Craig Scarborough points out that McLaren have been experimenting in Barcelona with an exhaust system that has no discernible exit. Now, this is slightly mysterious because the exhausts cannot exit through orifices in the underbody of the car, as stipulated by the 2011 technical regulations, designed to ban double-diffusers and exhaust-blown diffusers:
3.12.9 In an area lying 450mm or less from the car centre line, and from 450mm forward of the rear face of the cockpit entry template to 350mm rearward of the rear wheel centre line, any intersection of any bodywork visible from beneath the car with a lateral or longitudinal vertical plane should form one continuous line which is visible from beneath the car.
3.12.10 In an area lying 650mm or less from the car centre line, and from 450mm forward of the rear face of the cockpit entry template to 350mm forward of the rear wheel centre line, any intersection of any bodywork visible from beneath the car with a lateral or longitudinal vertical plane should form one continuous line which is visible from beneath the car.
The notion of a continuous line is interesting here, because solid substances are really discrete, rather than continuous, collections of atoms and molecules. The intersection of an imaginary vertical plane through a section of bodywork can only form a continuous line at a macroscopic level of idealisation. Nevertheless, it's difficult to see how modern materials science could exploit this ambiguity. In principle, one might be able to construct part of the underbody from a permeable, but stiff and macroscopically continuous material, such as a rigid polyurethane foam, through which the exhaust gases could then be diffused. However, the rate of diffusion would surely be far too low to influence the airflow aerodynamically, even though diffusion occurs faster at higher temperatures. One wishes to blow the exhaust gases under the car, not diffuse them!
McLaren could conceivably exit the exhausts through holes above the front splitter, or below the sidepod inlets as Renault have chosen to do. In neither case, however, does there appear to be any photographic evidence yet to support this. The latest suggestion, rather, is that McLaren are trying an inward blowing exhaust slit on the top surface of the floor, just in front of the rear wheels. Intriguing.