Conventional wisdom holds that Formula One's first exhaust-blown diffuser appeared on the Renault RE40 in 1983. “The exhaust-blown diffuser first appeared in F1 28 years ago…first conceived by Renault’s Jean-Claude Migeot…Migeot was a 29-year old Renault aerodynamicist when he conceived the exhaust-blown diffuser in 1982. The system made its debut on the Renault RE40 at the Monaco Grand Prix a year later” (Technical Focus: Blown Diffusers, Edd Straw, Autosport March 10th 2011, pp32-35).
Certainly, Renault were the first to blow their exhausts into the diffuser, and in fact, Migeot actually piped both exhaust and turbo wastegate flow directly into the diffuser. However, exhaust-blown diffusers include not only those which blow into the diffuser, but also those which blow over the top of the diffuser, and it could be argued that the first such device actually appeared on the 1982 McLaren, the MP4-B, as seen in the contemporaneous drawing below by Giorgio Piola.
Unlike most of the ground effect cars of the generation, the diffuser tunnels on the MP4-B extend all the way back to the trailing edge of the rear suspension. The engine cowling and rear deck is fitted from above and also extends to this point, but a gap remains between the two. The exhausts exit into this slot, above the diffusers, and beneath the rear deck. In fact, the exhausts are almost invisible on the fully clothed car, and do not protrude into the ambient airstream.
On most cars of this era, the exhausts were clearly treated, at best, as aerodynamically neutral elements, or at worst as disruptive devices. This is understandable because, in general aerodynamic terms, exhaust jets are particular examples of round jets, a type of paradigmatic turbulent flow. Exhaust jets are sources of heat and turbulence, neither of which appear, at first sight, to offer any beneficial contribution to downforce. The nature of a round jet is succinctly explained as follows by P.A.Davidson:
"The instantaneous interface between the turbulent jet and its surroundings is highly convoluted...since the turbulence is a manifestation of the vorticity which is extruded from the inside of the nozzle and then swept downstream...When Re is large, this vorticity is virtually frozen into the fluid and so the convoluted outer edge of the jet, which marks the outer limits of the vorticity, is an inevitable consequence of the eddying motion within the jet. As with a planar jet there is entrainment of the ambient fluid as the convoluted outer edge engulfs irrotational fluid. Thus the mass flux of the jet increases with [axial distance]," (Turbulence, Cambridge University Press, 2004, p154)
Bearing this in mind, Williams used an interesting approach on the FW07 (1979-1982) and FW08 (1982), where they extended the exhaust pipes into the centre of the rear deck, and pointed them in double-barrelled style at the central rear-wing pillar.
This is clearly not ideal for structural reasons, and indeed it ultimately caused Keke Rosberg’s rear wing failure at Monza 1982, when he was close to sealing the championship. One has to conclude, then, that this configuration was aerodynamically motivated. In particular, it seems likely that Williams wanted to remove the exhaust jets as far as possible from the diffuser exits of the sidepod venturi. The reason for this is perhaps best explained by these comments from Frank Dernie:
“The rear wing was a significant part of defining the wake pressure which drove the underbody performance, it is impossible to separate them. The rear wing was not at the legal limit because overall performance was better with the underbody and wing coupled.
“Most people’s diffusers stopped at the [leading edge of the] rear suspension. It was very difficult to keep the flow attached any further back, so all you got was more weight and less accessibility.” (X-ray spec: Williams FW07, Motorsport, November 2004, pp75-77).
Which brings us back to the McLaren MP4-B, and its extended diffuser tunnels. Directing the exhausts over the top of the diffusers might well have disrupted the link between the rear wing and underbody, but whilst exhaust flow creates turbulent kinetic energy, it also locally increases the mean-flow kinetic energy. Hence, it might have been McLaren's intention to use exhaust blowing to keep the airflow attached to their extended diffusers.
Whether this hypothesis is true, is something that perhaps only John Barnard or Alan Jenkins can answer...