Er, well I was trying to find out for myself, by asking someone who knew. I usually find that's the best way to understand these sorts of things.
So again, any idea what the benefit is of varying chain tension by engine oil pressure?
The early Busso V6 did have an oil pressure actuated tension reduction device but Alfa removed that in favour of a simple sprung tensioner for the 24v version of the engine so assume they also questioned the benefits?
In the GM application, does the hydraulic pressure act to decrease the chain tension like early Busso or increase it?
The Busso is different.The mechanical belt tensioner has passed into legend as the static pressure is greater than the hydraulic, as I understand it. When the engine oil pressure is up to working value, it effectively "Backs - Off" the mechanical tension. So when oil pressure drops, as revs com off the engine, the mechanical (spring) tensioner re-applies it it to maintain the tension. The belt slack is on the back face in such circumstances, so the mechanical (spring) tensioner, re-applies the static tension to the belt. Perhaps this why Alfa initially said 72,000 mile belt change; I think. But that did not take into account belt wear or deterioration of pulleys. "Busso gave them an inch and Alfa took a mile". There are so many brilliant aspects to Busso's design and historically most failures are not attributable to the "Great" Snr. Busso. Every modern V6 engine owes so much to this man's engineering talent. It has also to be said, he had considerable concern when the 24 valve version, quad cam was introduced as he did not think there was sufficient contact area between the belt and pulleys. And if one looks at his original design with a single cam per bank, it would have been very easy to adapt to the new heads of the, "Twin Air", where a single exhaust cam is used and the inlets are operated by rod with variable hydraulic valve advance/retard. The bore and stroke would have suited this turbo method and V.V,T would have taken care of emissions for the regulators. I think it is Twin Air, but I have lost touch with modern Fiat inspired Alfa's.
The Gm engine springs back up the chain tension when the oil pressure drops and when the engine is switched off. Operational pressures for the oil pump is given as 4 - 6 Bar. but at low revs/tick-over, the mechanical pump cannot cope with the low speed bleed through the crank and journal shells, so without the springs, there would be slack. It should work well when pressures are adequate. When they are not, they are having to do work that they were not intended to do. And the more work they do, the weaker they become. If you have a Brera, your intentions are clear. With inadequate oil pressure, the springs will suffer fatigue and the consequences of that are well known. Where as boring old f---s like me, have never hit the rev limit. But poor gallery balance also exacerbates the G.M. engines ability to maintain tension.