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You're talking about a tiny FE gain. Look at the size of the air intake on a 4 cylinder Honda. That's the amount of air stream you're affecting, if you're putting the petal to the metal. After making other more cost and time effective mods, you're going to be using so little engine power and hence air that I doubt you could really see an effect.
However for something like a satellite rocket, this plays a big role, and is the reason that the bottom of rockets can flare outwards with little effect on their drag.
It's also worth noting that the exhaust gas isn't constant, and so it comes out turbulant, which doesn't usually bode well for aerodynamics.
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