|ProjekTT Roadster: Transmission Upgrades
When we first introduced this TT as a project car, we mentioned that we had plans to modify and upgrade all aspects of the car. From performance to looks - we wanted to play with it all. In the beginning it seemed like a simple software upgrade might suffice, but somehow a chip has evolved instead into a full blown turbo upgrade. And truth be told, there were probably bigger plans than just a chip when the huge front mounted intercooler was installed.
Having made the decision to go with APR's Stage 3 turbo upgrade for the transverse motor, it meant that it was time to start thinking about getting the chassis ready for such a big increase in power. Thus we have arrived at the next installment of the ProjekTT series, which covers the installation of much-needed transmission parts. Since our Audiworld project car is a front wheel drive we really needed to beef up the transmission in order to get the power down through the wheels and onto the pavement. Knowing that a a GT-25 turbo capable of generating 300hp would be installed, we were certain there would be more than enough power to overwhelm the factory setup. It seemed to make sense to get the tranny overhaul out of the way first since there was already going be downtime with the Stage 3 install. It only made sense to do it all at the same time while we had the transmission torn apart.
The two main areas of concern were the clutch assembly and the differential. The original clutch had only 19K worth of spirited miles on it, but with the kind of horsepower we are about to install we had a feeling that it was not going to be up to the task over the long term. Researching various options, we also talked to the guys at NGP who were going to be doing the entire install. After sorting through our specific requirements, NGP came forth with their recommendations.
The main component of the setup was the clutch itself, which was sourced through Clutchnet. The clutch that was chosen for the application was a 6 puck, sintered pad with a sprung disc. Clutchnet designates this clutch as a Street Competition package. It spec'd out with the promise to hold the power we were looking to install and got the thumbs up. Having settled on a clutch, we needed to get the rest of the items to complete the package.
The pressure plate choice was simple as we had found out that the OEM VR6 pressure plate was in fact a heavy duty unit compared to the transverse 1.8T. As a bonus it just happened to be moderately priced and so we decided it was a good choice and not to try to find something better. We did want to keep the stock pedal feel for the purpose of daily driving (since this car is really not setup to be a drag racer, but rather more of a highway cruiser). If we were planning to spend more time running at the track, we would have probably looked into a more aggressive setup.
The final part of the clutch assembly was maybe the toughest choice of them all: the flywheel. There is so much information, both good and bad, on lightened flywheels that we were quickly overwhelmed in our research. Sorting through reviews, websites, product information and visiting with other enthusiasts, we affirmed a few things about installing a lightened flywheel. Basically we expected to experience a very fast revving motor that was quick to respond when the hammer was let down due to the lower rotational mass. We decided to go with Autotech's lightened flywheels for a couple of reasons. First, we had been warned about buying flywheels that are simply stock cast units that are then machined to lighten them. Because of the varying qualities of the castings, sometimes the machining process can really degrade the strength of the unit causing stress fractures and eventual failure. The Autotech flywheel is a cast steel unit that is then CNC machined for an exact balance and lightening. We wanted to have the most durable setup possible so this seemed like the best match.
After coming up with a good compromise between being able to handle the soon-to-be-present power and also having a daily-drivable vehicle, we thought that there was one more worthwhile trick for the FWD tranny. Since our estimated HP would be in the 300 range, we were looking to take advantage of any product that would help us put the power to the ground. Over the years of owning a FWD car, we had always read about the benefits of installing an upgraded differential and figured it would definitely worth looking into since given our requirements. We made another phone call to Autotech, the US importer of Quaife differentials, to find out more about what we had been missing.
The Quaife ATB (Automatic Torque Sensing) differential is designed to prevent the complete loss of drive that occurs with a conventional *open* differential when there is wheel slip. The Quaife unit is progressive in action and never completely locks one wheel or the other, but rather power is transmitted smoothly to both drive wheels. Quaife ATB differentials are ideally suited for high powered front wheel drive applications where optimum power is required, but there are many other applications from 4 wheel drive applications to Formula One racing.
The differential operation is automatic and the torque capacity of the unit is increased or decreased by varying the helix and pressure angles of the gear teeth. Sets of floating helical gear pinions mesh to provide the normal speed differential action. To preload to the gear packs, there is a selection of center spring discs available. In the event of a slipping wheel, torque bias is generated by the axial and radial thrusts (red arrows) of the pinions in the pockets; the resultant friction's force enables the driving road wheel to receive a greater proportion of the torque. Like mentioned before, all of this action is progressive and at no point does the differential completely lock up for one wheel or another. This makes the power and traction much more predictable for a street driven car.
Another reason that we had considered upgrading the differential is because of the horror stories we have heard over the years about exploding differentials. The VAG group has been using an open differential in their FWD transmissions that could be prone to failure in high horsepower applications. The differential often fails because of the cost cutting design whereby it uses rivets to hold it together whereas the Quaife uses heavy duty bolts to secure the unit. The rivets are usually good enough for OEM levels of power, but not nearly enough to handle the added twist of nearly 290 ft lbs of torque that our pending turbo upgrade promised. The rivets that are used in the OEM unit tend to shear off, causing the differential to simply fall apart and wreaking havoc on the rest of the transmission as a result. Hard launches or even strong shifts have been known to destroy these transmissions. It looked like we had found a good bulletproof design that would be up to the task of putting as much power to the ground as possible.
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