It’s simple: If you want some rear bias power delivery with a Haldex AWD, make sure that the rolling circumference of the rear wheels isn't smaller than on the front wheels!
The problem is, in most cases I've seen, the recommended tire pressure has led to exactly that.
You can have the most aggressive Haldex controller on the planet; smaller rears leads to front wheel bias.
In order to get the most out of the Haldex, it is important to keep the tire rolling circumference between front and rear as equal as possible (within 0,5%).
You can set the correct circumference via tire pressure (up to a certain point).
If you want some rear bias, make sure you have 0.2-0.5% more circumference at the rears.
Often this is neglected and simply put the manufacturer's recommended pressure in the tires.
(That’s basically a safe way)
But often tire tolerances, wear, axle loads, etc., cause big deviations.
That may sound exaggerated now:
You should always measure the circumference!
The easiest way is to mark both tires with a soluble paint or something and move the car (straight) one tire rev.
Then measure the distance between the marks.
In any case the difference between front and rear should be in the range of +/- 0.5%.
Anything beyond this range just increases the stress in the powertrain and may affect the ESC/ABS!
At least that’s my experience, we already done this on some cars and it really improved the rear engagement...
I'm not an expert but I try a (very simplified) explanation:
Due to the smaller circumference of the rear wheels, the rear differential runs a bit faster than the cardan shaft. When the Haldex clutch engages now, the closing force of the clutch slows down the rear axle. The rear wheels are pulled by the front wheels. If the engine torque exceeds the closing force of the clutch, the distribution changes to a even more front wheel bias.
However, if the circumference of the rears is slightly larger, the rear axle will now be accelerated by the closing force of the clutch. The rear wheels are pushing up to the point where the engines torque again exceeds the closing force of the clutch.
When that will be, depends very much on the car and the Haldex controller.
This results in a rear bias power delivery when accelerating.
Just give it a try...