Some of you may remember that I posted when we went to the Sand Hills Open Road Race that we had a cooling issue at high speed and had to withdraw. Once over about 130 mph, the tems skyrocketed and we had to shut the car down.

This came as a surprise because we'd driven it some and never had a cooling issue before, but never at high speeds. After the race, it even spent 5 hours on the dyno without any cooling issues. This lead me to believe we were having an airflow issue more than a problem with the actual radiator or cooling system itself.

Originally, and here is a picture of it, we put the air dam on with no ventilation at all, relying just on the opening of the aero nose. We did have the opening you see in the hood, but it is almost 1/2 covered up on the inside with the radiator itself and since we had an issue with the headlight doors popping up around 120 mph or so even with the vent, I don't think it did a lot of good. (BTW, the headlights are removed and the doors are now pinned solidly)




This caused us some issues at the race two years ago, so after lowering the car I also drilled a series of 1.5" holes in the dam.



However, there was virtually none of the stock rubber seals in the
front (the car wasn't originally an aero nose car) and my thoughts
were that could be improved a lot.

I sealed the front up and added a pan to direct air into the radiator. I also built aluminum deflectors in the turn signal lights tying them into 3" aeroflex hose that also aims at the radiator. Everything was sealed up and the holes opened to 2" diameter, as well as being made two ovals with a screen to keep some debris out.




In the pictures, you can barely see the tinwork inside the nose, but I've done all I think I can do to make sure all the air coming in the openings goes through the radiator. For the straight line, brief runs, we'll probably tape off the lower openings because for short duration runs they're not needed. We had no cooling issues in the 1/2
mile runs last year.

Now that I was getting the air in, I needed to get it back out. I debated on a lot of openings and finally decided on trying to put a modified NACA duct, as large as would fit, in the hood. NACA ducts work better to draw air in than out, but they're better than just having a hole there.

I found the ordinates I needed (and if anyone would like them, let me know and I'll post them up as well). The size is as wide in the front opening as the radiator, as long as the hood, nearly, and opens just to the top of the swaybar.

I made a plug and did my layup right on top of that sitting on the underside of the hood. I used a layer of 12 oz cloth, two layers of matte, and one more layer of 12 oz cloth. The scoop is extremely rigid. I drew the pattern out on masking paper, taped it to the hood, and used the jigsaw and Fein sander to cut it out, then smoothed by
hand and with the Dremel and a small drum. With the cutout made, and the underside scoop done, I bonded it to the hood by using some flox, pop riveting, and then laying 3 layers of glass and matte at the seams.





It's nowhere near show car quality, but perfectly acceptable after smoothing and painting for it's intended purpose.

The front support was cut out to allow the scoop to pass through it, and some simple aluminum ductwork made and installed that mates to the scoop with the hood closed.



I still have to take it off and finish painting the lower hood, maybe make a couple of adjustments and seals (the tin is about 3/4" short of touching the underside of the hood, and I'm debating if I want to seal that or not).

If we still have cooling issues, I'm fairly certain now that it won't be due to packing air under the hood or not getting enough airflow into the radiator, but an aluminum thick core race radiator might still be required. Time will tell.

John Stricker

Edited to add:

I'm out at the shop now and found the NACA ordinates of the duct. With these ordinates, the vent will end almost exactly at the cowl edge, it narrows enough you miss the brake booster and master cylinder, and the front edge is on or near the raised portion on the underside of the hood that would normally seal the trunk compartment. IMHO, it's about as large as you can get in the Fiero hood. It is more efficient than straight sides, but the NACA profile was NOT designed to be an extractor, it's much more efficient as an air inlet. If you want to make more straight sides, always either make them perfectly square to the front opening or narrowing away from front opening. Never make it wider as you go back as that makes the duct grossly inefficient.

[This message has been edited by jstricker (edited 12-07-2008).]

Looks like you've done the logical thing, thumbs up

Even on my old 2.8, after I installed a chin spoiler (much smaller than that one) I noticed higher temps when cruising on the highway. I added a rubber addition to the wind deflecter under the car so that it "scooped" up more air to the rad and it worked nicely to bring the temps back down to normal. I love the "purpose" built items on the car. Have you thought about making extensions for the rockers so that air cannot rush in under the car from the sides as well? You might see some benefit there.

Also, what kind of clearance does your front chin spoiler have from the track? From what I understand, the lowest clearance you should have at all times is about an inch. You do not want all of the air flow blocked from going under your car. I'll try and find the article about it if you're interested.

Any place I can find more pics of your car?

If you opened up your front grille, you wouldn't need to drill holes in your air dam. Plus, one large opening in the front would be aerodynamically more efficient than a bunch of little ones. Just a thought.

