Size of hood vent
 

I'm thinking the next mod to my Focus Wagon is to vent the cooling air through the hood and then fit a belly pan. I've installed a air dam (here's the air dam) and modified my driving, with the help of a ScanGauge, to average above 30mpg. That includes time stuck in traffic, I hate seeing the 0.0 mpg on the ScanGauge.
This thread was close to answering my question but I don't have room to modify flow after the radiator since the exhaust manifold is only a couple of inches behind the radiator.
I've read the great articles at Autospeed.com and have done some testing with a Magnehelic gauge to find low pressure areas on the hood.
What size should the vent be? Is there a theoretical size based on the size of the grill opening?

I don't have a direct answer for you, but I do have some thoughts:

*Outlet area should roughly equal inlet area. If you know that value, it would probably make a decent baseline.

*You are thinking of chopping up a kind of expensive part of your car. Perhaps a junkyard hood could be obtained for your testing phase, then transferred to your "good" hood when you achieve satisfactory results.

*Or, make your own temporary hood out of coroplast or something and use tape and/or coroplast pieces to optimize the size and location of the outlet.

You could try what Ive seen some turbo cars do for cooling. Just use some washers as spacers to lift the back of the hood up near the windshield. Not sure if it would mess with aero or help it. Only a few bucks to try it though, and if nothing else its a windshield wiper block.

http://i112.photobucket.com/albums/n...l/DSC00644.jpg

FYI, the base of the windshield is a high pressure area. Not actually a good place to try to vent air from an engine compartment. That's exactly why cabin fresh air intakes are typically located there.

http://us1.webpublications.com.au/st...4/2455_4lo.jpg

Perhaps some is spilling out from the sides of the open hood in that Honda pic though. I don't know.

I am pretty sure you want the outlet to be around twice as large as the inlet (or maybe it's 50% larger, I've seen this somewhere). The exiting flow has less velocity, so it needs more room to escape at the same rate. You need the extra space so it can move the same volume of air as is entering.

That type of hood venting is primarily used for drag and drifting cars. It's great for allowing a lot of heat to leave the engine compartment... when you are not moving. Drag cars sit a lot and drifters are sideways at low speeds / high load most of the time, neither situation is helpful for moving air through the engine bay.

http://ecomodder.com/forum/showthrea...drag-2886.html

Probably better to go with what Phil submitted than what I said about area...

but I'd still go with my other two points.

Thanks for the information.
I looked into getting a hood from a wrecking yard before I start hacking at the perfectly good one on my car. I found one for $175! I wasn't expecting that. Makes the temporary hood seem like a good idea.
Based on the reading I've done, I understand that the windshield area is not the best place for a vent.

In order to make the vent large enough I'm going to have to cut through some of the reinforcing on the hood.
What do you think about a smaller hood opening along with venting out through the wheel wells?
I haven't found anyone post any results for this type of mod.
I'm beginning to wonder if this is worth the effort...

Search for posts by basjoos on wheelwell venting. He's done it successfully.

hood vent safety mention
 

I've driven cars old enough that,on occasion, they entertained me with exploding coolant hoses,and radiators.---------------------

Should you do the hood exit,in the event of a cooling system breach,you might find yourself blinded by a windshield covered in glycol and hot vapor attempting to condense on the glass if below it's dew-point.------------------

The mix won't wipe away like rainwater,and if,say you were right in the middle of a lane change in heavy and fast close-quarter freeway traffic,it might all end in a mishap.---------------

This may be a reason why auto makers have "neglected" hood-cited exits.--------------------

If you were to cite the exit closer to the A-Pillars,you'd be dumping air in to very fast-moving,lower static-pressure air than in the centerline,and should the system open up,center vision might be maintained.---------------------------

A kid threw a rock off an overpass in Los Angeles which shattered the windshield of my CRX once.I was in a situation as mentioned above,totally blinded,and it's a miracle that I escaped the situation in one piece.-------------------

Plan for the worst,hope for the best.

Hi,

My interpretation of the drawing you posted is the opposite -- the base of the windshield has some negative pressure, and so it is the best place to exhaust the cooling air.

interpretation
 

Neil,the graphic is a bit confusing,but you'll notice that in the yellow portions of the image,the arrowheads are directed "away" from the car (signifying lift,or negative pressure),while within the black portion ,the arrows are directed "at" the car,signifying positive ram pressure.The image is "dyslexic" are far as intuition goes.Maximum lift is occuring over the leading edge of hood and roof while stagnation is occuring at grille and cowl area.

my hood setup
 

This is my modded hood with vents. I've cut original stock hood and place vents from old pontiac. I also tilt radiator about an inch back for better air exhaust and made air ducting to vent directly from fans. My radiator intake is about same size what vents. When I put my hand by vents while radiator fans is blowing, I feel hot air coming out. I like this setup works very good no problem so far.

Hi,

If the front edge of the nose is low pressure, I'll eat my hat. I think that drawing is bogus.

do you prefer ketchup or mayo?
 

