[M_a_t_t]

I was actually thinking about this the last few days. I was reading Road Vehicle Aerodynamics 1st ed. and there was a section (6.3, pg 115 if you have the book) about "BASE DRAG REDUCTION BY INCREASING THE WAKE PRESSURE" and it was very interesting. The main way he offered was with the exhaust gas from the engine, but for it to work well you needed a large displacement or turbo charged engine (high volumetric efficiency) to make a significant difference in terms of what is possible with this method. You also need to slow the exhaust gas down so that instead of ejecting the gas out of the wake it "pumps" it in, increasing the pressure in the wake. Also don't forget that with the exhaust it would work optimally during acceleration as it won't pump out as much gas at low throttle cruise.

I thought it was interesting regardless though.

Originally Posted by Scibor-Rylski

Consider the total volume of air displaced by a car in motion as the product of its maximum cross-sectional area S and its speed V, i.e SV is a 'measure' value of the air flow rate. Thus the exhaust gases from a 1.6 litre car engine produce a gas flow of order of .67% of the total value of SV, the ventilating system may produce .167% and the engine cooling system about 1.2%. All these sources of air or gas can therefore produce a combined flow rate in the wake of about 2% of the total air displacement SV.

In the case of a vehicle with an engine of 5 litres capacity the exhaust gases alone can create an injection rate of about 2.1%. As the exhaust gases are ejected from the exhaust pipe at high speed a plenum chamber is required to reduce the momentum. By losing momentum the gas increases its pressure. From this plenum chamber gas or air, or a mixture of both, is ejected through a grid to ensure the correct rate of flow into the wake...