NACA HOODS FOR '71 TO '73 MUSTANGS. CORRECT OR INCORRECT!

[Ron Tanzi]

I read that article. I posted a comment Re: Why our cars were left off the list. Several others commented that our hoods are not true NACA inlets because they are not flush to the surface. I fired back that our car look better than the 69 Shelby they chose. I always thought the 69 Shelby's styling was too busy.

Ron

 

[tony-muscle]

I also read that article and was surprised to not see out NACA hood mentioned. I understand the theory about the surfaces being flat but if that's the case I agree that the Shelby will not count and neither the R5. I posted a comment in response to the explanation from NovaResource.

I have the capability to open/close the RAM air louvres on demand, so in my own experience, based of my fuel injection live screen, I see no difference in manifold pressure when I open or close the louvres. What I definitely can see is a big difference in intake temperature. In hot 90 degree days I have seen a drop of up to 10 degrees in inlet temperature a couple minutes after I open the RAM air. From what I have read, at highway speed you may see a 1 psi increase in a good day from a non-NACA air scoop. That said, I don't care, I agree that they look cool and they are functional because they can make an impact in reducing intake temperature.

 

[BigBlue]

I hate to say it, but the author is right. Our ducts are not NACA ducts. It was great marketing by Ford, but what we have are hood scoops. Not only do our scoops not have the correct triangle, ramp, and inlet shapes, but they also interrupt airflow, which goes against the point of a NACA duct in the first place. The point is to draw air without disturbing the boundary layer flowing over the surface. Ford's scoops protrude above the airflow of the hood line, especially toward the outer edges of the hood. What we have are sleek hood scoops, but scoops nonetheless. Hey, at least they're functional But calling these NACA ducts is like saying a 428 Cobra Jet is a fighter plane engine. It's a fancy marketing term to get people to cough up more money. That said, the author also got it wrong with the '69 Shelby. Those are also not NACA ducts, but they're a hell of a lot closer to the real thing than what we have (though I'd argue ours look cooler).

 

[tony-muscle]

I hate to say it, but the author is right. Our ducts are not NACA ducts. It was great marketing by Ford, but what we have are hood scoops. Not only do our scoops not have the correct triangle, ramp, and inlet shapes, but they also interrupt airflow, which goes against the point of a NACA duct in the first place. The point is to draw air without disturbing the boundary layer flowing over the surface. Ford's scoops protrude above the airflow of the hood line, especially toward the outer edges of the hood. What we have are sleek hood scoops, but scoops nonetheless. Hey, at least they're functional But calling these NACA ducts is like saying a 428 Cobra Jet is a fighter plane engine. It's a fancy marketing term to get people to cough up more money. That said, the author also got it wrong with the '69 Shelby. Those are also not NACA ducts, but they're a hell of a lot closer to the real thing than what we have (though I'd argue ours look cooler).

What about NACA-like hood scoops?

 

[BigBlue]

What about NACA-like hood scoops?

Bingo!

 

[Austin Vert]

Hi again,

Thought i would jump in here and give my final verdict on all the great feedback and comments put forward by the Forum guys so far. Many thanks there!

Regards my two questions - i'll start with the second one first up. (NACA OR NASA?) Correct?

So history shows that the organization started out as NACA, but got name upgraded to NASA many years later. The invention and product was founded originally under the NACA organization. In that sense, calling them today, NACA ducts is perfectly 100% legit, just as calling them NASA ducts today is 100% legit and justifiable as the organization officially upgraded its title to NASA. So bottom line here is changed organization name, same invention/same product. There is no correct/incorrect name title thing going on, as both are correctly applicable. Call what ever title suits you. Case closed Doctor Watson!

Re the first question --------- Are our 1,2,3 hoods really NASA hoods?

