I've thought about this, on and off, for many years -- specifically with respect
to its application in drag racing.
Since dragsters have, for decades, been accelerating horizontally at more than 1G,
it's obvious that the coefficient of friction between the rear tires and the pavement
is more than 1. So it made sense, to me, for the exhaust to be as vertical as possible
when leaving the vehicle, provided, of course, that traction limitations were the thing
keeping the car from going any faster/quicker.
At Bonneville, for the more powerful wheel-driven vehicles, traction certainly limits speed
attained, so it seems to me that having the exhaust exiting vertically would help more than
aiming it toward the rear.
I suspect that the amount of exhaust thrust from a wheel-driven vehicle is significantly
less than was implied earlier in this thread. Here's why: Only a severely-limited amount
of combustion takes place, resulting in a severely-limited amount of exhaust thrust. Why
is the amount of combustion so limited? Because the desired traction is not there. Remember
that a funny car's horizontal acceleration is about 4G, so the coefficient of friction must be
at least 4. This is about 8 times what's available on the salt, barring the use of clever things
like spoilers, which I strongly encourage. I think the Carbiliner is, in the foreseeable future,
going to impress us all by going well over 500 mph while using "negative lift" provided by the
airfoils that contain the rear axle(s).
While putting out limited horsepower (because of traction limitations), one is only burning a
limited amount of fuel; therefore, the exhaust thrust is going to be limited also. A dragster
can get away with lots of all this stuff -- power/fuel consumption/thrust -- because there is
so much traction available on a drag strip.
For Bonneville, I think it would be helpful for a fast car to use some of its horsepower --
okay, maybe a LOT of its horsepower -- to generate downforce at the driving wheels. And
then maybe less ballast (or mass in general) would be needed to keep the car "on the ground."
With enough available horsepower being used to provide traction, then aiming the exhaust
rearward should help accelerate the car horizontally.