Neil,I know it seems crazy,but you can find these studies going back decades.Dr.Timothy Maxwell of the Texas Tech Aero Lab once explained to me that with the 1st-gen Ford Taurus,that the hole in the front of the car where Ford placed their oval emblem,actually had air coming "out" of it!.Remember,as you displace forward,the air is accelerated from rest,to it's maximum velocity as it rounds the leading edges of the car.The "airspeed" at the leading edge of the hood could be as high as 50% greater than your road speed.Same for over the roof.----------------------

From conservation of mass,and energy,the only way the air can gain velocity, is if it trades off pressure at the same time.------------

It's how a perfume sprayer or carburetor works.Daniel Bernoulli researched all this and published long ago and its in all fluid mechanics text.On race cars,the radiator exit duct may be positioned on the forward radius to both reduce lift,as well as dump air to this low pressure area.Also,this is a race car,and by moving the fully-ducted lightweight cooling system forward,heavier components can be compacted and nested close together,closer to the CG of the car,for lower polar moment.

The drawing is good; you need to think about your interpretation of it some more.

Air pressure profile per Aerodynamic flow may have been a better label.

Hi,

Velocity means that air is being deflected, or pushed up and over the front of the hood, and this is where drag is most easily created. The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.

So, the diagram makes sense if you think about velocity (changing direction), and this also means pressure against the surface of the vehicle. Low velocity is closer to still/stagnant air, and therefore is lower pressure against the surface of the car (because it is closer to the way it was before the car pushed it out of the way).

The vectors are showing lift, I guess. They only shade the parts that have a positive Y value. I guess it's the X values that I have a problem with -- why are the vectors below the neutral point (on the front bumper) going the right (positive X) and the vectors above the neutral point go to the left? Is this area of the car pulling the car forward?

I'll draw a picture later that is more helpful, to me at least.

Hi,

I've attempted to show the air flow lines, overlaid on the "velocity" image (if that is what it is showing):
http://img.photobucket.com/albums/v7...esVelocity.png

And here it is with just the flow lines:
http://img.photobucket.com/albums/v7.../FlowLines.png

As you can see the greatest velocity occurs where the air is being pushed aside at the greatest angle, and this is what gives it the greatest velocity. I think that the nose pushes the air up above the main part of the hood, and then the upper part of the windshield again pushes it up; creating another higher velocity zone.

holypaulie, I like the hood mod. Did holes of that size effect the hood's strength? Does it flex under normal use?

I've played around with the a Magnehelic gauge and the aerodynamic flow profile makes sense to me. I found lower pressure 6 inches back from the front edge of the hood, and higher pressure up by the cowl.

I'm going to next check along the side of the front fenders and further off the center line of the hood as suggested by aerohead.

Thanks again for all the input.

There is a pocket of high pressure air there that deflects the flow.
Hence the maple seeds sit there undisturbed.

It also depends on the vehicle.

drag
 

The velocity will be imparted to stagnant air as it is displaced by the moving vehicle,going from rest,and atmospheric pressure,to some local velocity determined by the time/displacement function,and lower static pressure.---------------- Drag will be imparted by viscous shearing forces of the air of the boundary layer,induced from positive or negative lift generated by the body of the car, along with attached- vortices ( all from pressure differentials ),and primarily from form-drag ( profile-drag ),associated with separated flow and it's attendant turbulent wake,with low base pressure acting against the back of the vehicle.--------------- The drag is not created by the impact of the vehicle against the air,but rather the pressure differential "across" the vehicle.

My hood is still strong because I didn't remove the main vertical hood support which is locatated between vents. I cut very small portion of front horizontal support. No flexing at all. Everything was well planned out. I'm glad that I could share.

velocity
 

Neil,I think a smoke-rake image of the car would better show how the body deforms the airflow.I think the car is a Porsche 944,and I've seen imagery from Porsche,and it's a little different.------------------------------- To illustrate "lift" associated from velocity,I've gleaned the following: In the windtunnel development of Sunraycer,too much air went under the car,causing negative lift under the very long,cantilevered tail.The "lift" would pull the tail down,causing the nose to lift.Fairings behind the wheels would catch the air and yaw the car sideways,and then the car would liftoff,flying.This was at 20-mph(32 km/h).For the World Solar Challenge,the nose was re-configured and wheel fairings deleted,trading increased drag for better stability.-------------------------- A 1969 Z-28 Camaro was tested and found to generate 500-pounds(227-kg) of lift at the nose at 128-mph(206 km/h).Addition of a chin spoiler cut lift to 275-lbs (125kg).-------------------------- A half-million pound (227,272 kg) Boeing 747 leaves the ground at 165-mph (266-km/h).------------------------------- Racing Beat's 3,000-lb (1,363 kg)Mazda RX-7 LSR car "flew" at 235-mph ( 379 km/h),landing on it's roof at Bonneville.-------------------------- In 1989,Art Arfon's "Green Monster #27" ( which looks like the fuselage of a 1951 Grumman F-9F-2 Panther ) took off at 300-mph (484 km/h),did a complete flip,landed on it's side after 300 feet in the air,grinding to a halt after a half-mile(0.8 km) on Bonneville's salt.----------------------------------- Lastly,during the Monte Carlo Grand Prix,when the Formula-1 cars go through the tunnel at Monaco,they experience oversteer,do to the ceiling of the tunnel,20-feet overhead.It's been found that the rear wing can suffer lift losses from any disturbance within up to about 30-feet.Using inverted logic,one can deduce that an F-1 car disturbs the air up to 30-feet away.---------------------------- If windtunnels were much larger than they are,we could better appreciate how much our cars really affect the air around them due to "lift" and the implications with respect to mpg.

