Thanks Blanchimont.
So with 71.8mm bolt to bolt, I am getting the following tailpipe diameter estimates:
Mercedes: 98.2mm
Ferrari: 124.9mm
Renault: 94.4mm
And cross sections:
Mercedes: 7534mm2
Ferrari: 12252mm2
Renault: 6999mm2
Hmmm... that's less that the allowed cross section for the Renault engines. The diameter estimate from the last of the 9 pictures is a bit larger and would come very close to the allowed cross section.
The errors in my method are potentially quite large (measuring single pixels, up to 2% error per pixel, applied 4 times over), the reflections in the bolts, etc. So I guess all one can conclude is that the Ranaults and Mercedes are very close to the inferior limit (I'd guess exactly in it), that the Ferraris are close to the largest limit with a cross section area of about 5/3 that of the competition. And that I need a better measuring method or better pictures
Well, it was worth trying.