What looked like a simple solution to ban off-throttle blown diffusers turned out to be a nightmare for the FIA at Silverstone - but what went wrong and will the end solution prove satisfactory?
The legality of off-throttle blowing is down to an interpretation of the rules, as is so often the case in F1's controversial technical situations. There is nothing to suggest it is not acceptable as an integral part of engine operation, but there are interpretations that suggest it is not acceptable when it is done to gain aerodynamic benefit from the increased airflow through the exhaust outlets, as has become the case over the last year or two.
The trend towards off-throttle blown diffusers led to two concerns - one that the conceptual design development increases cost and benefits teams with greater budgets and the other that progressive development could lead to radical designs and potential safety issues.
The cost issue has certainly created a challenge for the mid-grid and backmarker teams, as while all have engines that can offer off-throttle blowing, the time and resource required in the design office to benefit from this within the overall car concept is still costly.
Equally, it is obvious that any team that is not making as much of the concept as their rivals would look to push the interpretation of those rules more to the illegal side.
For all these reasons, the FIA felt they had to act immediately, and so came the new ruling that, before Silverstone, everyone had appeared to accept.
That ruling allowed that when the driver was not accelerating, the throttle could still open but to a much lower level, with a sliding scale based on engine revs, from 10 per cent up to a maximum of 20 per cent at maximum revs of 18,000rpm. This way, the benefit of the off-throttle blown diffuser is virtually negated.
However, the FIA was forced to allow for several special dispensations because some teams argued that some of the approaches being outlawed had been an integral part of the engine concept to improve reliability way before the off-throttle blown diffuser idea was conceived.
And this is where it all fell apart.
Mercedes argued that they needed to "fuel on the overrun" to relieve crank pressure within their engine. To explain their situation, when the engine is not under throttle its pistons are still running and so a vacuum builds inside. Allowing fuelling to create combustion in the engine off throttle can alleviate this, so to avoid Mercedes suffering engine failures during the race the FIA allowed them to add fuel to four of the eight cylinders.
Renault brought up a precedent of their own, namely that they have traditionally had their throttle open up to 50 per cent when off the throttle. Christian Horner stated this was for "purposes of throttle blip and reliability" (which includes cooling the exhaust valves) and that it was "a necessary part of the operation of their engine, otherwise it would cause serious issues". So before Silverstone, the FIA also decided to allow that to continue.
Both precedents, however, also deliver the benefit of increased airflow through the exhausts, which can then be maximised by the development of the blown diffuser. In Mercedes' case, the ignition of the fuel produces faster airflow through the exhausts while for Renault it is simply the partially open throttle that increases airflow.
The problem is that different engines work differently, and while the FIA did what they could to balance the benefit from each dispensation fairly, that was hard to determine.
While Renault's open throttle sounds like it would be more beneficial, Horner argued that Mercedes' solution was giving them more performance gain - so when Renault's dispensation was withdrawn again on Friday the arguments began.
The effect could be seen clearly in the times, as on the first timed session, where Renault engines did run with 50 per cent throttle, Mark Webber's Red Bull was some 0.66s ahead of second-placed Michael Schumacher's Mercedes with Felipe Massa's Ferrari almost a second off.
From the third timed session onwards, with the Renault dispensation removed, Sebastian Vettel was just 0.063s ahead of Fernando Alonso's Ferrari in final practice and when Webber claimed pole just ahead of Vettel, Alonso's Ferrari was only 0.117s behind.
By Sunday lunchtime, the teams and the FIA came to the sensible conclusion to cancel the ban and return to Valencia conditions, where teams cannot use radical engine maps for qualifying but can optimise the blown diffuser concept - but only for the remaining races of the year, the argument for its reinstatement being that with half the season already gone there is only limited development potential remaining for the lower teams in any case.
Next season, then, the permitted exhaust exit location will be defined clearly in the rules in a place that will not benefit the diffuser - but with the technical ingenuity in Formula One, it will surely only be a matter of time before the next innovative approach causes more controversy.