Why does the DC-8-70 have the same MTOW as the DC-8-60, despite its more powerful engines? The DC-8-70 is a more-or-less straight reengine of the DC-8-601, replacing the obsolete JT3D-3B low-bypass turbofans of the DC-8-60 with the CFM56-2C1 high-bypass turbofan.  According to Wikipedia, the maximum takeoff weight of the DC-8-70 was unchanged from that of the DC-8-60 (which resulted in the DC-8-70, with the heavier CFM56, having a slightly lower payload capacity than the DC-8-60):


  The DC-8-71, DC-8-72 and DC-8-73 were straightforward conversions of the -61, -62 and -63 primarily involving replacement of the JT3D engines with more fuel-efficient 22,000 lb (98.5 kN) CFM56-2 high-bypass turbofans with new nacelles and pylons built by Grumman Aerospace and fairing of the air intakes below the nose. The DC-8-71 achieved the same end but required more modification because the -61 did not have the improved wings and relocated engines of the -62 and -63. Maximum takeoff weights remained the same, but there was a slight reduction in payload because of the heavier engines. All three models were certified in 1982 and a total of 110 60-Series DC-8s were converted by the time the program ended in 1988. DC-8-70 conversions were overseen by Cammacorp with CFMI, McDonnell Douglas, and Grumman Aerospace as partners. Cammacorp was disbanded after the last aircraft was converted.[10]


Emphasis mine.

But maximum takeoff weight is determined by engine power,2 and the CFM56-2C1 has nearly a quarter again as much thrust (98 kN; 22 klbf) as the JT3D-3B (80 kN; 18 klbf), which should result in a proportionate increase in MTOW for the CFM56-2C1-equipped DC-8-70.

So why does the DC-8-70 have exactly the same MTOW as the DC-8-60, despite its significantly-increased engine power?



1: The DC-8-72 and -73 are literally just reengined DC-8-62s and -63s; the DC-8-61 required some additional modifications to make it into the DC-8-71.

2: It's the maximum weight at which the aircraft is physically capable of attaining liftoff speed on an infinitely-long frictionless runway at zero density altitude with one [for twinjets] or two engines [for more-than-twinjets] inoperative; beyond this weight, the aircraft has insufficient one-or-two-engines-out thrust to push it fast enough to generate enough lift to lift off.
But maximum takeoff weight is determined by engine power


not by engine power alone. The airframe also has to be strong enough for the intended takeoff weight. There is a structural strength margin, but this margin is mandated by the certification authority so you can't use it. The design won't have much more than the required margin because that would make the aircraft heavier than necessary and less competitive. 

At a minimum, you'd have to get the aircraft certified at the new MTOW, which requires expensive testing. Worst case, you have to modify the design of parts like the wing box/spars, landing gear, and fuselage structure. 

By 1988 the writing was on the wall for narrowbody 4-engined aircraft, with ETOPS-180 coming into force. 


  It's the maximum weight at which the aircraft is physically capable of attaining liftoff speed on an infinitely-long frictionless runway at zero density altitude with one [for twinjets] or two engines [for more-than-twinjets] inoperative;


No. That would make the definition of MTOW useless in the real world. Compare to the FAA definition: 


  The Federal Aviation Administration has defined Maximum Takeoff Weight (MTOW). The MTOW of an aircraft is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off, due to structural or other limits. 


The MTOW you see in manufacturer specifications is a figure established in the certification process. This will be below the physical maximum weight the aircraft can handle, to make sure there's a margin of safety.

In many circumstances (airfield runway length, altitude, temperature) 


  an aircraft may not be permitted to take off at its MTOW... The maximum weight at which a takeoff may be attempted, taking into account the above factors, is called the maximum permissible takeoff weight, maximum allowed takeoff weight or regulated takeoff weight.