I have reached the conclusion that they definately are. Why? Ok, the true original classic shape of a NASA duct when viewed from looking down, is essentially triangular shaped with the drop down happening on the bottom floor of the duct. This drop down floor creates an open slot design for the incoming air flow.
See my photo of a classic NASA duct. That said, it would be fair to say that most NASA ducts used on cars feature mounting the ducts on a flat or slightly curved styled panel. However, if the duct mounting design incorporates a raised profile section above the back of the duct, then that is perfectly acceptable as well, and can still be called a NASA duct or hood. ( See my photo of the Shelby with the three ducts in the hood. The center duct is flat mounted, and the other two are raised mounted. All three ducts are called NASA ducts.) I have included another photo of raised duct panel mounts design.

So getting back to our Mustangs, our ducts are based on a raised profile hood design, with a slight variation of the flat mounted original, using a stylish offset triangular delta shaped entry floor. There is still the classic air entry slot happening. There can be no mistake. Our Mustang ducts can definately be called NASA ducts, and so Ford was fully justifiable in calling them NASA hoods. Raised or flat panel mounted ducts, they all come under the banner of being called NASA hoods. (Review all my photos for evidence) For example, my photo of a '71 Dodge Challenger hood shows the use of hood air scoops, not NASA air ducts.

The difference being that the entry shaped floor is not delta triangular shaped the way a NASA duct is designed. Our Mustangs have the classic delta shaped triangular entry floor, and so, are a true NASA duct design with a slight variation twist to the offset entry floor compared to the classic original semetrical design. Final conclusion - 1,2,3 Mustangs come with optional NACA/NASA hoods. Case closed Mr Holmes!

Thanks again guys for all your great input to help solve these mysteries and put it all to bed. If you disagree with any of my conclusions, please feel free to comment.

Reference .............................
Photos 1. Example of an air scoop not a NASA duct. 2. Example A - Front view of our Mustang NASA hood ducts, (note offset triangular floor entry design) 3. Example of an original NACA air duct. 4. Example of flat mounted NASA air ducts on a Euro car. 5. Example B - another pic of the front view of our Mustang's NASA hood ducts. 6. Example of the Shelby with the three NASA air ducts, flat and raised design. 7. Example of a raised NASA hood duct design on a Euro car.

Greg.

 

[BigBlue]

Hi again,

Thought i would jump in here and give my final verdict on all the great feedback and comments put forward by the Forum guys so far. Many thanks there!

Regards my two questions - i'll start with the second one first up. (NACA OR NASA?) Correct?

So history shows that the organization started out as NACA, but got name upgraded to NASA many years later. The invention and product was founded originally under the NACA organization. In that sense, calling them today, NACA ducts is perfectly 100% legit, just as calling them NASA ducts today is 100% legit and justifiable as the organization officially upgraded its title to NASA. So bottom line here is changed organization name, same invention/same product. There is no correct/incorrect name title thing going on, as both are correctly applicable. Call what ever title suits you. Case closed Doctor Watson!

Re the first question --------- Are our 1,2,3 hoods really NASA hoods?

I have reached the conclusion that they definately are. Why? Ok, the true original classic shape of a NASA duct when viewed from looking down, is essentially triangular shaped with the drop down happening on the bottom floor of the duct. This drop down floor creates an open slot design for the incoming air flow.
See my photo of a classic NASA duct. That said, it would be fair to say that most NASA ducts used on cars feature mounting the ducts on a flat or slightly curved styled panel. However, if the duct mounting design incorporates a raised profile section above the back of the duct, then that is perfectly acceptable as well, and can still be called a NASA duct or hood. ( See my photos of the Shelby with the three ducts in the hood. The center duct is flat mounted, and the other two are raised mounted. All three ducts are called NASA ducts.) I have included another photo of raised duct panel mounts design.

So getting back to our Mustangs, our ducts are based on a raised profile hood design, with a slight variation of the flat mounted original, using a stylish offset triangular delta shaped entry floor. There is still the classic air entry slot happening. There can be no mistake. Our Mustang ducts can definately be called NASA ducts, and so Ford was fully justifiable in calling them NASA hoods. Raised or flat panel mounted ducts, they all come under the banner of being called NASA hoods. (Review all my photos for evidence) For example, my photo of a '71 Dodge Challenger hood shows the use of hood air scoops, not air ducts.

