From 1983 onwards the only allowed wing profile at the bottom of the car was behind the rear wheels' axis.
First came the coke-bottle rear end. This reduction of side-pods' width as they approach the rear area ensured a smooth flow over the top of the diffuser and rear wing.
More pressure difference can be achieved by speeding the air below the wing. Exactly this was done by Renault in mid 1983. They made their exhaust pipe blow into the diffuser.
Remember: A wing works because of the pressure difference between the top and the bottom. The higher this difference the more downforce this wing will produce.
This increased the downforce produced substantially, but only as long as the driver was pressing on the accelerator. When you had to lift off, even slightly, meant no exhaust and sudden loss of downforce. This made the cars quite unstable.
In 1986 Lotus used their active suspension to slightly lower the rear end of the car so that the gap to the ground could produce ground-effect or a venturi effect.
By the 1990s diffuser shapes became more and more intricate and sophisticated.
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