BTW, that hood vent is bad-ass!

With normal passenger weight in the car we have 1.8" of ground clearance between the ground and the chin spoiler. That's not a scientifically arrived at distance, it's how much we have when we lay our air hacksaw on a 2X4 in front of the car and cut it off. At that clearance, we still have issues getting the car on and off the trailer at times. I need a longer ramp extension.

Side skirts are planned and they'll have a cutout for the jack location. With the stock location, we can still pick up one side of the car completely (both wheels) if someone leans a bit on the front end. Not ideal, but this ain't NASCAR pit stops either.

You're right, you don't want it ALL blocked, but you want enough that you avoid turbulent air under the car. Looking at pictures, at speeds over 100 mph, it appears we settle from downforce on the car to about 1" of clearance. Definitely less than we have static.

We're thinking of lowering the car another 1". We can do that by cutting the front coils one coil and then adding spring rubbers to get them a touch stiffer. We have coilovers in the back so we can set it to whatever we like.

You have a PM (in a few minutes).

John Stricker

quote Originally posted by doublec4: Looks like you've done the logical thing, thumbs up Even on my old 2.8, after I installed a chin spoiler (much smaller than that one) I noticed higher temps when cruising on the highway. I added a rubber addition to the wind deflecter under the car so that it "scooped" up more air to the rad and it worked nicely to bring the temps back down to normal. I love the "purpose" built items on the car. Have you thought about making extensions for the rockers so that air cannot rush in under the car from the sides as well? You might see some benefit there. Also, what kind of clearance does your front chin spoiler have from the track? From what I understand, the lowest clearance you should have at all times is about an inch. You do not want all of the air flow blocked from going under your car. I'll try and find the article about it if you're interested. Any place I can find more pics of your car?

I know the chin spoilers are pretty popular however, the stock air dam location scoops in a fair amount of air from what I can see. Probably more than the nose vent. Perhaps moving the chin spoiler back and having it function as a lower scoop with a corresponding hood vent would cool things alot. Just a thought. Nice looking racing car though as it sits. Nice lines.

Arn

While that's true, I like the looks of the stock grille. Doing it this way means when we make the mile and 1/2 mile runs, we can put two strips of racer's tape over them and not have any extra drag we don't need. At least that's my story and I'm sticking to it.

Oh, a bigger opening is only more efficient if it's an optimized opening, and that would be hard to do without a lot of body work. I'm trying to keep the body work as little as possible and keep the car instantly recognizable as a Fiero.

I'm still toying with the idea of an IMSA style over the roof scoop for the air intake. The air the engine is breathing now is HOT (oil cooler is behind the stock inlet location with the little ram scoop on it). I just haven't figured out how I would like to do it and have it work and look decent.

John Stricker

quote Originally posted by Blacktree: If you opened up your front grille, you wouldn't need to drill holes in your air dam. Plus, one large opening in the front would be aerodynamically more efficient than a bunch of little ones. Just a thought. BTW, that hood vent is bad-ass!

John -

While the results may still be OK, I don't think your outlet is going to work the way you expect it to. By far the best reference I've found on the "NACA Inlet" is NACA RM-A7I30, An Experimental Investigation of the Design Variables for NACA Submerged Duct Entrances. Once you fully understand how the NACA inlet actually works (mainly by manipulating and thinning the laminar boundary layer), you will see that the design offers no advantage whatsoever as a duct outlet. Just two specific problems to get you thinking further:

1) The sharp corners between the plane of the hood and the sides of the duct (an important but often overlooked detail of the design when used as an inlet) actually inhibit the lateral flow of air up out of the duct and onto the hood. (i.e. Air doesn't like to flow around sharp corners.)

2) Even if the duct succeeds in accelerating the air mass down the centerline of the duct, it will be exhausting right into the high-pressure stagnation point at the base of the windshield. This means that there will be a positive external pressure gradient along the entire length of the duct (i.e. increasing ambient pressure as the air moves aft) ... exactly the opposite of what you'd like to see for an outlet.

One simple thing you can do in the field to fine-tune your outlet is possibly to attach (even 100 mph tape will probably do) a deflector "lip" or "fence" across the hood just forward of the leading edge of your outlet. A deflector like this will probably add some drag, but it may also significantly lower the ambient pressure that your outlet duct "sees."

I really don't mean to rain on your parade. I admire you for being willing to experiment to this degree, and I hope you'll report your real-world results back to all of us.

[This message has been edited by Marvin McInnis (edited 12-08-2008).]