The difference being that the entry shaped floor is not delta triangular shaped the way a NASA duct is designed. Our Mustang has the classic delta shaped entry floor, and so, is a true NASA duct design with a slight variation twist to the classic original design. Case closed.

Thanks again guys for all your great input to help solve these mysteries and put it all to bed. If you disagree with any of my conclusions, please feel free to comment.

Reference .............................
Photos 1. Example of an air scoop not a NASA duct. 2. Example A - Front view of our Mustang NASA hood ducts, (note offset triangular floor entry design) 3. Example of an original NACA air duct. 4. Example of flat mounted NASA air ducts on a Euro car. 5. Example B - another pic of the front view of our Mustang's NASA hood ducts. 6. Example of the Shelby with the three NASA air ducts, flat and raised design. 7. Example of a raised NASA hood duct design on a Euro car.

Greg.

Sorry Greg but I have to disagree with your second point about our cars having legitimate NACA ducts. What makes a NACA duct a NACA duct, is its function, not its form. Just like titanium colored steel wheels won’t be as light as actual titanium and half of the exhaust tips on a BMW don’t actually do anything… these inlets are functionally not NACA ducts even though they resemble them. Yes they look similar, but that’s pretty much it. If you analyze the way the air flows over the front of the hood and into the “nostrils” you’ll notice that not only is the airflow interrupted, but toward the outer edges of the scoops, the aerodynamic characteristics are kind of sloppy. For these to have been actual NACA ducts, the inlets would’ve needed to be placed in an area with smoother airflow so that air enters the duct without affecting the boundary layer flowing over the hood. Instead (anyone with aerodynamics experience feel free to chime in) the scoops are placed directly facing the incoming air and toward the outside of the hood bulge, effectively creating turbulence. They’re aerodynamic looking scoops in the shape of NACA ducts, nothing more. As much as we’d all love for them to be the real deal, Ford gave us the looks but none of the functionality. I don’t mean to be the Debbie Downer and poop on everyone’s party, but the whole NASA thing was a marketing gimmick. To be fair though, it did give us one of the coolest looking hoods ever created. And that’s good enough for me.

P.S. Im not an aerodynamics expert, so if my assessment is wrong please feel free to correct me!

 

[Austin Vert]

Sorry Greg but I have to disagree with your second point about our cars having legitimate NACA ducts. What makes a NACA duct a NACA duct, is its function, not its form. Just like titanium colored steel wheels won’t be as light as actual titanium and half of the exhaust tips on a BMW don’t actually do anything… these inlets are functionally not NACA ducts even though they resemble them. Yes they look similar, but that’s pretty much it. If you analyze the way the air flows over the front of the hood and into the “nostrils” you’ll notice that not only is the airflow interrupted, but toward the outer edges of the scoops, the aerodynamic characteristics are kind of sloppy. For these to have been actual NACA ducts, the inlets would’ve needed to be placed in an area with smoother airflow so that air enters the duct without affecting the boundary layer flowing over the hood. Instead (anyone with aerodynamics experience feel free to chime in) the scoops are placed directly facing the incoming air and toward the outside of the hood bulge, effectively creating turbulence. They’re aerodynamic looking scoops in the shape of NACA ducts, nothing more. As much as we’d all love for them to be the real deal, Ford gave us the looks but none of the functionality. I don’t mean to be the Debbie Downer and poop on everyone’s party, but the whole NASA thing was a marketing gimmick. To be fair though, it did give us one of the coolest looking hoods ever created. And that’s good enough for me.

Hi Big Blue,

Thanks for your reply.

OK, you have bought in a new scientific/ technical element to this debate regarding duct wind aerodynamics design and wind/air flow capture efficiency dynamics, in relation to calling our Mustang true NASA hoods.

