[Dominik]

I found that NOT shrouding the radiator completely helps airflow through the engine bay a lot. Of course, not so much at idle. I leave it open at the top.

I suggest to help air to escape by having gaps between bonnet and body. Ie do not run rubber all around.

Are your side vents open? I have seen cars where they were just ornamental.
Also, I never had inner fenders which helped airflow too. Do you have a scoop where eat can escape past the air filter?

I am surprised though that your fuel doesn't boil...