Bonnet vents

[Lou Girardin]

Hi Matt,
If you install vents they must be placed in a low pressure area in order to
suck hot air out. This is about halfway between front and rear edges.
Drawing cold air into the engine bay will not give the results you want and
may create extra front end lift at speed. A downside of vents is that warm
air will be drawn into the heater inlet this will get quite toasty in
summer, if we ever get one.
Lou

[Mathew]

Hey Lou, you've confused the hell out of me! (never was that great at physics!). I was thinking that the vents would let the hot air escape when the car is doing very low speeds around town as this is when the heat is building up. Why will drawing cold air into the engine bay not give me the results I want?, and how will the vents draw hot air into the heater inlet? isn't the outside air always going to be cooler than the engine bay air?

ps- I think summer was on Tuesday morning, but before I could get outside to enjoy it, it disappeared and the rain returned.

[Lou Girardin]

Hi Matt,
Sorry to confuse you, a car bonnet has high and low pressure areas,
generally there's high pressure at the leading and trailing edges and low
pressure in the middle. So you use high pressure areas for intakes, etc. and
low pressure areas for extracting hot air. I've found on my car that
appreciably increasing air flow via a high pressure area into the engine bay
seems to create extra lift probably because the engine bay is vented below
the car, this made the car feel less planted at the front. So, I think it is
better to extract hot air by placing vents in the low pressure area, this
could also increase air flow through the radiator and should have the effect
of reducing lift as well because you are reducing the low pressure area. All
these systems work better the faster you go like all aerodynamic aids.
However, because you are drawing hot air into the airflow over the bonnet,
this will be drawn into the heater vent and will be warmer than ambient
temp. Whether this will worry you or not, who knows. If you have air con
then it's no problem. Have a look at vent and intake placement on various
cars, E types, Austin Healey's, Wrx's Evo's etc. they all seem to conform to
this practice.
Lou