It's always a good idea to check the content of CUP in Monty's very well done manual
You're right Korrigan, still much useful to read a manual ! I did it, but I must admit that I unfortunately forgot most of the info listed
One thing is sure is that I was looking for engine startup flames + exhaust blue flames, which I didn't get with CUP. Don't know if such a mod is included but if so in any case it never worked in my game. That's why I have added Crazyflak's/Jeronimo's and Co cowlings mod, and why I don't remove it.
To solve my issue I have just disabled the He-111H-2 in the 3do folder of the mod, and it works fine now.
There's only one problem left : startup and blue flames shows at the back of the cowlings, under the wing as if the engine's got a single exhaust pipe on each side, or a exhaust damper. Well it doesn't matter, much more acceptable than a see-through engine
Btw I 'm just thinking about something while writing : unless I'm mistaken there was a He-111 with exhaust dampers in the original game or am I wrong ? In my modded game I can't no longer see such an equipment on 111s. Was it inaccurate ?