davek9

Looked through a boat load of the Hood Mod threads, and chose this one to reopen, and it already had an Air Flow chart

I've bee looking at ways to get cooler fresh air to the SC or a stock Intake system for that matter by modifying an easily removal-able and replaceable part (like the hood).

Has anyone tried cutting off the front lip of the hood (about 1") leaving the latch and that support structure in-place, then opening up the underside structure and creating a cavity and venting it to the area where the stock Air intakes are.
The "look" i'm going for would be somewhat like one would find on a newer 911, pulling Air into the front edge of the hood.
Looking at newer performance cars with "cold air" intakes, most of them utilize some sort of scoop to an under hood cavity and the air box connects w/ that, leaving all the channeling on the bottom of the hood.

Thoughts on this, would it work?

Thanks,

Dave K

Edit: don't think this would increase any down force, could help w/ cooling, other better titled threads were closed

GT6ixer

Hey Dave. If you look at that pressure distribution chart where the values go negative, this represents a region of low air pressure (i.e. suction). Your idea, if I read it right, would put the opening about at point #5 on that chart, an area of pretty good suction. The "scoops" in this region would need to be NACA ducts. NACA ducts are a tried and true way to introduce air inlets in negative pressure regions like this with relatively low drag compared to a ram air scoop that sits above the boundary layer. The best position for ducts like these on the hood would be at position #7 since there is a little dip in the suction there. This looks to be in generally the same location in the ducts in post #36.

mark kibort

again, the entire concept is based on venting the incoming air to the hood area, about same horizontal as the middle of the front half of the front wheel. if you want a higher pressure zone, you need t be very close to the hood latch area or front edge of the hood. anywhere near the radiator top and you are going to be fighting a lower pressure region vs other spots on the hood . the high pressure regions are at the base of the windshield and again, the area around the front half of the front wheel . extending the stock intakes horizontally to the front edge of the hood, might look ok and be function. in looking at it again, you need to grab air at the nose, not the hood area at all. its all lower pressure there back to the rear of the front tire. you would be better off grabbing it from the wheel wells or infront of the radiator , and not make any hood mods. my hood mods were functional downforce, increased flow purposed.

ptuomov

mark kibort

good luck with that....NACA ducts work on areas of laminar flow. it does create a slight pressure differential of the surface flow, but it doesn't change the fact that that flow pressure vs the nose or base of windshield pressure is 4-5 x higher. because of this, this is strictly for looks and would not provide gains, and certainly would provide loses. it would be MUCH better to use the stock intlets fed by the higher pressure nose area. ive done measuring of this and the pressure is retained without much loss even right above the radiator, but under the hood. think of it like this. you are putting naca ducts in a low pressure zone. the zone pressure doesnt change, even if you can entice flow into intake tubes.. but because the pressure is so much lower in the intake tubes due to the pressure differentials caused by the intake stroke of the pistons, there would be no gains and only losses compared to an area that has higher pressure to begin with. the nose can have near .11psi at 80mph.. the front hood area has a pressure less than ambient.

as another data point. if you didnt connect the intake tubes to the naca ducts, flow would STILL be forced out of them (not pushing into them) , which would tell you something about their effectiveness. the faster you go, the worse it would be.

there is no cavity to be filled that isnt happening already by the lower intake louvers in the bumper. if you measure pressure, the face of the radiator is near what the the nose of the bumper sees. again, about .1psi at 80mph and near .3psi at 150mph

its counter intuitive, just as the base of the windshield is the best source for high pressure air to feed the intake, but it changes direction of incoming air.......the opposite is true to for the forward facing naca ducts.. they look like they will trap air, but in reality, they will release what is behind them, which is higher pressure than air moving over that section of the hood. this is very easy to test with tufts. (and ive done a lot of this type of testing)

GT6ixer

I agree with Mark on this one, and I'd be surprised if that Strosek design did not have the inlet connected directly to the opening of the NACA ducts. NACA ducts were developed for aircraft which travel at significantly higher speeds than cars. Because of the airspeeds involved simple ram air scoops add too much drag, thus the reason for NACA ducts. On street cars where more power for better acceleration at lower speed is almost always a premium over absolute top speed, a ram air scoop placed in a high pressure zone would be the best way to increase HP if looking to redesign the air intakes. The highest Cps are at point #3 according to the graph above. Point #3 is where the air inlet is now. However it uses the engine bay as a plenum to pressurize as apposed to directly feeding the intakes. Bringing intakes to that area would require a lot of rearranging I imagine. One way to increase the engine bay pressure would be to completely seal it off with a formfitting bottom tray, seal the hood gaps and plug any holes in the inner fenders and firewall. Of course doing all this would cause a big issue with cooling and therefore is not practical. Alternatively a ram air scoop at base of the windshield is in a region that only provides half of the pressure recovery of the nose region and I'd venture to guess equal-to-or-less than where the stock inlets are at now.

