Just a point of interest I've recently spoken to the guy who had my 412 S1 before me and he was the person who removed the blanking discs in order to create a “venturi” effect through the bottom slots to improve the exhaust flow.
All I can say is that I very often ran with the back section of the soft top down but with the middle section of the roof in and never had the hint of fumes in the cabin.
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