum directly upward.

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22. Loosen all three bolts for the AC compressor. Pull the AC compressor out and carefully set it aside, a towel underneath is recommended. Careful not to drop the spacer for the rear bolt, shown below.

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23. Now push the lower C-clamps for the center plenum aside as well. Undo the clamp below on the oil tube and slide off to the right. This helps make the lower flap clear.

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24. Slide plenum out of the car all of the way, being careful to not break the lower vacuum actuator.

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25. Refer to the flap removal and installation document and follow the process for removing flaps and installing components. There is also a video of removing the lower vacuum actuator off of the flap shaft.
26. Move the vacuum lines over to the new plenum and it is ready to install into the car. Slide plenum into position and push it toward the top of the car.


27. Put clamp back onto oil tube.


28. Install AC compressor making sure to put the aluminum space back in for the rear bolt.

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29. Push the plenum into position making sure that the seals are correctly in position all the way around the plenum. Roughly rotate try and make the throttle body sit pointing directly out toward the back of the car. This will make the plenum alignment perfect.


30. Put the front bolts into the plenum clamps. Tighten the front and rear clamps down alternating front to back to make sure that they clamp down evenly. Make sure both right and left clamps are tightened down properly, while the seal doesn’t pinch at all.
31. Install vacuum lines as they were removed there are some detailed pictures again below.


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32. Install and torque throttle body down to the specs that are listed at the end of the document.

33. Connect and install the supplied adapter harness.

34. Now we can install the Dundon Motorsports airbox. Place 4” coupler onto throttle body with both clamps on the coupler. Then install the airbox into the car. Tighten the clamps to the throttle body inlet clamps.

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35. Make sure you attach hose as shown below, regular 991 has a similar tube, this is for the vent to the flaps.


Throttle Body Install

The throttle body above is installed once the center plenum is in the car. Make sure the O-ring is well seated. Using your stock throttle body bolts off of old plenum, install new throttle onto plenum. THIS IS A 2 STEP TORQUE PROCESS. The throttle body bolts need to be torqued with an E10 socket to 70 Deci-NM. Let the plenum sit for 15 minutes with the throttle body torqued on. Loosen the bolts. Re-torque the throttle body bolts to 75 Deci-NM.