I think the cam shaft variable on only 4 cylinders is part of the cylinder deactivation system. It shuts off fuel and spark and apparently air to 4 cylinders when driving gently for more fuel economy. Thats all I can think of.
The only thing I've heard of is sometimes an injector gets partially clogged and the air/fuel mixture leans out only on one cylinder causing damage to one cylinder and all others remain okay. Could be other things too but I'm sure you'll have it figured by the next video
If anyone wants to know what happened to this engine, it's in another of Tasos videos. It seems they still have it on a pallet up on one of their storage shelves. The customer bought a new engine and they don't know what to do with the old engine. "Maybe we'll make a coffee table with it". ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HrIw16gPkWM.html
common fsi/tfsi injector failure, need to flow test and i bet that cylinders injectors flow rate is down compared to the rest? Also one of the big end bearings has a rifle drilled oil hole but the rest aren`t?
The engine detonated... which means that the compression-ratio(cylinder pressure) was too high for the type of octane. low octane (i.e; 87-regular) fuel in a high compression engine will cause detonation; which is blowing a gaping hole threw piston crown. The more cylinder pressure you make, means more power u make, but also u make more heat in the combustion chamber. This is a twin turbo engine- either the boost was turned up too high for the type of fuel used or not enough fuel to cool the combustion chamber in btw fire strokes. Also an efficient cool air intake is necessary when turning-up boost ...blah blah blah ... i could go on. but the answer to your question; "why that piston melted?" answer: Detonation.
Injecter failure on cylindre 2 2 echauxt valve burned and piston got melted + that chamber is not variable So ? Injecter is giving more fuel more heat on big torques boom melted
