I would like to change the subject slightly.
The only piece of circumstantial evidence that supports the AL-31 theory for the J-20 is the appearance of the nozzle. Nobody has actually seen a J-20 engine removal or installation with AL-31. Nobody can verify an AL-31FM2 contract with Russia.
Some people have also claimed that the so-called "Frankenstein" engine theory is impossible. Is it?
I want to put some facts out there and let people decide.
The F100 engine has five major modules that can be removed from the engine for repair or overhaul, serviced separately, and then reassembled to produce a whole engine:
(1) inlet/fan
(2) core (compressor, combustor, and compressor-drive turbine)
(3) fan-drive turbine
(4) augmentor and exhaust nozzle
(5) gearbox
Each module is completely interchangeable from engine-to-engine at the intermediate maintenance level.
The modular concept allows for minor engine problems to be fixed directly on the flightline. For example, let's say the F100 has suffered slight damage from a minor birdstrike. You change the fan module, test, and return the engine to service within a short time period without the need for a long shipment and wait time for another location to address your issue. A supply of modules are kept on base. In more serious cases, the engines go to an Air Force depot.
The photo below is the F-15 engine shop at RAF Lakenheath airbase (48th Fighter Wing). All the F100 augmentor/exhaust nozzle modules are separated before your very eyes.
My point is that the augmentor/exhaust nozzle is not some sort of inseparable element of the engine. Some engines don't even have augmentors. Other engines (like in the X-35 prototype) combined the F119 core with an interim nozzle. The F135 continues to share a common core with the F119.
So the "Frankenstein" engine theory is impossible? Why is it impossible? China could have easily created a lookalike AL-31 augmentor/nozzle module to expedite testing of the WS-15 core and other more critical components. As long as the module is tailor-made to fit the engine, everything should be fine. Perhaps the final stealthy, thrust vectoring augmentor/nozzle is still in development. It is far easier to create different modules that are interchangeable than different whole engines that are interchangeable. When the final stealth augmentor/nozzle module is ready, they remove the interim AL-31 module, and pop in the new one. Some additional testing will be required, but the intakes and engine bays can stay the same. The WS-15 core was being tested inside the engine bay the whole time.
What have we seen so far from the J-20? Sometimes the nozzles look like AL-31. Sometimes they are black. Sometimes they are silver. Are they applying different coatings to the nozzles, or are they rapidly changing augmentor/nozzle modules? I have no idea.
But I will be watching closely at several things. The size of the intakes do not lie because they are directly tied to the airflow requirements of the engine. The rear fuselage will change if they change the dimensions of the engine bays. Lastly, look at the nozzles.