Just use the Aerolip and a bigger Wing. That will do the job.

 

Sorry, I cannot send pics at this moment, I do not have a lift at home.

The under trays are from the dealer. The rear trays have ducting to aid in the cooling of the rear rotors (order the 997 rear brake ducts and trim if you are using 18" wheels, these were designed to work with the 19" on the 997's)

 

Highly highly highly doubt that will provide any aerodynamic benefit by looking at it, it seems more of a show piece. Additionally, our cars don't really lend themselves to being able to get the correct geometry for a functional rear diffuser without some cutting and making things look ridiculous.

Nice looking piece, but I'm going to go with that is a cosmetic upgrade rather than performance.

 

Forgot to mention the rear fender liner needs a little trimming for the under tray ducting for the rear rotors.

 

If you are running a GT2 bumper cover (or replica), you will need the flip and angle the center radiator forward along with the proper ducting to divert air out over the front hood thus creating down force. Without this, the front bumper is just for show and aerodynamically useless.

Here are pics of the 997GT3 rear under tray on my car. It has NACA ducts (purchased separately) that clip into it and divert air to the rear brake calipers via air guides (also purchased separately) that attach to the rear thrust arms. All factory OEM parts.

Rear 997GT3 under tray: 997 504 629 92
Rear 997GT2 brake NACA ducts: 997 575 183 00 and 997 575 184 00
Rear 997GT3 brake air guides: 997 331 487 02 and 997 331 488 02
Rear 997GT3RS brake rotor ducts: 997.575.531.90 and 997.575.532.90
Rear 997GT3RS brake duct clips 997.575.631.91 and 997.575.632.91 (these are for the PMNA cup arms - smooth finish. There are other clips that fit the textured OEM cast arms)
Brake duct screws (4 total needed) 999.073.516.01

You will need to do a little modding to make the rear 997 attach properly on the 996. The center attachment points are the same on both car but the end attachment points do not match. I ended up cutting the ends off of both trays and riveted the 996 ends onto the 997 tray. It's a perfect fit but you need to make accurate measuring and cutting to make it fit right. In my case I also added a scoop to the 997 tray to divert air to the transmission cooler which is directly above it (a 996TT does not have this obviously). You will also need to trim the rear fender liner to allow the back of the NACA duct to be exposed.

I also added 997.2GT3 LCA brake ducts which divert air to the rear brake rotors. These are superior to the 996GT2 ducts but will require adjustable drop links for perfect clearance. The 996GT2 ducts, while not as effective, will be easier to fit without much hassle.

997GT3 under tray. Note the NACA duct and the rivets where the 996 ends were spliced

[url=https://flic.kr/p/ezQFJw]

NACA duct exit. Note the trimmed fender liner and the air guides attached to the thrust arm

[url=https://flic.kr/p/ezMrvv]

997GT3RS rear brake ducts. The smaller GT2 heat shield needs to be trimmed further for these ducts. Note the 997Cup caliper studs.

[url=https://flic.kr/p/p4vRKB]

Rear view of the 997GT3RS brake duct. The attachment bracket is visible. Note the 997Cup toe arm.
[url=https://flic.kr/p/oMH2A5]

 

john what lug studs are you running?

 

Here ya go.

Rear PMNA caliper studs/nuts (20ftlb into upright)
999.061.004.02 studs (4x)
999.076.083.02 nuts (4x)

 

Wow, that's a really nice setup! Thanks for sharing and providing part numbers! We're these parts difficult to get or particularly pricey? You've done GT2 air guides and whatnot up front correct? Really looks effective!

 

From rough memory, all the rear parts were probably $600-700 give or take. In the front I have 996cup brake ducts along with GT2 fender liners and 997GT3 upper air guides to channel more air to the front brakes. The RUF bumper channels air over the hood the same way a GT2 bumper does.

 

I wonder if this is what's going on in the Pontiac forums. Grand Am to Trans Am conversion. I hope mine turns out better than this!

 

7GT3 undertrays are fairly expensive but fairly cheap compared to bumpers/decklids/spoliers... I have a new full(front to rear) replacement set waiting to go on 7GT3 if anyone needs pictures

 

I've been running 997 gt3 under trays since buying my car from Sambo. They aren't that expensive. Mine fit without any modifications. I have additions Naca ducts installed in mine to feed fender intakes. Works very well.

The 997!brake ducts are also a nice improvement.

Lastly - I'm running a gt2 Carbon from bumper. Very lightweight and great finish. I think I paid around 2k for it from Getty.

 

With regards to the rear diffuser i agree with SeattleTurbo.
To get any downforce from the diffuser, vacuum needs to be created, and the lower the fins are the better, and when the fins are 5inches or more above the ground the effect will be close to zero.

So if you go for this you will have to be really careful when driving around in the streets as one speed bump will probably scrape it.


PwdrHound: do you have any photos of the linkage between the front air duct and the underside if the car? i guess theres a flexible joint there?

Also is there any recommended swap for the centre undertray?
The tray sitting under the midle drive shaft.

Reason i ask is that im about to make a carbon mould from my standard 996tt piece, but if there are any improved undertrays available, id rather use this.

Edit: its this piece im thinking of #4

 

There is no reason to swap the center #4 under tray. The the 7GT3 tray connects with the OEM center under tray with minimal modification from what I recall. The 997 center tray is not interchangeable with the 996 cars from what I remember.

 

Thanks!