Chassis and Suspension setup
This is where the real magic is for this kind of racing. It requires a stiff platform, top notch tunable damping, and lots of wheel and tire on the ground. We need to be able to pull 1.5Gs all the time or we will be too slow. Piglet the Cayman had 18x10.5F and 18x11R, running 295/315 Hoosiers, on Moton/Tarett suspension. This car will need more…


First step is to document what we started with. Cobb took baseline alignment and ride height measurements while the car was the setup rack. This will be the target (maybe a bit more camber) for the initial setup.




We purchased a used set of GT3 Moton club sports, which have been rebuilt locally by Maxcy Speed, who are former Moton USA techs. New C4/Turbo sleeves for the uprights and 2.25" spring perches have been ordered to convert them for the 997TT.


We will start with the standard Moton valving, which can handle a wide range of spring rates that we will be testing with. Currently the car has 400/567 rates installed on the Damptronics. We will be bumping that up, to around 700/900 to start with.


A lucky ebay search found a deal on a new Tarett blade adjustable front bar. This also comes with the extended length strut mounts to reduce the end link deflection angles. A GT2 rear swaybar will be used to balance this .



Front Suspension
Warthog will have the same front setup as the Cayman, to run the 295’s without fender modifications (because my wife will not allow them to be cut!). We repositioned the strut shafts down to move under the clearance hole, and further inboard to get to 3.5 degrees or more - adjustment had to be done with a pick tool.


The TPC(?) camber plates on the car now are maxed out at around 3 degrees (actually a good thing given the warning below) That worked for our first test , but we will need more adjustment range.


On the 997TT, we found you cannot move the strut too far inboard without risking ‘bottoming’ out the front CV joints. So an offset lower arm bushing will be used to balance the driveshaft travel and camber setting. Why not use GT3 style arms? The strict SCCA rules allow only one method of camber adjustment – adjustable lower arms OR camber plates. But bushing changes are legal – go figure.

Vorshlag Motorpsorts, who are also nearby in Plano, TX, did some forensic work with our front control arm bushing to determine the rubber to metal volume. SCCA rules require the replacement bushing to have the same or less metal % than the original.



Then they ordered in this trick powerflex adjustable offset bushing, measured it, and confirmed it satisfied our metal % requirement.

Vorshlag will be building some new 997 camber plates to give us more range for camber adjustment, and true caster adjustment. They should be similar to this design, and with their in house CNC machining capability, the new plate design will be ready to install in just a few days after measurements are taken.

The other key component for big front tire fitment is a very short, small diameter, spring package – 4” x2.25” main springs and low rate tenders. This moves the perch and spring further away from the wheel and tire, so that it can be pulled further inboard. This is the spring stack on the Cayman below, you can see at the red arrow how high up the perch is on the strut body. We will setup the GT3 front struts the same, with the same 18x10.5 wheel, with 295/30/18 Hoosier A7.


Rear Suspension
For the rear, we are super sizing to 345 Hoosiers in the rear. This is quite a bit bigger than the 315s we used to run.




We think the 345 tire is slightly pinched on the 18x12 rim we tested with – so we will be changing lips and barrels to get them to 18x13. 3pc CCW’s win here for making the size changes easy.


The 345s fit at around 47mm offset, with just slight rubbing on the rear brake ducts. Some trimming there, and maybe switch to the cup car rear ducts should improve clearance.
Also in queue will be solid subframe bushings like the GT cars, and a delrin or urethane inner control arm bushing to match the front ones. No changes to the other rear links yet. SCCA rules do not allow heim jointed links, so we have to be careful not to make any bushing changes that would bind up the kinematics.

 

Great thread

Would be curious how this beast would compare to say Autox stock 997 gt2rs

 

Nice progress guys. Call me Monday or Tuesday and I'll go over some front track and camber tricks with you.

 

Great write up. Subscribed. Thanks for going thru all the details.

 

On the budget side, it will come in at about 30% of the cost
The main downside are the weight aspects. With a sunroof body, awd hardware, we will be 200lbs +heavier. At autox, we may have some advantage in power down out of the corners, as the GT2 will be traction limited with 2wd.

 

Great Thread!


