Some thoughts on exhaust energy recovery (jet thrust):  Most of the energy, and all of the "free" energy in the exhaust, is released during the blow-down event just after the exhaust valve opens.  This is when most of the exhaust leaves the cylinder and it results in the highest velocities.  If the exhaust pipe is short and has minimal volume this pulse arrives at the tip with little loss and can be used for thrust.  The rest of the flow, which occurs during the exhaust stroke, is at a much lower rate and is therefore not very useful.  And during the rest of the engine cycle there is no flow, so the pipe is nothing more than drag and weight.  Since the pulse is so short multiple cylinders can be combined to use one exhaust pipe - however, the total volume of the pipe then increases and much of the pulse energy is lost by expanding back up toward the other cylinders.  It is logical, but probably not much more beneficial, to combine cylinders so that  the necessary minimum volume is combined with evenly-spaced pulses.  This works well with a 6-cylinder engine since each bank is even-firing, but on a 4 cylinder HO engine two cylinders from opposite banks must be combined.  The pipe must be as small as practical to produce the highest velocity at the exit and a surprisingly small pipe can be used to advantage.  This is because the backpressure that is important to engine power is that during the exhaust stroke, not during the blowdown, at which time the piston is still going "down" and increased pressure actually helps power (don't bother trying to take advantage of this, though, since the piston is barely moving and some of the backpressure will certainly remain when the piston starts back "up").  Excessively small pipes are probably not good because of the backpressure caused by the discontinuity at the exhaust port - might as well keep the pipe about the same size as the port or maybe just a little smaller.  So individual stacks are best, combining cylinders on each bank next best, combining two cylinders on opposite banks third best and combining all cylinders is the least desirable.  Remember, combining cylinders thinking that the net velocity will be higher is incorrect - it's just the free pulse energy that can be used effectively.  The angle of the exhaust to the slipstream is important for two reasons:  The exhaust "plume", if directed across the free air stream, creates drag just as though a pipe were there.  The thrust is proportional to the cosine of the angle to the direction of flight, so the angle should be kept below maybe 30 degrees - being exactly zero isn't important.  A 30-degree angle will recover 86% and a 15-degree angle is 97% effective.