No worries Simon: it's a pleasure taliking about cars and possible mods...
I really like the way your A3 is set up the S-Line kit looks great in white, and the RS6s have grown on me.
Your RS6s, which are specific to the A3, are 7.5x18 inches, with a 54mm offset (ET54). That puts the centerline of the wheel 54mm towards the inside of the car or 54mm away from the mounting surface of the wheel.
The lower the number, the more the wheel "sticks out" of the car. Normally, we should be expressing our offset as negative offset and place a "-" before the number, because inherently offset is positive, whereas we're talking about negative offset. Usually, the higher the number, the further OUT the wheels goes... There you go: more info than really necessary, ha!
If you want to compare wheels and how much they'll "stick out" and move the centerline away from ideal (aka OEM, aka as factory-designed), then here's one way to go about it: my original wheels were 7.5x17 ET56, and now I have 8x18 ET45.
56-45=11, so the outer lip of my wheels stick out 11mm further out (and we have moved the contact patch of the tire that much further out, which modified the scrub radius, and which in turn influences on bump steer, effective spring and dampening rates etc...).
But wait, we also have to factor in the wheel's width. Going from 7.5" to 8", we have added 1/4 inch to each outer/inner edges. So our outer edge really sticks out 11mm + 1/4", so a total of 17mm further. (phew!)
The GenII HPP (2nd generation Haldex performance part) is this blue thing:
Haldex is the name of the Swedish company which produces the electro-hydraulically-controlled clutch pack which turns our fwd cars into awd cars.
Picture the front diff, which distributes power between the front wheels. Now picture a 90° power take-off followed by a longitudinal driveshaft, which runs under the car towards the rear diff. This is what turns the rear diff and thus the rear wheels.
But, before reaching the rear diff, the shaft turns a clutch pack. When this 'pack is fully open, the rear diff sees no movement, no torque. When fully closed, the rear diff sees just as much torque as the front diff, and the car is now a 50:50 all-wheel-drive automobile (yay!!!) lol
Now, what opens and closes the Haldex clutch pack is 1) a difference in rotating speeds between the front and rear diffs and 2) an electronic signal from the Haldex Control Module.
In 1), picture the car on a grippy, straight bit of road. The rear wheels, simply dragging along, rotate at the same speed as the front wheels and thus both diffs turn at equal speeds. There is no speed difference to build pressure within the clutch pack, and the car is essentially fwd.
Now picture it accelerating hard, and the front wheels starting to rotate faster than the rears (slipping!). Well, that speed difference builds pressure within the clutch pack, which closes it, and which sends torque to the rear wheels. Now we're awd, and we're not slipping anymore.
I hope the following gif shows on your screen. As you can see, this MkIV R32's front wheels start to slip, then the rears catch up (GenI Haldex). Our GenII works the same way.
The pressure is built mechanically within the 'pack by pumps rotated by the the difference in speed, but it can also be augmented electronically by independant, electrical pumps. That is where the control module comes into play: plugged into the car's CAN system, throttle, steering, and engine torque inputs are taken into account to apply +/- pressure to the 'pack. In effect if you mash the throttle, the control unit won't wait for slip: it will lock up the 'pack before slip can even occur. (Not to be confused with the "Pre-X" system used by Volvo, which can actually lock the 'pack at a full stop: ours still needs some rotation).
And that's where this "GenII HPP" thing comes into play. It is a different control module, sold by Haldex, which has a different program built into it, and more agressively valved pumps. The program tells the 'pack to close sooner and with greater force, and the pumps are calibrated to do just so. The end result is that the car is now MORE awd, MORE OFTEN. And that, my friend, means more neutral acceleration in turns, less understeer, and generally more fun. Still with me?
Here's the unit:
Here are the instructions. The neat stuff is on page 5, and I'll get right to it:
The original Haldex unit, as seen under the car: The black driveshaft comes from the front diff and feeds torque to the Haldex unit, which sits right in front of the rear diff:
Remove and replace: simple plug & play:
The new unit in place:
OK, see those lovely graphs? Notice how there are three "modes" for this HPP? Well, it needs a switch installed to access all three modes, which I haven't gotten around to installing yet. By default it is in the "sport" mode. But, driving with this intermediate mode is already a lot more rewarding, so I can only imagine that the "race" mode will be even better.
If we are very lucky, then this race mode keeps the 'pack locked when braking (the control module opens the 'pack during braking, even with the HPP in "sport" mode). Keeping the 'pack locked under braking would make for very fun tail-happy antics under braking in slippery conditions...
As soon as I have my switch installed, I'll let you know if that is the case or not.
Cheers!