One idea that is kind of out there would be to design ram air inlets that take the place of the flag mirrors. Those mirrors are high drag anyway with the flow stagnating on the front side of them now. A u-duct would need to be used to bring the ram air back to the inlet over the top of the fender and into the rear of the hood. It'd be a pretty funky looking set-up.

Another wacky idea would be to use the headlight buckets as ram air scoops and blend a duct off the back side of them through the hood and directly into the existing air inlets. You could then put new headlights (probably illegal) in the front bumper. And if you where really inclined, you could design the ducts to lay flat in the existing headlight cut-outs and then when you get challenged at the stop light just turn the head light switch and deploy the POWER!

ptuomov

Well Kibort will probably occasionally disagree even with himself, if he reads his own posts as carefully as mine.

I think that the best place for inlets is in the bumper cover or at the intersection of the hood and windshield, that the best place for outlets is in the wheel wells or about in the middle of the hood, and that other things can be made work with things like NACA scoops or diffusers but if you aren’t fighting the pressures to start with everything else is easier.

In terms of the Strosek design and NACA ducts, I suspect that the air is not flowing out of those NACA ducts. This is because of the following reasons. First, NACA ducts are reasonably effective in converting the kinetic pressure into static pressure even when installed on a surface that is parallel to the flow and has no angle of attack. They aren't as effective as scoops sticking out, but reasonably effective. Second, and more importantly, the NACA ducts have a non-zero angle of attack in the Strosek design, which helps a lot. Steeper the surface on which the NACA duct is placed, the better it works. Third, the low pressure created by the belly pan and by the engine intake will make the air flow in thru the Strosek NACA ducts even if the stock bumper cover inlet converts kinetic pressure into static pressure more effectively than the Strosek NACA duct. Fourth, the air flow from the stock bumper cover inlet thru the radiator and engine air inlets never completely stops so it will always have some kinetic pressure pointed away from the NACA ducts. For these reasons, I suspect (but don't know) that the Strosek NACA ducts are functional in that they flow air in at speed.

mark kibort

ptuomo
I read your post carefully, and am in no way disagreeing with "myself" or you on the fact that the base windshield area and bumper nose are the highest pressure levels. Keep in mind, discussing , even simple aerodynamics here on a internet board can be a little difficult, so ill try and keep it simple.
you understand that the midde of the hood area is a low pressure zone compared to ambient. (flow around the car, under the car, etc) belly pans by the way, do not inherently , lower pressure under the car, they reduce turbulence, and if you measure the flow under the car , you will see that hat flow is very close to ambient pressure. the key thing to look at , is the differential of pressure from top to bottom of the car. (down-force discussions can be even more involved, so lets stick with the fact).... yes, there is flow though the radiator that is high pressure, near nose pressure and it has pressure drops along the way . the engine compartment is pressurized too and the release of this flow is usually under the car, raising pressure underneath. any hole in the hood, regardless of its design , will be an exhaust port. the differential pressure is too great. now, the intake ducts might be attached to them, , so the inlet pressure could be raised vs no NACA duct being present, but the comparison to under hood pressure , or base of windshield pressure would be much higher than its ability to create a differential pressure, locally. in other words, the pressure source of the inlets will ALWAYS be lower pressure, than the nose, windshield area, or inside the engine compartment , near radiator. picture an extreme case of a naca duct on the side of the airplane cabin . if the airplane cabin is pressurized, the naca duct will vent air if it was open to the cabin. regardless of speed. if the cabin was not pressurized, then it would draw air in... sure, not as effectively as if it was located at the nose of the airplane. in the car hood example, it would draw air from a low pressure source, but the source will always be a lower pressure sourse, regardless of the shape of the naca duct inlet vs most anywhere else on the car. so, no, the air cant flowOUT of the NACA ducts, because they are attached to a flow pressure MUCH lower than the air around the naca ducts. (the engine air flow ) BUT , the sourse of the flow would be lower than any place else on the car , generally. In other words, this is a bad design for a car's intake inlet,although it looks good,it is not functional. the amount of HP loss would be proportional to the difference in pressure.found at the nose vs at the middle hood area.

you confuse the design of the NACA ducts and its ability to create pressure differential and flow, with overall pressure differentials. another way to compare what you are saying is to look at a hoodscoop,that actually faces flow... if it is in the lower pressure range, and there is higher pressure below it, it still will act as an exhaust porrt. Air moves in proportion to differntial pressure. some of the intuitiveness is tossed out in most cases.

read my post "carefully " and let me know where you think there is disagreement . especially the prior post which i just re-read and I stand by the descriptions i made. right or wrong, i dont see the contradictions you allude to

ptuomov

Pressure questions are usually questions of magnitude. If the aircraft cabin is pressure-normalized to certain level with a pressure regulating outlet in the tail, the example you use, whether a NACA duct flows in or out of the cabin absolutely depends on the air speed at which the aircraft is traveling and on the altitude (i.e., atmospheric pressure) at which it's traveling. How could it not depend on those parameters? Say if the aircraft is flying at 0.7 mach at 200 feet from the ground (a helluva ride that would be, talk about riding a cruise missile) and has an cabin pressure regulated from the tail, then how could the NACA duct flow out if the cabin is pressure normalized to ground level? If I read your post literally, I think I have to disagree.

