Generally, no. Because of airflow through the radiator opening, most production based automobiles develop a higher pressure in the engine compartment relative to the airflow over the hood. Auto manufacturers are making a lot of progress in reducing this pressure differential through better flow control within the engine compartment and reducing radiator opening sizes.

Also, its important to define where within the engine compartment and hood we are talking about. The forward part of the engine compartment tends to have a slightly higher pressure then the rear. That's because most auto manufacturers exhaust engine compartment air under the car, behind the front wheel centerline. Conversly, the air pressure on the hood increases near the back of the hood as the air packs up at the base of the windshield.

 

I think you may be right about the forward portion of the hood - not disputing, but I found the article I remember reading about stock car aerodynamics (link). Here is a quote:

Now that I re-read this though, I don't understand their explanation as to why the air under the hood would move faster (lower pressure) than the air above the hood (higher pressure). Maybe it's a misprint? I would think it's just the opposite since there is less resistance to flow above the hood (more velocity, less pressure). Need to think about this more...

 

You can't compare the aerodynamics of a purpose built race car with a production car. Despite the term "stock", there's are hundreds of thousands of hours in CFD simulaiton and wind tunnel testing behind the aero management on these cars.

Nissan has done a number of things to improve the engine compartment aero. The wider front of the car and the square cut off at the wheel wells helps create a low pressure area in the front wheel well. This increases the flow of engine compart air out of the compartment into the wheel wells.

If you look at the undertrays you'll see a number of small tricks to induce vortices which drops the air pressure under the front of the car. This gives a little more front downforce and, depending on where the vortices trail off, might also help pull air out of the engine compartment.

Some of the things we can do to help is to increase the low pressure area in the wheel wells are: Adding fender top vents, limiting the amount of brake cooling air to the minimum needed to cool the brakes, front wheel well Gurney lips, and opeing up the plastic undertrays separating the engine compartment from the wheel wells.

 

RC-
if you want more front end downforce there are alot of things you can do, raking the car, adding a front splitter, varying the pitch of a canard, elongating the nose, adding an undertray with a swept up diffuser, etc.

as for the physical forces pushing down on the hood/car, there is a good pict i found in my Z magazine i scanned on the nismo Z..



red is higher pressure, blue is low (i think. i cant read japanese, i'm just trying to think logically)

where the vents go, on the hood, there will be high pressure in that relative area, hence i put the gurney flap there to direct the flow upwards, also the vents are louvered in reverse so whatever airflow is hitting the vents, will travel up and out, and not into the engine-

if you're lookin to vent directly INTO the engine and force air in, you might want to look into NACA ducting if you're trying to direct air into your intakes or what not-, typically high pressure areas, air flow will go wherever you direct it...

 

Most CFD systems show higher relative pressures in blue and lower relative pressures in red.

 

picked up the new thinner vents from stan just now-
with the holes cut out . looks much much better than the first set-


will post picts later tonite-

as for the pressure color. then that much high pressure behind the car in the wake?

 

Actually, I'm not sure anymore. A couple contradictory examples:

http://www.mulsannescorner.com/techarticle2.html
http://www.mulsannescorner.com/techarticle1.htm

 

^^

i guess ill have to find someone japanese to read the chart. haha. yeah a legend on the graph would have helped..

thought: wouldn't u want a low pressure under the car as to suck the car down for less lift?

 

Pics and whats the word? price? is this happening i know a couple here are interested but its taking forever lol.

 

You want low pressure under the car. That's why the bottom is deep blue and green. A clue to this is that the front of the tires are red

 

Looking forward to the pics Man.
I appreciate all the work your putting into this idea.
This is exactly what I'm looking for.
If its possible, could you take a pic with one of them maybe sitting on the hood just for reference as to its total size?
Thanks...

 

pricing wise, i'd like to this price target low. it is labor intensive for the guys to cut all the vents out.. how does this sound:

non cut FRP louvers, ~$120.
cut FRP louvers, ~$140

if you wanna beta test for me. PM me. i might be able to cut you a deal...

for dry carbon pieces that what stan is telling me would weigh a few ounces (less than a pound each), would be a little more pricey- ill find out about it.

still keeps overall cost cheaper than the current line of vented aftermarket hoods, and this way you guys can keep the stock hood latch, not use pins, plus get cooling.. and the labor at a speed shop to cut open the stock hood should be around $75-100.


picts coming later tonite.. on and off my stock hood-

thanks
--cheston

 

Cheston, do you know if there will be a template for this? So we would know where to make the cut in the hood.

~Robert

 

Pricing sounds good to me...very reasonable...

 

I will be making all the instructions for installation, as well as the cut out template...


the prototype edges are little longer, since i didnt know how close to cut it to the edge- i think we might be leaving at least a 3/4" flange all the way around.

dimensions are approximately 19.5" long (at the longest point) and 10.5" wide (at the widest point). Gurney flap/edge in the front is approx 1/2" tall, the rear louver is 1.25" deep, the middle louvers are 0.75" deep.

here are the picts of Prototype2.. much more flexible. its stiff but rivetable to the hood and will conform.







ontop of the hood.. these are the approximate areas where i would suggest, just make sure the rear vent clears the strut bar.. from the looks of the underside, it does not go that much 'deeper' than the factory support bar thickness.








on the bottom. (note where you will need to cut and remove material. should be fairly easy. the factory skeleton underside is just glued/pressed together to the top..

 

Love what I am seeing. I hope my Crawford Strut Bar won't be in the way. I can only tell once it is done. No big loss if I have to remove the Crawford. Rough estimate these will be made to the public?

~Robert

 

they're cleaning up the mold to make both sides of the louvers more symmetrical, and i need to finalize hole sizes... and i'm getting married in a week. so these will be available in April for sure.

if you guys are interested: start a list, and i'll take a $50 deposit.. PM me if you'd like...

 

hmmm thats a decent chunk out of the factory skeleton bracing? im curious to see if that has a effect on the hood. and for 20 bucks more hell yeah i would have them cut them

 

grats bro

Just enter the world of dating again and I guess crashing your wedding would be a good start in meeting people

~Robert

 

Geez I did not read the whole thread but they just look like EXACT copies of the CWest vents. Did you get them from Adam or something? Seems like a huge PIA to cut the stock hood to accomodate them though.