With all due respect, this is an area which i am not qualified to comment on or to debate with you, as my background knowledge on these matters is quite limited. We start getting into highly technical research and scientific areas here which are beyond me. I have not done enough research into these areas sorry. Wind tunnel experiments and testings would come into play here as well to put factual arguments or points of view forward i would think.

That said, and based on what i know to date, i will still stick with all the comments i have made in my last post as being correct. If your self or any other Forum members can educate me with hard scientific proof/ facts/ figures or evidence regarding the wind collection, capture efficiency rates of the original design NASA ducts compared to our 1,2,3 Mustang ducts, i will take it in with a open, learning mind.

Greg.

 

[BigBlue]

Hi Big Blue,

Thanks for your reply.

OK, you have bought in a new scientific/ technical element to this debate regarding duct wind aerodynamics design and wind/air flow capture efficiency dynamics, in relation to calling our Mustang true NASA hoods.

With all due respect, this is an area which i am not qualified to comment on or to debate with you, as my background knowledge on these matters is quite limited. We start getting into highly technical research and scientific areas here which are beyond me. I have not done enough research into these areas sorry. Wind tunnel experiments and testings would come into play here as well to put factual arguments or points of view forward i would think.

That said, and based on what i know to date, i will still stick with all the comments i have made in my last post as being correct. If your self or any other Forum members can educate me with hard scientific proof/ facts/ figures or evidence regarding the wind collection, capture efficiency rates of the original design NASA ducts compared to our 1,2,3 Mustang ducts, i will take it in with a open, learning mind.

Greg.

Same here. I've done all kinds of research on aerodynamics, but it is in no way my area of expertise. So if anyone can present hard data to prove one point or the other, that would be great.
That means we're going to need a wind tunnel. And some engineers. Possibly some folks from NASA.
C'mon people, I know some of you have a sister's best friend's uncle that works in this kind of stuff. WE NEED ANSWERS!

 

[Johnny57]

There was one Mustang that actually had 1 NACA scoop, but it was a 1970 Shelby

 

[tony-muscle]

Same here. I've done all kinds of research on aerodynamics, but it is in no way my area of expertise. So if anyone can present hard data to prove one point or the other, that would be great.
That means we're going to need a wind tunnel. And some engineers. Possibly some folks from NASA.
C'mon people, I know some of you have a sister's best friend's uncle that works in this kind of stuff. WE NEED ANSWERS!

Let's start a GoFundMe page to collect for renting a wind tunnel for testing. There are many other features that I would like to get tested.
Curious on how much improvement will be made by having body panels fit tight together? windows flush with body? removing or making drain rails flush with body? How much downforce is really created by the front wing and rear wing? How much is gained by smoothing the undercarriage? Is there any gain from adding a seal between hood and radiator support? and now.... how does air flow around NACA hood?
My guess is that $50,000 will get us there

 

[sewolla]

I’ll second that.

 

[Ajh1973]

I can tell you subjectively that they work because my brother-in-law and I both had 71 mustangs with no NASA hood. We would both race from New Orleans to Pensacola to go to a car show there. On the interstate we were dead even neither person could pass the other. One year I installed a NASA hood and Plenum and duck work on my 71 and the race was on. When we got to the 90 miles of interstate, I walked away from him slowly, but confidently because of the hood. more subjective information one year I removed my hood and we were once again did even. I love those hoods. I don’t care what you call them.

 

[AcesArneson]

Well AJ’s imperical data seems to win the argument. Switching the hoods and seeing a performance gain that is tough to measure but certainly improved is the only fact/fiction that compares.NACA to non-NACA with any accuracy. Pending any further data from other sources my vote goes to the NACA for the performance gain. Whether it is a true to form NACA or just a hood scoop is really not the issue. The real issue is ‘does it improve performance?’

 

[mrgmhale]

I do not give a rat’s rail feathers whether or not a NASA/NACA hood somehow improves performance. On our 69 GT500 Shelby, and in our 73 Mach 1. The hood openings and beautifully sculpted design look awesome. Even were the scoops to slow down either car I would still want them just for the look if nothing else.

