Already mentioned by Christian Horner in the run up the first race, McLarens slotted rear wing was well known through out the pit lane from its testing appearances. It was even planned for the FIA's technical delegate inspect the wing at the teams factory before the race weekend. When this could not happen due to flight problems, it was left until Bahrain that the legality of the McLaren system would be initially decided.
Thus the sport held a collective breath while the McLaren was scrutineered on the Thursday before the race. Once the wing was given the all clear, more details emerged from team principal Martin Whitmarsh about the system. This gave clues to the how the system might work. It was already debated within the paddock that the slots in the McLaren rear wing flap were not to act as a blown slot to reduce separation, but rather blown to create a ‘stall' condition around the wing. What was not clear was how McLaren were able to tune the wing to stall only above certain speeds. This is where the snorkel on the chassis top comes in. Although rumoured, it was only when the snorkel was fitted with sensors at the same time the car ran with the rear wing aero testing rig, that a clear link was established. It is now understood that the snorkel leads to a duct inside the cockpit, this duct leads back through the monocoque and passes out the back, through the shark fin and into the rear wing flap. However the duct has an opening that allows air to blow inside the cockpit. This has the stated purpose of ‘cooling' the driver, but if the driver covers up this hole the flow instead passes through the duct to the rear wing flap. Exiting through the slots near the base of the flap, causing the wing to stall reducing both drag and downforce. The loss in drag is enough to gain several MPH on the straight.
The exact routing of the duct and the method of how the driver blocks the duct is not known, perhaps the term "f-vent" is a sign that the driver Foot closes the ducts opening. As the duct is only used on longer straights, the drivers left leg is free to operate the duct, until he removes it to prepare for braking into the next corner. McLarens set up is legal as it contains no movable aerodynamic devices and the rules do not specifically ban stalling aerodynamics. It is now left to the rival teams to either protest or copy the F-vent. Creating the snorkel and the hollow slotted flap is not a major problem, but finding a hole in the monocoque to route the duct work through will be an issue, as the teams have already homologated their monocoque. Meaning no changes can be made to their structure except for safety or reliability reasons. Even if allowed, many teams might feel that the cost of designing, producing and crash testing a new pair of monocoques is not worth the investment. Leaving McLaren with a season long straight-line speed advantage.
Aside from the much talked about F-vent, the McLaren was largely in the format that ended testing in Barcelona. One feature of which is the new central section. Rather than dip the middle section of diffuser, McLaren kept the bodywork straighter and steeper. As this section is so steep and not fed as well from beneath the floor it could be prone to separation. In order to keep the flow attached McLaren expanded the hole used for the starter mechanism to create a slot to feed higher pressure air from the diffuser deck above. This slot effectively makes this section of diffuser into two pieces. However the rules demand continuous bodywork in this region and the expansion of the starter hole was deemed excessive by the scrutineers and needs to be changed before the next race. One interesting point to note is that the starter shaft hole has been offset on one side on the McLaren gearbox for many years. This is a result of their seamless shift mechanism, however this year the starter is again placed conventionally on the cars centre line.
Source:racecar-engineering.com
