Before the race there was much talk about the value of the F-duct. Williams technical chief Sam Michael said it was a "no brainer" that it should be used, but others thought the benefits might not be big enough to justify running it, given the low downforce nature of the circuit.
The F-duct is most beneficial when the car is in a high downforce configuration, with a large and highly raked rear wing, so as the need for downforce - and therefore the size of the rear wing - is reduced the blown rear wing becomes less efficient.
Due to its positioning, running along the top of the engine cover, the F-duct also adds extra weight higher up, and that is a bad thing for handling as the lower the car's centre of gravity is, the better its manoeuvrability. In addition, the bulky system also affects the airflow and reduces the performance of the rear wing in normal flow.
McLaren's engineering teams were split, with Jenson Button using the F-duct and Lewis Hamilton choosing to scrap it in favour of a much more slender profile for his rear wing, which gave him less downforce in the chicanes but more speed on the straight.
The effect of this was extremely noticeable. Button's much larger wing set-up lost him 14km/h to Hamilton on the main straight as the blown system was indeed not as effective as simply having a low downforce set-up. But when it came to handling the chicanes in the rest of the lap, the extra grip achieved through the additional downforce gave Button a significant advantage, gaining him two tenths of a second in each of the two remaining sectors.
Button's front row grid spot (and the fact that everyone else who had an F-duct ran it) proved the system was overall beneficial, a fact Hamilton reluctantly admitted after qualifying.
But McLaren's approach was perhaps just a little too simplistic. Their approach was to run a standard F-duct ant test its performance during the Friday sessions then decide whether or not to run it. Ferrari, in contrast, developed a unique F-duct specifically for the race as part of their Monza aerodynamic package and merely tweaked that blown rear wing package to perfect it during the Friday sessions.
The Italian team fitted a revised F-duct with a significantly smaller pipe running along the inside of the engine cover to the rear wing than used in their standard set-up. That made it less bulky and less heavy and, like most others who ran their F-ducts, it was combined with a much shallower rear wing than the one used on Button's McLaren.
And not only did that considered approach to their Monza package put Ferrari in a position to secure pole position in qualifying, it also gave them the advantage over McLaren in the race - because despite losing out at the start, once Fernando Alonso got ahead he had the straight line speed advantage to stay there.
- Jenson Button