 

[basstrix]

This is one of those discussions that seems to keep popping up...sort of like the "M" in 351M version of the 335 series engine. It's akin to the motor vs engine and 4-stroke vs 4-cycle.

I got an introduction into the NACA airfoils and ducts in college. The focus was on airfoils since it was an aerodynamics course, but the ducts were also studied to a much lesser extent. It is my understanding that the key characteristic of a NACA duct is that it can be placed in a moderately high pressure area and adds relatively low aerodynamic drag. Do our scoops meet that criteria? I don't know, but I do believe their design is heavily inspired by the NACA design. Below is the summary of a NACA report on them.

https://ntrs.nasa.gov/api/citations/19930093809/downloads/19930093809.pdf
Technically, I think they are still called NACA ducts, but Ford named them NASA ducts....as has been previously stated, I think it was purely for marketing and the popularity of the space program in that era. I really don't care what people call them, but I do think they look really cool.

 

[Marks1]

Well guys, the last guy to post on the article was this morning, and may be the expert you've been calling for;
Craig Hunter (NASA Aerodynamicist)
April 23, 2023 at 8:51 am
It would have been nice to put a few sentences in the introduction about how these work, but for folks so interested, here is the NACA report:
https://ntrs.nasa.gov/api/citations/20050061115/downloads/20050061115.pdf
NACA ducts were never meant to be high ram-pressure inlets, but rather, to be low-drag inlets with mild pressure recovery. In many cases on aircraft, this was sufficient to feed internal systems and low-flow / low-pressure systems. There are still many situations on aircraft where more intrusive inlets are needed to produce the flow rates and high pressure recovery levels for things like engines, etc.
When you consider the lower speed range of cars, the NACA ducts really had limited application and would have to be carefully placed on a vehicle to function correctly. That was not always done. But properly placed, a NACA duct can intake air with minimal drag, and provide a mild amount of pressure recovery (rise). Because ground vehicles are traveling subsonic and downstream pressure drives flow, the real action of a NACA duct happens upstream where its mild pressure recovery creates suction, drawing air in. Placed properly, this makes for a very effective intake.
Over the years, I have run into several sloppy applications of NACA ducts on vehicles where the flow was extremely poor or even going backwards! I chalked that up to designers not understanding how they work, or putting them in locations of low pressure on the vehicle where the overall pressure differential was not favorable. Step 1 in designing any sort of intake on a vehicle or aircraft is to understand where high pressure regions lie on the clean/watertight outer mold line (OML). That’s where you want to intake air because it already has a tendency to flow where you direct it. And if you can then route flow to exit into to a low pressure region elsewhere on the OML, guess what, you will have reduced drag, or at a minimum, not increased drag. These are the fundamentals designers need to understand before they go blindly sticking NACA ducts on the vehicle. It can’t just be a styling exercise.

 

[351c1971]

My final word on this subject is in the 1971 movie "Diamonds are Forever", Jill St. John liked the NASA hood on the red 71 Mustang. That is all I need to know.

 

[Bobby]