For those wondering about lifting the rear of the bonnet/hood with spacers - I tried this on a 3rd Gen RX7. The heat generated in the engine bay is massive, so any extra cooling is helpful.
I was fortunate enough to see where the heat was spilling out in the mornings when the bonnet was iced up.

The raised edges cleared first along the whole length of the bonnet, the de-iced strip being perhaps 2" wide from the edge. The next area to de-ice was directly above the turbos, followed by a criss-cross pattern that marked the underbonnet reinforcing. The *last* area to thaw was the very rear edge of the bonnet. It seemed that very little hot air was expelled from here.

I thought about raising the bonnet in a similar fashion on the Laurel to smooth the transition to the windscreen, however it looks like the turbulence generated by the raised edges might counteract this effort.

If it were mine, I would use no hood vent at all, and go with holes in the inner fenders to vent to the wheel wells. The wheel wells have low pressure air so they will suck the high pressure air right out of the engine compartment very well.

Just cut the holes there down near the axle so that there isn't a large amount of tire spray flying back into the engine bay.

Also use a grill block to keep out more air than you need for cooling.

Ever seen hot rods with the "scoop" pointed towards the windshield and ending right near the base of the windshield? They do this as a "ram air" setup due to the forces shown in the aerodynamic pic on p.1. At the base of the windshield when you are traveling at a decent speed air is trying to force its way down and into the back of the hood. Even firebirds came with this stock in the late 70's and 80's.

As for venting the hood by raising it. I believe this works well at slower speeds. Seen it help with cooling problems on 4x4s that saw trail use and slower speeds.

A person I knew had overheating problems in arizona while one a trip. He stopped at some podunk place and asked "cooter" if he could put louvers in his hood. They did it for a decent price and it fixed his cooling problem. It was a mid '80s toyota pickup that was hauling a huge load. The louvers were formed into the entire middle of the hood but not directly in the center line where the main support is. They didn't go within back 1/4 of the back of the hood (towards the windshield). Basically air would come through the radiator then up and over the motor and out the louvers.
There are louvers on a number of vehicles that comes that way from the factory. FJ40 landcruisers have some by the way on the back corners of the hoods but that seems to be to there to help more with slow operating speeds with high loads. seen them on other cars but can't quite recall which ones...grand am?

If I remember correctly, the forward facing scoop located midway back and just to the right of center on the 84-86 Mustang SVO actually flowed the opposite it was supposed to. It moved air from the engine compartment through the intercooler and out the scoop while the vehicle was moving instead of pulling in cooler outside air for the intercooler. I Googled it but found nothing to back up my memory.

A quick update, and end, to this thread.
I decided to not cut my hood. There are many reasons but one of the biggest is the expected impact on economy was pretty small.
Since I've joined ecomodder I've been faithfully tracking my mpg. The biggest effect on mileage, without a doubt, is time of day. If I leave home for work during "rush hour" and wind up stuck idling in traffic my mileage for that tank is crap. I have a flexible work schedule, so the biggest improvement to my mileage is to travel during off peak hours.
I've decided the garden edging on the bottom of the bumper was a good improvement http://ecomodder.com/forum/showthrea...mods-5835.html.
My next project is a 1985 Toyota MR2. If I'm stuck in traffic it should be more efficient to feed a 1.6L engine. But mainly more fun to drive...

I thought I shouldnt open a new thread that is why I am resurrecting this one

I/C Water Spray Test on Subaru GRB

0:55 onwards

subaru OEM intake scoop seems to reverse its flow (!) to the outside

wasnt subaru aware of that? how can that be? :confused:

Interesting!

I don't have an answer to your question but I've measured underhood intercoolers that either have no airflow through them (Peugeot 405 diesel) or reverse airflow (my initial underhood intercooler installation on a Nissan Maxima V6 turbo). The other day I was looking at a current turbo Mazda that has an underhood intercooler and no outside ducting to it at all!

Remember that in most road car applications, the intercooler acts as a heatsink basically as much as it does as a heat exchanger - and as a heatsink, it can transfer air back to the intake air when off boost.

Incidentally, for those looking through this resurrected thread, there's a fair amount of misleading information in earlier posts.

Yes they were aware of the interactions of the hoodscoop. On my 06 Forester XT they refuse to install a bug deflector Stone catcher citing that this device changes the Dynamics of the flow over the hood creating too many factory warranty claims from Turbo's overheating and failing due to this disturbance caused by the $250 factory option now(07) only available for naturally aspirated models.