bigad4x4

Why do people think fitting bonnet raisers helps with under bonnet temps?

Okay sitting in traffic yes but not driving along at 60-70mph

What are peoples thoughts on it?

BigMouthBillyBass

several cries of "oi mate your bonnets open!"

is probably the only difference your going to see

bigad4x4

Nick.

Yes!!.. when sitting in my mates escort which has them we normally get people shouting it

THEYGOTMETOO

They look crap IMO but I do believe they actually work.

RS500/364

i rasied my saff rear of the bonnet as i worried about the temp sometimes and now the air though the heater is a lot cooler so i would have to say yes

bigad4x4

In traffic yes but on the road no, had a long chat with Karl about them and

he too said basically they were crap, how is the hot air going to escape

from under the bonnet when the wind force or whatever you call it is

holding it in the engine bay

nilrem

they draw the hot air from under the bonnet and let the air flow more freely towards the back off the engine bay, in a similar fashon to bonnet vents, hence dropping under bonnet temps! dont know how efficient they are though...

bigad4x4

Where is Karl when you need him

nilrem

thats the way i understand it... thats if you can understand the diagram... lol

bai1ey

erm have you ever seen a car in a wind tunnle the force of air comming at the front of the car hits the car and whirls round in a circle therefore pulling air from the back of the bonnet hot air is pulled out i think it will work at speeds

nilrem

is that agreeing with me or disagreeing?? lol...

tabetha

Diagram is WRONG as it assumes(incorrectly) that the air pressure is EQUAL on both sides of the bonnet, IT IS NOT EQUAL.
The underbonnet air is at a lower pressure due to not being pressurised, to such a large extent as it is not MOVING AIR, such as that flowing over the car is, the air then hits the screen area(just before it) and creates a HIGH pressure wave that pushes air into heater inlet etc, it also pushes the air through he gap made by the bonnet raisers and is suctioned out from underneath the car where lower pressure also exists.
They DO make a difference, they can do noting other than make a difference as this is basic laws of physics/airflow/pressure.
Note this is why bonnet louvres/vents are at the front edge of the bonnet where the high pressure wave does not exist due to upright screen, but air intakes/scoops(scooby) are towards the rear of the bonnet.
Air acts in a very similar way to water, ie BOW WAVE of ship, ahead of the ship further the faster it goes.
tabetha

nilrem

so the air would essentially push down teh back of the engine? rather than getting "sucked" out? you got any good info on this? i am very interested in the way things work and why they work! thanks!

melzieann

a mate of mine fitted his first every time he drove it the coolant temp would reach 93 degrees

with the bonnet raisers on it now only reaches 85 degrees, so that is as much proof as i need to show they work a drop of 8 degrees

TiB

iTrader: (1)

I have seen them work well on a fiesta turbo whilst moving but i presume they will have different levels of effectiveness on different cars due to different aerodynamic qualities.


Lee

bigad4x4

Stilll wouldn't dream of putting them on my saff

tabetha

For any air to be sucked it would have to be in a vacuum, the air here has a high side in front of screen area as it acts as a brake(to forward motion), hence spoilers to "spoil"/smooth the airflow around such things.
The high pressure side will ALWAYS over come the low pressure side, as it exhibits more energy, so yes air would indeed flow downwards.
The whole principal of why F1 are shaped the way they are, and not like a bus!!
A pressure wave is created when any object moves through a gas or liquid, the wave MUST go somewhere, and the wave goes to the point of least resistance, it does not produce a vacuum but a low pressure side, so high pressure side will force the low pressure side, air asside.
This is the same principal of why certain cars have flat undersides, the air moving underneath will move a lot faster(lower pressure), so force downwards from higher pressure air acting on top will push it down for added grip, same as a rear spoiler.
tabetha

dave_y

iTrader: (4)

tabetha so what you are saying is that when you have bonnet raisers its not the hot air flowing out from under the bonnet but the cool air flowing into the engine bay.

is this what happens to RS Turbo bonnet vents then also. they let air in rather than letting air out....?

All sounds confusing to me!

mechanic28

iTrader: (5)

sorry but these r the biggest load of wank!imo they look crap and r a total waste on raod cars

Jasesapphy

iTrader: (1)

We have them on our track maestro !!!!!!!!!!!