Well guys, the last guy to post on the article was this morning, and may be the expert you've been calling for;
Craig Hunter (NASA Aerodynamicist)
April 23, 2023 at 8:51 am
It would have been nice to put a few sentences in the introduction about how these work, but for folks so interested, here is the NACA report:
https://ntrs.nasa.gov/api/citations/20050061115/downloads/20050061115.pdf
NACA ducts were never meant to be high ram-pressure inlets, but rather, to be low-drag inlets with mild pressure recovery. In many cases on aircraft, this was sufficient to feed internal systems and low-flow / low-pressure systems. There are still many situations on aircraft where more intrusive inlets are needed to produce the flow rates and high pressure recovery levels for things like engines, etc.
When you consider the lower speed range of cars, the NACA ducts really had limited application and would have to be carefully placed on a vehicle to function correctly. That was not always done. But properly placed, a NACA duct can intake air with minimal drag, and provide a mild amount of pressure recovery (rise). Because ground vehicles are traveling subsonic and downstream pressure drives flow, the real action of a NACA duct happens upstream where its mild pressure recovery creates suction, drawing air in. Placed properly, this makes for a very effective intake.
Over the years, I have run into several sloppy applications of NACA ducts on vehicles where the flow was extremely poor or even going backwards! I chalked that up to designers not understanding how they work, or putting them in locations of low pressure on the vehicle where the overall pressure differential was not favorable. Step 1 in designing any sort of intake on a vehicle or aircraft is to understand where high pressure regions lie on the clean/watertight outer mold line (OML). That’s where you want to intake air because it already has a tendency to flow where you direct it. And if you can then route flow to exit into to a low pressure region elsewhere on the OML, guess what, you will have reduced drag, or at a minimum, not increased drag. These are the fundamentals designers need to understand before they go blindly sticking NACA ducts on the vehicle. It can’t just be a styling exercise.

All that being said and with the lack of any wind tunnel data, what do you think, at a glance, how the 71-73 hood scoops measure up? Are they more trouble than their worth?

 

[tony-muscle]

Well guys, the last guy to post on the article was this morning, and may be the expert you've been calling for;
Craig Hunter (NASA Aerodynamicist)
April 23, 2023 at 8:51 am
It would have been nice to put a few sentences in the introduction about how these work, but for folks so interested, here is the NACA report:
https://ntrs.nasa.gov/api/citations/20050061115/downloads/20050061115.pdf
NACA ducts were never meant to be high ram-pressure inlets, but rather, to be low-drag inlets with mild pressure recovery. In many cases on aircraft, this was sufficient to feed internal systems and low-flow / low-pressure systems. There are still many situations on aircraft where more intrusive inlets are needed to produce the flow rates and high pressure recovery levels for things like engines, etc.
When you consider the lower speed range of cars, the NACA ducts really had limited application and would have to be carefully placed on a vehicle to function correctly. That was not always done. But properly placed, a NACA duct can intake air with minimal drag, and provide a mild amount of pressure recovery (rise). Because ground vehicles are traveling subsonic and downstream pressure drives flow, the real action of a NACA duct happens upstream where its mild pressure recovery creates suction, drawing air in. Placed properly, this makes for a very effective intake.
Over the years, I have run into several sloppy applications of NACA ducts on vehicles where the flow was extremely poor or even going backwards! I chalked that up to designers not understanding how they work, or putting them in locations of low pressure on the vehicle where the overall pressure differential was not favorable. Step 1 in designing any sort of intake on a vehicle or aircraft is to understand where high pressure regions lie on the clean/watertight outer mold line (OML). That’s where you want to intake air because it already has a tendency to flow where you direct it. And if you can then route flow to exit into to a low pressure region elsewhere on the OML, guess what, you will have reduced drag, or at a minimum, not increased drag. These are the fundamentals designers need to understand before they go blindly sticking NACA ducts on the vehicle. It can’t just be a styling exercise.

What this guy is saying about reverse flow if a hood opening is not properly placed is very true. I have been reading a book where they discussed this same issue.
Look at this generic pressure profile. It is typical for the pressures at the front of the hood to be negative compared to the pressure where the hood meets the windshield. That said if the pressure under the hood is greater than the pressure outside the hood the air will flow out. That said, purely on this, air inlets should be placed towards the rear of the hood rather than the front. However, this pressure graphs is over a flat hood. If the hood has a raised spot like our NACA hood then it is likely that the pressure is increased - how much? who knows without wind tunnel data. Now, in our case, the hood air inlet is sealed to the carb so the pressure under the hood will be negative due to the engine suction.
As I said before, with my EFI I can measure inlet air temp and manifold pressure. I have seen a significant reduction in air inlet temperature with the louvers open so fresh air is coming in for sure. However, I don't see a measurable difference in air pressure which leads me to think that any change in the manifold pressure is negligible.