Is it possible to upgrade the Boeing 747-8 and Airbus A380's engines (upgrading to a newer engines) for greater speed, if the engines were more efficient and used less fuel at higher speeds relative to distance?
If so, what are the maximum speeds that the airframes or entire aircraft could handle or are designed to handle?
I just wanted to comment on this question since some of my code is on the A380's GE engine.
Those engines are optimized for fuel efficiency at their cruising altitude and speed. The tradeoff that you fight against when designing the engine for cruising altitude is that it makes the engine hard to start, and the stall margin (the pressure and fuel combo you need to keep the engine from stalling) is razor thin when they are on the ground.
Just to echo what Kevin said, any improvements to allowing the engine to start without stalling would surely go to the fuel efficiency, not to speed.
If the engines are upgraded to "better" ones, the manufacturer would make it result in increased carrying capacity or increased range (or both), but not increased speed.
The limitation is the airframe. If you try to push it faster, you will run into aerodynamic problems, for example part of the flow is supersonic around certain areas of the wing. It is undesirable and cannot be changed without redesigning the airframe.
However, other things are relatively easy. Even if you do nothing to strengthen the structural elements, an improved engine would allow better climb performance, shorter runway length under the same conditions etc. It will give airlines more flexibility. Better efficiency means lower fuel consumption, which translates to either more payload or more flight distance, again giving more flexibility to airlines.
Since there is inevitably a big change in the aerodynamic behaviour of any plane at Mach 1.0, there is a relatively limited speed gain to be had from pushing beyond Mach 0.85 before some parts of the airflow become transonic and the whole design would need to be changed.
The same applies to the air flow within the engine itself. Supersonic aircraft don't have large diameter, high bypass ratio, wing-mounted engines with (at least conceptually) simple intake designs.
From the customer's (i.e. the airline's) point of view, the simplest way to operate a plane is to be able to cruise at the same speed as everyone else when flying busy routes (e.g. transatlantic from Europe - USA), not by trying to jump the queue and save a few minutes.
It would certainly be technically possible to do what the OP proposes, but it's not economically sensible if either the customer or the manufacturer wants to run a profitable business.
Airfoils and frame are optimized for a certain speed. The newer aircraft are optimized to operate closer to the trans-sonic regime, if not in it.
The 747 is a great workhorse, and was designed to actually fly quite fast. Pan Am wanted the 747 to fly at 0.9 mach, but Boeing made a compromise of 0.87.
Realize that all of this was before the oil crisis of the 1970s and the current stringent regulations on engine emissions. The thing is the 747 was designed back in the 1960s and I am sure its drag profile is a lot bigger than current aircraft that can fly 0.87.
Now the issue one will have to take into account is cycle time for a 747. Fatigue will be setting in for older aircraft as opposed to one that is in mint condition... right off the factory floor. The wings will be a little less stiffer and aero-elastic effects can become more prominent for the 747 at 0.87 compared to several decades ago.
The answers here are BONKERS!
The sound barrier is the limit for speed on 747s and 380s. Therefore stronger engines will not make top speed higher. Rest in pieces.
For the more common case of cruising speed, new engines might make the airplane cruise faster for a given cost (eg fuel cost), but that would be a hard sell.
ASIDE: I have heard that 747s have the highest cruising speed.
