Some new BMWs have grills that open and close like venetian blinds according to needed intake air flow. Reason is for improved fuel efficiency. Closed blinds force more air over the aerodynamics of the surface of the car, whereas open blinds allow air into the engine bay, acting more like a parachute.

So cardboard in front of your rad may just improve your mileage.

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Cheers!
Scott McPhee

1987 300D