I'm sure Citroen's aerodynamics team approved of the cendrier location.
If I was going to add vents to my car......
(Hang on.... I have a '72 DS21iePallas, a perfectly standard car to conduct experiments on..... except it is green.
And I have several downgraded bonnets, and some downgraded front wings.....)
I would start with cendriers upgraded a little to suit the 3rd front.
Could I recover the costs from supplying kits?
I'll add it to the list of "important" jobs.
I did a little research and it's interesting to note that Citroen claimed that their "underhood" aerodynamic changes alone (for the 2nd nose restyle) were responsible for a 4 mph increase in top speed and a 4 mpg improvement in fuel consumption at high speed (over the 1st nose).
Also of interest (according to the French website that I got the photo of the cendrier off) was that the DSs with cendriers got a full length seal where the bonnet meets the guard wheras the ones without cendriers just had little segments of seal. It didn't explain why, but my guess is that air being pushed out of the gap under the bonnet's sides was meeting air coming out of the cendriers with undesirable effects.
My understanding is that cendriers were only fitted on the 1st nose cars, so Citroen's aforementioned claims of aerodynamic improvement on the 2nd nose must have come from guiding more air down underneath the car?
Based on this rationale the seal at the base of the screen on the 3rd nose, along with the full length guard seals on the 3rd nose are probably all there for the same reason (to make the under bonnet air flow down). Period Citroen rally cars don't seem to have any extra venting except for one photo I found of a car with gills in the guards behind the front wheels?
The purported reasoning for fitting the vents in the top of the guards was due to "extra heat from the DS's hydraulics"? The early cars also had about 3 foot of exhaust pipe making its way forward in the engine bay, so possibly a combination of both? Perhaps once they got rid of this heat source (the longer exhaust pipe) the need for cendriers became obsolete? They both seen to have disappeared at the same time?
No doubt some hot air would bleed out of any sort of upper engine bay venting at standstill, but what is happening at speed? They may need flaps to close them off once moving?
I don't think I'll be chopping any holes anywhere any time soon until someone else does it, with wind tunnel results ratifying some worthwhile improvements.
The aerodynamic team may have approved the cendriers in the first place but one would assume they also signed off on taking them away?