Flatlined

Is this a viable upgrade or a useable alternative on our cars? I searched but didn’t find any info. I can’t remember if the gt3 cup trans is weaker or not.

nbressette

What are you looking to change / upgrade? There are multiple upgrades available. Most are based around the 997 gt2 rs guts and adding a cooler.

A straight GT3 trans wouldn't be a good choice because of gear ratios imo. the 997 gt2 1st and 2nd are taller and 3rd is wider to be stronger.

2fcknfst

Does the GT3 tranny have an output shaft on the front of it?

Flatlined

The gt3 cup trans doesn’t have an output shaft so I would delete the front diff. I believe the cup trans already has an lsd in it. I also thought NA gearing were taller compared to forced induction cars. I do want to know if it would be ideal or not for the gearing but more so the strength of a cup gearbox. I will eventually be running 850 hp but I won’t go any higher than that.

nbressette

the gearing is a non-starter imo. This site lays out each box and what ratio's are in there

http://www.gboxweb.com/gt2specs.html

This path is pretty well paved. In my humble opinion, short of something exotic like a sequential holinger the best route is to run the 997 gt2 1/2 + mainshaft and 3rd gear. 4th is optional. Steel synchros in at least 3rd and 4th (1st and second are $$$$$). use the cup shift lever and if you want to be super super durable use the cup shift forks.

850 is an odd target number. you're going to be past what a single plate clutch can hold and if you're going to go beyond that then the path to from 850 1k is pretty easy. most either stop at ~750 in order to retain the street-ability of a single plate clutch or they go to a multi-plate clutch and shoot the moon.

nbressette

on the topic of LSD, I wouldn't run the factory unit. I'd get yourself a guard club LSD. The clutch plates in the guard unit are a lot more durable than the factory plates.

As a recent convert I can tell you that the stability of the guard on a rwd car is 2x what the open diff awd stock setup is. The way this thing tracks under braking is revolutionary compared to the stock setup.

powdrhound

The 996.2GT3 gearing is relatively tall and very much comparable to the 996TT gearing (1&2nd are same and 3-6 marginally shorter). 997GT3 gearing is much shorter. As with anything, it is not easy to swap those gearboxes into the TT due to the fact that you would need new coolant hoses in the rear or some custom ones made. You can not just swap the GT3/GT2 coolant hoses in as they would not fit the TT coolant pipes which are different. It's a bit of a snowball rolling downhill.

There is also no way to control the cooler on the TT and you would need to devise a way to open and close the cooler bypass valve once the engine and gearbox come up to temperature. Here is a picture of the GT2 gearbox out of my car. You can see the vacuum operated coolant valve there ahead of the cooler. It uses an N75 type solenoid valve to allow vacuum to open and closer the valve. Without a way to close the valve your engine would have a difficult time coming up to temperature as it would be the same as having the engine thermostat open 100% all the time. The Cup gearboxes do not have these valves but they are also run at high power all the time which allows them to come up to temp in a reasonable amount of time. Note there is no front output shaft naturally. The case is not machined for that.

With that said, the GT2/GT3/Cup gearboxes are superior to the TT boxes in that they have an internal spray bar that delivers high pressure oil to all the gears after it has passed though the liquid to liquid cooler. All these gearboxes come with proper LSDs, albeit weak ones. The can be rebuilt with Guard guts or a new Guard LSD swapped in. As Nathan has said above, a proper LSD does wonders for these cars and the Guard diffs are virtually bulletproof and maintenance free. LSD install is not easy and having someone knowledgeable is paramount.

On a TT that sees track use, I would recommend a proper LSD along with an air to liquid cooler using an external oil pump. It will get you 80% of the functionality of the full blown GT/Cup gearbox.

Flatlined

Thank you guys! Well explained as always. I was looking at the 997 cup box. But it sounds like the gearing wouldn’t be idea?

powdrhound

That would not work and its sequential to boot.

32krazy!

send your trans to guard trans in vegas and tell them what you want. best builder in the country

pete95zhn

John, I had no problems installing GT2/3 coolant hoses with turbo's engine when I reconfigured my transmission to GT2/Cup specs. I'm also running the heat exchanger without shutoff valve, Cup style. Maybe a little slower to reach normal operating temp, but OTH transmission oil warms up faster. At track both are just right.
OEM GT3 street LDD is quite weak, but 997 Cup unit seems to be up to it's task. I've run it for 8 years and it hasn't shown any symptoms of getting weaker. 800 Nm.

997GT main shaft (=1st&2nd) 3rd is straight cut one with custom (longer) gearing from Guard. I regret not getting similar for fourth too, ie 4-6 are std turbo ones. Steel synchros for 2-5 gears. No need to change to 1st or 6th at track.

powdrhound

Hey Pete, it's not on the engine side, all there Mezgers are the same there. I'm talking about the TT coolant pipes that run down the tunnel. Due to the driveshaft (and the saddle fuel tank), they are different than what is on the GT2/3/Cup, my car, and also your C2 narrow body chassis. These pipes are actually shared with the 986 Boxsters.

As far as the temp goes, when I hooked up the wiring to the bypass valve incorrectly (where it was open all the time) the car would warm up, albeit slower, while stationary or driving around at low speeds. Once I got on there highway cruising at about 75mph at low engine load, the coolant temperature would drop down significantly into the low 150F range. This was in 70F ambient temperatures so in the winter or near freezing it would be much worse. My car also does not have A/C so the main radiators are not blocked by the A/C radiators which will make them more effective and would thus make the situation worse. This was clearly evident by looking at the digital coolant temperature readout on the Motec dash where I get a precise coolant temp number taken off CAN bus. I'm not sure what you are using, but the dash manual gauge is heavily dampened and always shows pretty much straight up give or take 20-30 degrees so it's not a very good indicator. On the track it's obviously not a player as the engine sees a high load all the time and will warm up regardless. RSRs for example do not come with thermostats at all but can be retrofitted in cooler climates.

Anyway, once I hooked up the valve correctly and it was closing as designed, all coolant temps were rock solid around 180-185F in normal highway cruising. The logic on the GT2/3 is as follows:

Cooling circuit OPEN when coolant temp > 100 C or transmission temp > 105 C
Cooling circuit CLOSED when coolant temp < 90 C and transmission temp < 95 C

Cheers....