I did a "test" during my vacation trip and it appears that the 1cm wide slits in my grille block are still good for something at highway speeds (~95 km/h): On the way out I had the lower grille block closed (upper grille and IC unblocked) and saw coolant temperatures around 91-92°C with outdoor temp ~21°C. On the return trip it was much hotter (32°C), but the engine was cooler (81-82°C) with the lower grille unblocked.
More details about the test
here.
BTW My grille block now has more openings than on the old picture in the first post.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread