Underhood Airflow Management

[scopx]

All:

I have scoured the web and this site to gather info concerning engine compartment (EC) air volume and pressure with less than rewarding results. And, of the opinion (no basis in fact), that even at moderate speeds on a vehicle with average exposed frontal area , the EC becomes "stuffed" (pressurized) and the possible air flow is reduced by lack of escape route. The search here brought up two references, one about hood pushing up against fixture pins at speed. I would suspect that those that go fast for a living have done extensive research at great expense on the subject, but understandably hold their findings close.

My application is not for a missile seeker, but to a tow vehicle where behind grille area is heavily populated. My intent is to install "somewhere" a double pass intercooler of significant size to satisfy cooling requirements. The EC is pretty much stuffed also.

So, having said all that, it would appear my only remedy would be to place (freestanding) cooler above the engine, which would require cutting a portion of the hood, utilizing a scoop. However, if my premise is correct, it would be necessary to face the scoop backwards, as the positive airflow underhood, would, to seek equilibrium, actually be exiting out the top of the hood and over the windshield! (I am aware of the effects of cowl induction and high pressure area at base of windshield.)

Anyone have any ideas, hard info, discussion on relevant issues, directions to white paper to either prove up, or discredit my premise?

Some may notice this is my first posting. I have much to learn. And, find, the more I learn, the less I know.
Tom

[Kenova]

A front spoiler will create a low pressure area behind the rad and help draw out the hot air. The closer the spoiler is to the ground the better. Placing hood vents (similar to what GM has used on several cars) in the low pressure area just after the leading edge of the hood will also draw out some hot air and reduce the high pressure under the hood.

Ken

[S.Bretz]

If you do a double pass, your best might be to have a 2 layer system...one core in front of the other. Have the hotter mass enter the rear core first, then have the second pass as the front layer. This will help mantain a later temp differential between the cooling air and the fluid/air you intend to charge.

There is a 'happy spot' between length and height of a cooler.
If you where to take, say a radiator, and use the top 1/2 as one pass and the bottom 1/2 as the second pass...the 2nd pass will have minimal cooling and you reduced the flow rate of the system by halfing the amount of rows/passengers that the fluid can take.


You say its a tow vehicle....I'm assuming a truck. If so, have you thought about using the areas behind the corners of the front bumper? ...around where a fog light might go? You could use a pre-cooler there with maybe a fan on it before the main intercooler.

[scopx]

Thanks for the replies. Perhaps I should expound on subject.
The underhood and frontal areas of the old F250 are stuffed with the 7.3 components. And considering larger condenser to satisfy 143A freon conversion requirements along with the 22K (cement mixer style) hydraulic cooler used for the AT, space is at a premium. Under the bumper is a one quart power steering reserve/filter. In other words, there ain't much room. It was necessary to trim-rework grille to get mentioned to fit.

My musing led me to a possible solution and therefore posted question. IF IN FACT THE UNDER HOOD AREA has a positive pressure status at road speed, why not mount cooler (freestanding) horizontal over the engine, cutting out portion of hood for air to escape with open section scoop pointing to rear of hood to create low pressure area. This would reduce the 3.0 in piping requirements from about 10 feet of multiple bends to about 3 feet with fewer complicated bends. Due to confined space, it will be necessary to have cooler partially protruding thru hood plane and redoing hood support inner structure. As most cooler components are mounted with radiator hose or V clamps, removal would be easy to allow access to motor.
My thought was to fab alum scoop that would be open on both ends with divider. The front portion for cool air intake,a partition, and then the rear for hot air (cooler) dump exit.
My humble understanding of the whole process is there would be heat soak of the cooler at idle, but there is no load, and with the natural "rising" of heated air, this would actually help unload the engine compartment of heat when stopped.
Am sure that someone has studied under hood air flow and has knowledge to share on this subject.
Thoughts Guys?

[MadBill]

Sounds like it could do a credible job of venting the underhood area(and of course a number of factory high performance vehicles and race cars do the same), but as far as cooling, keep in mind that's all hot rad exit air you're trying to cool with.

[RCJ]

I don't think the pressure on top of the hood is uniform .Any ideals on how you can check the pressure of areas around the car?And when we say a wing has a higher pressure on the top than the lower side ,is this barometric pressure

[MadBill]

You could build a simple U tube water manometer (Google' U tube manometer') and run a tube from one leg to to various locations to compare. Blocking the end of the hose and having several small holes around the diameter should cancel out most velocity-induced errors.

[2seater]

The U-tube manometer can be used in differential mode too where each leg is placed separately, like above and below the wing in your example. It will give the pressure difference directly. Simple device with many uses. Clear plastic tubing taped to both sides of a yard/meter stick works well. A couple drops of food coloring in water helps visibility plus one drop of detergent to break surface tension.

[S.Bretz]

I think its Kentmoore that makes a gauge that is very accurate in low pressure situations.. It read something like 0-3psi but on a very wide sweep gauge. It is in another pressure unit, and I can't remember if it kilopascal or mbars...I dought its in torrs.

Any way, I think you would be better off using a forward facing scoop. If you use a rear facing scoop, you will be drawing hot air thru a core that you entend to cool. If you are making insance amounts off boost and have charge temps in the 300-400 range right after the turbo, the cooler with a rear facing scoop will help, just not as much as a forward facing one. A cowl scoop as close to the base of the windscreen glass would work nicely too, and placing a good fan on the bottom of the core to move some air while at stops wouldn't be a bad idea either to help prevent the hot air from rising up thru the cooler in traffic.




Another thought. You can make the lines for the a/c condensor longer and rear mount that some place. You will have to add more 134 to the system to get hte pressures where that are happy, but that will free up some prime real estate up front.

[MadBill]

I think its Kentmoore that makes a gauge that is very accurate in low pressure situations.. It read something like 0-3psi but on a very wide sweep gauge. It is in another pressure unit, and I can't remember if it kilopascal or mbars...I dought its in torrs....

The pressures are very low in this situation; only ~ one tenth of a psi (~ 3" H2O) maximum at 80 MPH...

[mystic1582]

What body style F250? 80-97? 99-up?
Why not just use a Banks type intercooler? Available and it works?
Would a suggestion to place a sizable airdam under the front bumper to deflect air around the truck be better? for fuel conomy/aerodynamics?
I'm guessing the hoods on these trucks already have ennough leakage areas, like fender wells, front of door/fender gaps, hood gaps etc to equalize any pressure.
Also I'm thinking a truck and max speed of mabey 80 w/trailer is not enough to generate any sizable under hood pressure?
I know a lot of people dislike the air dam on the 2011-present Superduty but i'm confident it helps this brick cut through the wind.

[scopx]

Good ideas All:
In review of all the responses, and pondering on all, have come to conclusion the best solution in this instance is to

1. install front dam, frontal area gaps such as between core support and rad are shielded on front side forcing air into rad

2. vent areas behind core support to lower pressure, have removed fender apron extensions (which are heavy ~1/8" rubber panels)to cut down down on noise

3. fab front facing scoop for incoming cool (precharge) and double pass intercooler (freestanding horizontal mount),

Mystic 1382
Banks does not list a cooler for the old 91 F-250. The listed are all front mounted and still has all plumbing nightmare. Will be using air dam as you suggested, Yes is old brick at that. Also changing out diff ratio to 4:10. Present 3:55 results 1950 RPM= 70MPH in top gear! Good at road speed, but really pooches belly button out when backing on soft soil when trailer loaded.


MadBill S. Bretz & 2Seater

I did not know Kent Moore was still in business. Perhaps is another KM as remember them in the 60's providing auto body shop aids.Will ponder manometer experiment for another time.

RCJ

Yes as slow as this will be going, think there will be lot of turbulent, but think 3" high intake should suffice to provide positive flow.
NASA website has wing simulator program to play with that gives one idea of flow over various configs. Find attached link for any interested. Can invert/alter shapes to use for auto down effects with little adjustments to program.
http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html
XXXX
The down side to this configuration that when stopped/idling air flow may be negative and engine compartment air may be backing up as MadBill said. (I have not had good luck with mounted electric fans.)
My humble understanding of the charge cooling premise indicates double pass non stacked coils type cooler to be more effective than radiator "selective flow" type at speed.

Am sure have forgotten to address all areas and issues,but will find when installing.
Thanks to All.
Tom

[mystic1582]

Scopx
I have a 95 F350 dually 7.3 DIT. It came with a/4.10:1 gear. Had 330k miles on the clock, old, worn out (6 cylinders had 410 psi and 2 had 375 psi compression) and mostly stock. I use this for towing a 7-8,000 lb trailer, mostly highway.
It would consume a great deal of fuel and rev pretty high @ 60-65 mph...like around 2100 rpm and have trouble keeping speed.
With some pretty basic modifications like injectors, tuning software, free flowing exhaust and intake it will produce 600lb-ft pretty easily without an intercooler (air charge cooler). I added the Banks kit to mine primarily because of the overfueling issue, black smoke and high egts. In reality I think the injectors needed another look. I went through them but never had them flowed or matched. I think this is a big issue. aS well as a weak injector pump (HPOP) which since has been upgraded w/an Adrenaline pump.

Since increasing the output of the motor i switched to 3.55:1 gearing and empty it gets around 20mpg average and 12-15 loaded @ 70mph cruising @ about 1900 rpm.
I'm in the process of building another motor, doing some port clean up work, new flowed injectors. I fixed an injector the other week; an internal spring broke inside the intesifier piston. Someone gave me biodiesel to run as well and I could not believe how bad the fuel section of the injector looked (had to replace a fuel tank and go through about 5 filters, fuel pump as well so who knows what was in that crap). The fuel rails must be poor looking as well. I'm tired of hanging over the fenders, and chasing leaks and throwing parts at this thing. I'm building the new motor and being done with it.
I also want to fabricate a decent sized air dam to attach to the underside of the bumper, or even try a 2011-up plastic air dam to see how well it fits and wht it takes to make it work.

[Bubstr]

I won't get into inter coolers or injectors but air flow management in the engine bay is determined by air in and air out. The differential of flow in and out can create high or low pressure in the engine bay.This is the key to cooling and front down force at speed.

There are two main inlets to flow. Frontal area openings and cowl or scoop openings. There are two main outlets to flow, under car and front area of wheel wells. These are separated by your core support. The core support is the area you put your cooling devices, radiator, oil cooler and A/C if applicable. The trick is to create low pressure in the engine bay. This is done by limiting flow in by air dams and sealing cowl and hood scoop off and promoting exit flow with rear wing and shaping fronts fenders to create venturi effect. Look at a super late model front fender or NASCAR fender. The edge at the front of the tire is wider than the tire and the rear wheel opening on that same tire. The flow over that, creates a sucking of air from engine bay. On some of these, it can create up to 700 lbs of down force. This low pressure not only creates down force it pulls air threw your cooling system.

Sealing is the key to trying to get a balance of in and not sealing is key to out flow. You do not want to restrict air flow at the cooling devises. A big 6 core fine finned radiator is a hindrance to effective cooling, rather than an asset. If you want more air threw, you open frontal area intake and increase low pressure at exits, always maintaining a low pressure. Look to air dams and fender flairs even side step flairs in the case of a truck, making a rear wing more effective.