991 GT3 Center Plenum Installation with O-Rings

This document contains information regarding the removal and installation of the 991 GT3 Center Plenum with O-Rings.

Created by: Anthony Butay                                                                           date: 06/19/2019

Edited by: Eric Rosu                                                                                       date: 07/22/2020

Center Plenum Removal

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

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


plenum 4

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

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.

5. Un-clip the 02 sensor harnesses on both the driver and passenger side, this is to
make sure we can lower the engine without damaging wiring. You do not have to
unplug the 02 sensor plug, just release the harness.

6. Remove oil filler cap and hose/assembly with metal pipe. Disconnect and clamp near TB.


plenum 12-1

7. Remove brake booster line from plenum/crankcase/body connections (mark and watch vacuum line connections).

8. Remove rear undertray 991.1 GT3/RS and 991.2 GT3 (RS optional)

a. Remove diagonal cross braces

b. Remove center cross member under bell housing

c. Flip cross member upside down and invert. Bottom side is up and top will now be hanging down.

d. Insert one 1/8" piece of spacer such as wood or pressboard on top of cross member.

10. Loosen O2 sensor wires from body side clips (enables free movement of engine).

11. Remove 18mm nuts and lower engine down on to the cross ember it will rest there.

NOTE*** (991.1 RS: Someone with medium to small arms can do this without cross member flip as no engine bay insulation gives 10 mm more room)

12. Remove throttle body, set bolts aside for new center plenum.

13. Remove crankcase and evap hoses from driver's side of plenum.

14. Unplug small flap vacuum line from solenoid.

15. *IF THE CAR IS A 991.2 GT3/RS also unplug vacuum line from large flap solenoid.

16. Loosen plenum clamps, front and back evenly. DO NOT REMOVE THE REAR SCREWS. Loosen about 6-10mm (1/4"-3/8") this is about 6-10 full rotations. Remove the front clamp screws completely.

17. Move clamps and loosen plenum and move upwards all the way.

18. Loosen rear AC bolt, remove front AC bolts. Pull the AC compressor as far
forward and to the side to gain room for plenum. WATCH REAR BOLT SPACER

19. Loosen oil filler hose clamp on tube side behind AC compressor. Slide the clamp
off of the rubber hose. This allows more flex when removing the center plenum.

20. Loosen coolant bleed line from clips. From the coolant tank to passenger coolant

21. Loosen bolt for fuel line on driver’s side plenum. Using an E12 unclip fuel line
and pull rearward to gain more room.

22. Start to slide plenum out and watch bottom vacuum actuator on the hose/pipe for
oil fill. IT IS A VERY TIGHT FIT AND CAN BREAK EASILY. If needed use a pry
bar to carefully push the pipe down to gain more space.


1. Place 1 3/32 large diameter O-ring onto oval plenum surface where there is an O-ring groove on each side of the center plenum. Once on the center plenum, lubricate with Vaseline or another
lubrication product to help the large oval plenum gasket to seal on the plenum surface. Make sure that when installing the center plenum that the gaskets stay on the gasket sealing surface. We do not want these to roll off out of the groove. 

2. Follow the removal instructions in reverse with the new Dundon Motorsports Center Plenum.

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.



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