I have to disagree on the hood scoops, too. You say "any hole in the hood, regardless of its design , will be an exhaust port. the differential pressure is too great". I say that if you take the spot on the hood that per your graphs has the lowest pressure and put a raised, forward facing scoop there, it will flow air into the engine compartment at high speeds. This is because cars that have functioning radiators have a meaningful total pressure drop over the radiator and thus a relatively low total pressure under the hood compared to the total pressure that the forward-facing hood scoop can harvest.

mark kibort

I said "pressurized " cabin. this means relative to the outside, so talking 15,000ft or higher in the example.. the point was to show that a near 7 psi differential pressure vs the naca duct would force air out not in through the duct. disagree there? In your example... yes, thats where a naca duct could work and does work in areas of the fuselage that are not at higher pressure than the surrounding air flow or what is induced by the naca duct.

now, you disagree on hood scoops not being an exhaust port? you are loading your conditions with a "hood scoop" being "raised" . out of the boundary layer , you will get a pressure change that could equal or be even higher than what is in the engine compartment.... i can easily make this test for you and with tufts and sunx sensors, give you actual values. what i have seen is not even close. air is pushed out of MY hood vent with authority. tufts nearly stand straight up out of the hood! how is that possible? because the pressure differential is SO great. ive provided the values, but you seem to ignor them.
now, you make MORE assumptions of what the pressure drop is across the radiator. you are incorrect there. there is a high pressure value even after the radiator pressure drop . you said over the top, and the radiator has a functioal seal above the radiator to allow pressurized air to build and feed the current intakes... Ive measured this too. if not sealed, the air flow would divert from the radiator and go over the radiator. So, with it, the high pressure air goes through the radiator and with a pressure drop, not much , it pressurizes the engine compartment. i have measured this too. you see, you are guessing and ive measured. so, if you are interested, i can gather my data and post it, or conduct the test with an intake scoop 1" above the hood and show that air actually is forced out of it,and not into the engine compartment. you are going off intuitiveness and not using engineering logic. the pressure over the hood is much lower than in the engine compartment, even after the pressure drop through the radiator. i can provide those values. if true (and it is) the flow will not go into the engine compartment , it will exhaust because thats how flow works.

mark kibort

example:

ptuomov

mark kibort

Funny.. the pics are just pointing to manufactures that know the pressure zones, putting vents to help with air flow out of the higher pressure engine bay, fed from radiator flow. Now, if you are comfortable with what you wrote, then basically, you need to show that there is a way to take lower presure air, and force it into a zone of higher pressure. or do you doubt that pressure is not higher under the hood than on top of it. and if you agree, you might say that the NACA duct, even forward facing will have enough molecules hit it, as to cause a pressure rise, higher than the pressure below. i am willing to fabricate anything you think might prove this to yourself. So, if you think that pressure is generated by a forward facing duct , i should be able to put a simple solid fence up (a 1" plate maybe the width of the inlet that i have) and measure pressure on it. i can do that test. currently, i have measured the differential pressure under and over the hood while sealed and over the hood is much lower than under the hood in that region. how high you need to make a duct over the hood, to get high pressure is the question. im guessing a couple of " might do it . but 1" , i dont think so.. and certainly a NACA duct , i dont believe can overcome the pressure of the area underneath the hood in that area. you have to remember,ive had sunx sensors all over the car with tufts and video of the tufts. Nothing seen or measured was anything other than projected, or what im saying here.

have me do a test to try and validate your perception...... i have no problem with doing a test for you. I can put a scoop near my hood vents and i can put tufts before it and see if they are sucked into the scoop or are forced around it. i can video this with a gopro camera too. As i have done in the past with other tests.

ptuomov

This I believe is functional, for example:

mark kibort

yes, i was going to post a couple that looked like that.... some of these are functional and some are not. this one is for several reasons. out of the boundary level slightly, and the shape of the car has not a lot of low pressure area zones on top of the hood. we are talking about the 928 and i can pretty easily show what would happen with NACA or hood scoops in the area we are talking about. a key think to also understand, is that the speeds most cars are driven do not benefit from any ram effects.......after all, we are talking about.08psi in the most efficient hood scoop possible, at 80mph... that's less than .5% or 1.6HP on a 300hp car!

[QUOTE=ptuomov;15241110]This I believe is functional, for example: /QUOTE]