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Old 02-14-2009, 02:32 PM   #1 (permalink)
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Raising bonnet /hood -help airflow?

Wondering whether or not raising the rear of your bonnet/ hood could decrease or perhaps increase drag ?

A few people that i know of have raised the rear of the hood to stop the airflow hitting their wipers and pushing them up the windscreen ( on crx's at higher speeds ) perhaps reducing the drag wipers create too ?

( While im on the topic - when it rains, i get LOADS of water sat below the wipers / base of windscreen just sat there spinning around in turbulance )


Also quite a few people do it to lower under hood temps... It definately lowers the temps on the crx.

Then some people do it for the cool look ( Right, now I understand people doing this mod for cooling reasons, but I've never heard of anybody actually doing it because they're 'loving the look' of body panels that don't align properly... what's next? Misaligning the doors? )


Any opinions ?
aerodynamically speaking, from what i can see :

+ Its making a better transition of airflow from the hood to windshield
+ Its making the airflow flow over the wipers rather than into them.

- making larger gaps at the side of the hood ( could be easily rectified )

Also perhaps promoting a little more flow through the back of the engine bay , making less air pressure inside the engine bay. Whether that is good or bad may vary on application.

Also lower engine bay temps, which could be good or bad depending on application. ( Perhaps slower warm up time, but maybe also could get away with a bigger grill/rad block ? )



Ok his wipers ( looks like he has only got one blade in ? )are already part of the way up his windscreen to start with lol, but you get the idea.


Last edited by chipX; 02-14-2009 at 02:44 PM..
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Old 02-14-2009, 02:35 PM   #2 (permalink)
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Some people do it to help cooling. Someone is going to say I'm wrong because the base of the windshield is a high pressure area, but airflow is still going backward and the bernoulli effect applies.
It might result in better flow into the grille and up over the windshield?
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Old 02-14-2009, 02:38 PM   #3 (permalink)
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Old 02-14-2009, 03:49 PM   #4 (permalink)
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bonnet

I feel that the residence time,between when the air first enters the intake snorkel,and then passes into the throttle,is so brief that any heat flux which might exist under the hood simply would not have enough time to impact the charge density of the air/fuel mixture.You could calculate this and compare.-------------------- Dr.Morrelli's work at Pininfarina required extensive windtunnel time to develop proper engine bay discharge duct architecture.My opinion is that something as primitive as raising the trailing edge of the bonnet could exacerbate flow conditions and lower performance.Not an easy call!
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Old 07-14-2015, 06:11 PM   #5 (permalink)
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Yes it works

Quote:
Originally Posted by winkosmosis View Post
Some people do it to help cooling. Someone is going to say I'm wrong because the base of the windshield is a high pressure area, but airflow is still going backward and the bernoulli effect applies.
It might result in better flow into the grille and up over the windshield?
The problem here is people are talking about lots of different applications.
If you are running a NASCAR yes it'll mess up the air flow, dynamics and control of your ride 'at worst'.
If you are a regular joe that just has a car/truck/suv that is running hot in the summer, yes raising your hood 1 inch in the back will help the air escape. In fact if you don't believe it, simply raise it for a test, close the hood so only the rear 1 inch crack you created exists, start your engine and feel. You will notice the air coming through the crack even with the car standing still.

If heat is your enemy, anything you can do to eliminate it is excellent.

If hood scoops and or louvers were affordable to us then that would be the best because during the summer they could be open and during the winter they could be closed.

CYa
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Old 07-14-2015, 08:05 PM   #6 (permalink)
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I can't be bothered to find an example (anyone else?), but highly streamlined late model cars have a hood-line raised above the side-window sill. What that does is effectively lower the rear-view mirrors, moving them out of the flow around the A-pillar.

Quote:
Originally Posted by chipX
I've never heard of anybody actually doing it because they're 'loving the look' of body panels that don't align properly... what's next?
Rat rods.
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Old 07-15-2015, 02:26 PM   #7 (permalink)
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raised hoodline

Some of what we're seeing has mostly to do with NHTSA Pedestrian Safety Regulations which help prevent a struck pedestrian from contacting hard points (like the engine and firewall upper bulkhead) when landing on the hood.
The Honda Accord was the 1st production vehicle to market the new feature.
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Old 07-16-2015, 01:04 AM   #8 (permalink)
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Quote:
Originally Posted by aerohead View Post
Some of what we're seeing has mostly to do with NHTSA Pedestrian Safety Regulations which help prevent a struck pedestrian from contacting hard points (like the engine and firewall upper bulkhead) when landing on the hood.
The Honda Accord was the 1st production vehicle to market the new feature.
And it would also help aero? Because i.m thinking about it too. I have foam forming "under" the wipers because of air (see pic). And was thinking of a small lip to attach in front of the wipers
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Old 07-16-2015, 01:39 AM   #9 (permalink)
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Quote:
Originally Posted by DocHaley View Post
The problem here is people are talking about lots of different applications.

...

If you are a regular joe that just has a car/truck/suv that is running hot in the summer, yes raising your hood 1 inch in the back will help the air escape. In fact if you don't believe it, simply raise it for a test, close the hood so only the rear 1 inch crack you created exists, start your engine and feel. You will notice the air coming through the crack even with the car standing still.
That pretty much means nothing, especially if the cooling fan is on. Things change dramatically once it's moving. If what you said is true, cowl induction hoods would be fails. Since the base of the windshield/cowl area is a high pressure area, it is probable that air flows into the engine compartment at that point instead of out. If anyone cares enough about it to find out, a yarn tuft test should reveal what's going on.
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Old 07-16-2015, 02:36 AM   #10 (permalink)
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I have this type bonnet guard with a slight raised lip on my Tucson and my younger brother has it on his car golf mk1 (rabbit shape).
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We use to travel together for work and share cars and in both cars i noticed the drops flying off the guard striking the window about 1/3 up the windscreen at speeds of up to 70 kph. cant help but wonder how much of the air is actually deflected over the windscreen wipers. I then did a further test. A few days ago it was raining quite big rain drops but not a lot of them so i looked at how the patterns of drops flow off the windscreen while tailing a truck without switching the wipers on. When the truck turned off i noticed the drops moving a lot faster (predictable). i then sped up to about 65mph and noticed the drops in the bottom 1/3 to 1/4 of the windscreen not moving at all and the drops above streaking off to the top.

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