Thrust
Exhaust exiting a typical exhaust pipe does not create significant thrust. When a mass is accelerated in one direction thrust is the force in the opposite direction. This happens at the point where the acceleration occurs which is typically not the exhaust tip. Most of the thrust occurs when the exhaust goes from stationary in the cylinder to vMax as it exits the exhaust port. There are only a few PSI in a properly sized exhaust pipe, releasing that pressure at the tip creates minimal thrust. There is typically very little fuel still burning/expanding in the pipe as it approaches the tip. Air already moving at speed is not thrust.
Inertia
“So when I put my hand at the end of a tailpipe I can feel the thrust.” No, that is the inertia in the moving air. Just because the air is moving fast does not mean that there is thrust present. Analogy: Toss a medicine ball. As you throw/accelerate the ball your body falls in the opposite direction so you have to brace myself to counter that. That is thrust. Once the medicine ball is done accelerating it no longer has any effect on you but it still has inertia. When the ball encounters the hands of someone who is catching it, that transfer of energy has no effect on you, the person who threw it. Energized exhaust still may be useful for things like filling the negative pressure behind the car (reducing drag), or drawing air thru a venturi to create negative pressure in ductwork or even to help seal a diffuser. It may also have negative effects like separating the air already attached to the body of the vehicle which creates drag.
Zoomies create downforce therefore thrust?
The above statement demonstrates a misunderstanding as to why zoomies create downforce. It is the bend on the tube creates a centripetal force changing the direction of the exhaust. It’s the same force that presses your butt into the seat and tires into the pavement when you drive fast thru a dip in the road. If thrust was the principle by which zoomies worked TF exhausts would exit straight up in order to maximize downforce, but they don’t. They are angled upward at approximately the same angle as they went downward. This is because the direction of force they create is the difference in those two angles and not in-line with the exhaust exit.
Exhaust Thrust, P&W R4360-VDT
Most articles refer to the VDT engine make it sound like they just pointed an exhaust pipe out the back of the plane and it provided significant amounts of free thrust. A misleading oversimplification of the design. At cruise speeds it ran at wide open throttle, had no wastegate and controlled boost with a variable nozzle at the exhaust tip. The design intentionally created massive exhaust back pressure so they could release it at the nozzle and create thrust. The purpose was to gain high speed cruising efficiency not max power. When they wanted to create max power they would open the exhaust nozzle, cutting the ‘thrust’ and allow the turbos to create boost and put peak power.