The availability of good Boxsters that have an engine issue requiring an expensive repair was a big driver in the decision to do this as a kit, there are just too many great cars being broken for parts due to a £10K M96 engine repair bill.... if for the same £10K you can have a car which is capable of hanging with GT cars in performance terms it gives a much more convincing case for someone to spend the money vs breaking the car for parts or selling it to someone who will.
With regards power and torque, it's important to first understand that power is just torque X RPM / 5252 - essentially its a made up (but very useful) thing that we use to measure the amount of work an engine can do. The gearbox doesn't care about power, what concerns it is RPM and Torque.
RPM is important because most production gearboxes are splash lubricated so the bearing systems inside them will have an upper lubricity limit based on the available oil supply. Torque is important because it is the actual twisting force provided at the gearbox input shaft which the box then multiplies accordingly through the ratios within it and the final drive.
The 5 Speed on the Boxster is actually an Audi 012 gearbox, which is made by Aisin. Only the 6 speed is Getrag. The 012 gearbox is a very strong gearbox for its size and has been used in various aftermarket GT40 builds and other mid engined performance cars as well as having sat behind a huge amount of VAG group applications. When doing this conversion you're actually reuniting the gearbox with the engine it was designed to be behind.
The 012 Gearbox has an upper torque limit of somewhere around 300ftlbs, it can do a little bit more than this in 3rd/4th/5th due to the lower torque multiplication. As such we cap the torque based on boost pressure and gear position to avoid excessively loading the gearbox. The ECU we recommend has excellent boost control so is more than capable of drawing a flat torque curve at exactly the desired figures. In practice this means at 400bhp spec we run 1bar boost in 1st, 1.35bar in 2nd gear and 1.75bar boost in 3rd and up. This both keeps the gearbox happy and optimises traction with peak torque at high boost circa 330ftlbs at 5500RPM. Here's our boost by car table for example:
When we want more power than this we simply raise the RPM limit and change the cam profile to suit, the turbo charger on the validation car is massive so we have no issues filling the engine with air up to north of 500bhp an the engine is built to rev to 8250RPM with titanium retainers, double valve springs, supertech valves etc. What this means from the gearbox perspective is the input shaft turns faster but the load on the gearbox remains the same as it previously did, from our BHP perspective we've now done more "work" in the same period of time (as there's more cylinder firings in the same time period) so our power calculation gets bigger accordingly. Here's the math:
330(ftlbs) X 6500(RPM) / 5252 = 408bhp
330(ftlbs) X 8000(RPM) / 5252 = 502bhp
In both these scenarios the torque on the gearbox remains identical, we're just doing more work in a given period of time.
The other two things to consider in this application are clutch and driveshafts, both of which obviously see the engine torque in a related way. For clutch we designed a new flywheel for the 20V engine which carries the standard production clutch from the Audi RS4, this clutch is a larger diameter than the standard clutch and is rated at 350 ftlbs in the OEM application. Using this gives us an OEM/Production car pedal feel, great clutch life and all the torque holding we need.
The driveshafts are subject to very large torques due to the torque multiplication produced by the gearbox, the biggest multiplications are in first and second so limiting boost and therefore torque in 1st and 2nd gear keeps the driveshafts in a sensible loading window.
The really nice thing is how the car drives with this sort of setup. It's super linear with power rising across the rev range, it's not spikey or hard to manage and with a decent set of tyres on the car (Michelin Cup 2 in the VP cars case) it has fantastic traction and puts the power down without any issue at all. Here's a power plot at 400bhp:
You can see how linear the car is, arriving nicely into boost in the mid 3000's with a linear rise in power across the rev range. This is one of the reasons it's so quick, there's no having to get out the throttle - in the dry it grips and goes
Hope that wall of text is of interest and explains a little more how we've tried to engineer reliability as well as performance into this.