Talk:PORSCHE 917K/@comment-26845160-20180515093132/@comment-31668816-20180522171000

It's the cooling fan, the engine being air-cooled, yellow as it's 'raw' fibreglass so you're seeing the colour of the resin. It's driven directly by the motor, you can see the gear mechanism in the centre of the picture, so it will turn at a speed proportional to the engine's rpm: