To folk with bonnet raisers

[C.A.R.]

Yea;

Don't.

Naw just kidding. Thing is, there isn't a proven benefit for actually doing this, whether it helps dissipate heat any quicker from the engine bay. Also, if your HDi is getting that hot, something is wrong!

Basically there is an area of high pressure at the bottom of the windshield which is what generates the airflow for your air-vents in the cabin on most cars (inc. 306s). This area of turbulent air won't necessarily help 'extract' heat from under your bonnet, since the air will typically want to move where the pressure is lower. At a standstill or low speeds there may be a benefit, which is why 'bonnet lifters' are common place on drift cars as they are sat stationary for a long time and need to keep cool to maintain their performance.

The other problem is their appearance, they look silly. It just looks like your bonnet hinges are broken or the car has had an accident at some point...