Looking forward to see if your GT2RS intercoolers hold up at the track! You may want to check them after a track day or two.

 

Yeah, we will be pressure testing them between events to see if they hold up. The light weight was the big attraction for them, and the track tune may get restricted some to keep boost and IATs down.

 

The class rules I run only allow swapping between 996 through 997.2 cars, not GT2s. 997.2 vtg's may be an interesting upgrade over the .1's, if they can be controlled. Honestly I think the power levels should be ample, just need to tune with some margin to limit heat buildup on longer runs.

 

Hi 997.2 and GT2 turbos are very similar i.e. bigger - only difference I believe is that the GT2 turbos have a recirculating valve. You'll benefit from a larger hotside so less heat generated.

No probs controlling them as I have GT2 turbos on a 997.1 and have covered 10,000 miles with no issues. Difference is meaningful in my experience when both on track and dragging down a straight.

 

Controlling VTGs, whether they're OEM or upgraded has never been an issue and seems to be something that some tuners seem to have propagated on the forums or out there in general in the past. Its no harder to control VTGs than it is a conventional wastegate. You have a boost control PID and duty cycles you're working with.

Dzenno@PTF

 

GT2 turbine housing is a little bigger than the OEM 997.2. In fact we have a local 997.2 Turbo here that went with a 68mm upgrade and a GT2 hot side to get a touch more flow out of it. We never got a chance to do any back/back testing on the dyno with this change unfortunately.

In terms of recirc/diverter valves the 997.1/GT2 simply have them built into the compressor housing while the 997.2 they're sitting up in the y-pipe area. I wouldn't really use the 997.2 turbos on a 997.1. Its far easier to get the OEM 997.1 turbos upgraded with larger compressor wheels and clipping the turbine wheel slightly to get a bit more flow as well as slow the turbine shaft speed down at higher boost levels.

Dzenno@PTF

 

997.2 turbos have both a slightly larger compressor wheel and a clipped turbine. They're not as big as the GT2 but the difference is not night/day.

 

That's great info Dzenno, at some point we may move up to a higher competition class which allow upgrading the turbos, then we can have our cake and eat it too. Probably 1 or 2 seasons away I think.

Cobb Plano has the engine out of the car now so we are addressing the coolant pipes, clutch and diff upgrades now. Can't wait to get it back on it's wheels!

 

Cool, you're in great hands there!

 

Picked this up from a post Mke at AWE put up a while back and a few others posts cobbled together.

The standard compressor wheel is different on the 997.1 turbo, the GT2 wheel is larger. It is the same on the 997.2 but the compressor housing is different.

Info on wheel sizes and turbine material from the BW catalogue:

GT2 (RS) Borg Warner turbocharger specification:
BV50-2280DCB426.10BVAX0

997.1TT Borg Warner turbocharger specification:
BV50-2277DCB405.10BVAX0

997.2TT Borg Warner turbocharger specification:
BV50-2280DCB426.10BVAX0

The four numbers in RED indicate the compressor wheel size. All have the same size exducer, 56mm. The 997.1 inducer is 43mm and the GT2 and 997.2 inducer is 45mm.

The last letter that is bold and underlined indicates the turbine material. In this case O stands for Austenitic stainless steel, which is specially formulated to withstand high EGTs.

The 2270 designation appears to mean 2.2" (56 mm exducer) and 70% of that for inducer (= 43 mm). Similarly for 2280 (= 45mm).

There's seems to be differences between std .7 turbo and 7. GT2 (RS) and 997.2 turbo's hotsides:

GT2 (RS) and 997.2 : BV50-2280DCB426.10BVAXO
BV50 = turbo type
2280DCB = cold side where

22 = Dex(ducer) 22/10" = 56 mm
80 = d/Dex in % (TRIM) -> 56x0.8 = 45 mm inducer
D,C,B = wheel geometry etc definitions

426.10BVAXO = hotside where

426/405 = turbine housing area in cm2 (4.26 or 4.05) towards the turbine wheel.
10 = housing definition
B = wheel geometry
V = housing geometry
A= design stage
X = housing inlet flange
O = Housing material

So the hotside's scroll's opening to the turbine's inducer is 5% larger and the wheel has 15 dec clip.

Think this is correct but not 100%