Fook all to do with engine temps, we run twin carbs and carnt fit the bonnet on without them

Mark V8

iTrader: (2)

They look soo
I would rather drive a car that over heated than have people point and laugh

Mark

TiB

iTrader: (1)

Anyone want to explain how a non gravity spray gun works as surely this is the same effect?

Also as said earlier... i've seen it work for myself regardless of whether the heat goes up or down.

Lee

nilrem

i see what you mean... so in reality cossy & turbo vents should idealy be the other way round, or just holes???

nilrem

from what i understand, non-gravity spray guns work by dawing the paint from the paint hopper, one of the reasons i thought the heat was drawn out from the top of the bonnet (on top of the fact that hot air rises)...

TiB

iTrader: (1)

I'm no quantum physacist but my money says that the hot air is drawn up from the bonnet gap.


One way to be sure is some how let off a smoke bomb whilst driving and see where the smoke goes

TiB

iTrader: (1)

I'm now going to buy some raisers just to see if it works on the 3dr aswell as it worked on the fiesta.

I'll post the results.

Lee

bai1ey

i carnt really understand the diagram mate sorry

tabetha

You don't need to let of any smoke bombs!!
Flow tests have already done this for you, vents work in different ways according to where they are, front mounted vents, are at a much lower pressure area, on an almost flat surface(bonnet) so hot air has no problem rising out of vents, as we get towards the screen area, there is a large upright, called the windscreen, which tries to resist being pushed through the air, this then MUST and does create a pressure(bow) wave in front of the car, the faster you go the further out in front of the car/screen it projects, as the pressure has nowhere to go becuase it is constantly being pushed (by car) it goes to point of least resistance which is downwards, this is one reason why your heater air flow gets better the faster you go WITHOUT the fan on.
The screen is pushing aside 00000's of litres of air every minute.
It is only theory that the air will in fact above a certain speed be pushed downwards, due to high pressure(air) spoiler(screen), but it must happen at a certain speed.
tabetha

nilrem

makes sence! thanks for explaining it!

Thrush

iTrader: (1)

I can see how they work for sitting still, as it's a larger "outlet" and hot air rises, so it allows it to escape - but I do personally think they won't help when moving?

Why?

VORTEX

Similar in explanation to Tabetha basically, the effect is called a vortex - and I have seen many a big power V8 yank dragsters running air "ducts" that "faced the wrong way" - ie the opeing for the air to go into facing the screen, not the front of the car like you'd expect. Again the reason for this is the vortex created from the air hitting the screen.....

TiB

iTrader: (1)

Where are these flow tests?? what car were they performed on?

Different cars have different aerodynamics yes/no?

Were these tests done on a car with raisers on as they will change the airflow angle to the screen yes/no?

I have tried them on a fiesta and they worked!! i will try them on the sierra to see if they work on that too.


Lee

TiB

iTrader: (1)

I also understand what your saying about the pressure thing from the window but that effect will not be the same if you create an opening (such as fitting raisers).

Twellsie

iTrader: (1)

i would suggest that they do not work at speed as having the bonnet raised up in a differant plane to the shape of the front of the car will cause a vortex (rotating air), vortex's are likely to be higher in pressure than the flow of air also exiting the under bonnet, thus not allowing it exit, also the vortex combined with the wave due to the incline of the wind screen will completly change the air flow enough that you will get insufficient draw from any bonnet vent you mat have

smudge_lad

if you look at the vents though, the holes are angled towards the windscreen, well on an RS Turbo they are, so this would tie in with what tabetha is saying.

although they are closer to the front of the bonnet than the back where the least resistance is, but surely they were designed with the holes angled that way for a reason???

I always wondered why they faced that way

tabetha

You only get a vortex where there is a trailing edge such as on a wing profile.
tabetha

Sexy-RS

I put them on my car as had issues with act's and Grove / Ahmed told me to chop some out the bumber and stick them on don't care what it looks like apparently it helped alot

Lo@der

iTrader: (8)

I just used them as my bonnet was a bad fit. when raised it doesn't have to be a good fit.

Twellsie

iTrader: (1)

with the bonnet raised, you do have a trailing edge

Phyco

i fitted bonnet raisers to my sapp and noticed a big difference in temps. i like the look but i fitted them for the reason of cooling cause its easier and cheaper than buying a vented bonnet or cutting up my old bonnet to fit vents