This is just my opinion, but, I think you put that vent in backwards. With it going from large to small, wouldn't that create a larger pressure area as the air gets pressed together? That's what I think anyway.

I have given something like this much thought. I would want to create a vacuum to pull air through, yes? What I thought was something like on a Lancer Evo. You have the holes in the hood, but there is a small rise along the leading edge of the vent which would create a vacuum to pull air through. Taking into account the angle of the hood, the way it looks from the pictures, air is going right into the vent hole, dropping it's effectiveness.

(Disclaimer: I could be wrong.)

Marvin,

You are correct. NACA ducts were not designed for exits, they were optimized as inlets. So perhaps I should explain my thought process.

First off, the research I've seen shows they are still slightly better than a straight sided duct. Not a lot, but a little. However, the lower, "Floor" portion of the duct IS more efficient at directing the air up and out than a straight floor, from everything I've read.

Second, straight sides on the outlet were not an option if one wants to keep the ramp out as shallow as possible. The reason is it will hit first the brake master cylinder and further back, the brake power booster. It would have to have some kind of a curve in there to get by those two items.

Third, although the pictures don't really show it, the edges between the duct and the hood are NOT sharp, knife edge corners like you'd want in an inlet, they have been rounded over to about a 1/8 - 1/4 inch radius.

Fourth, I gave some thought to building a slight fence just behind the original "hole" I put in the hood a couple of years ago. Since we're dealing with boundary layer here, I decided not to do that and am just going to see what happens and if the original opening will "trip" the flow enough (it's recessed into the hood, the screen is on the BOTTOM side) to disrupt that layer of high pressure air.

Fifth, I really don't see a way around the high pressure area at the tail end of the duct. That's going to be a high pressure area no matter what I do, but I believe that will be localized to that area and (hopefully) not cause a huge amount of interference although it's bound to create some.

Last, I don't expect it to accelerate the airflow. I did it hoping that it would offer as little resistance as possible just by the fact that all of the curves are smooth coming up the floor. Yes, the boundary layer will tend to spill over into the opening. A more efficient form would have been to put some slight "fences" on the edge, but to be honest, I think I have enough exit area that it's not really going to be much of a factor given all of the air coming out of the radiator has to come out of a 16" round opening (the cooling fan, roughly 200 sq inches) and my outlet at the front is 14 X 19.5 ( about 275 sq inches). By the time it gets to an area that is equal to the original opening, it is over 1/3 of the way up the duct.

My primary goal here was not to attempt to accelerate the airflow out of the area. I don't believe given the constraints (not wanting to interfere with the normal side view among them) that's possible to do with any degree of success. I wasn't even really planning on an NACA type duct, but when I had the car on the hoist, and drew around the booster and master cylinder with a square, I had what nearly amounted to an NACA duct anyway, so I decided to go ahead and lay it out that way, knowing full well it is far from optimal, but probably the best I could do given the constraints I imposed on myself and that the vehicle imposed on me. Primarily, I just wanted a big azz hole to keep the air out from under the car, allow it a ready place to go with as few impediments as possible, and not have it look like crap.

John Stricker

quote Originally posted by Marvin McInnis: John - While the results may still be OK, I don't think your outlet is going to work the way you expect it to. By far the best reference I've found on the "NACA Inlet" is NACA RM-A7I30, An Experimental Investigation of the Design Variables for NACA Submerged Duct Entrances. Once you fully understand how the NACA inlet actually works (mainly by manipulating and thinning the laminar boundary layer), you will see that the design has virtually no value whatsoever as a duct outlet. Just two specific problems to get you thinking further: 1) The sharp corners between the plane of the hood and the sides of the duct (an important but often overlooked detail of the design when used as an inlet) actually inhibit the lateral flow of air up out of the duct and onto the hood. (i.e. Air doesn't like to flow around sharp corners.) 2) Even if the duct succeeds in accelerating the air mass down the centerline of the duct, it will be exhausting right into the high-pressure stagation point at the base of the windshield. This means that there will be a positive pressure gradient along the entire length of the duct (i.e. increasing pressure as the air moves aft) ... exactly the opposite of what you'd like to see for an outlet. One simple thing you can do in the field to fine-tune your outlet is possibly to attach (100 mph tape will do) a "deflector" fence across the hood just forward of the leading edge of your outlet. A deflector like this will probably add some drag, but it may also significantly lower the ambient pressure that your outlet duct "sees." I really don't mean to rain on your parade. I admire you for being willing to experiment to this degree, and I hope you'll report your real-world results back to all of us.

quote Originally posted by jstricker: You're wrong. If you have a chance, read the reference that Marvin posted in his post. Again, NACA ducts were never intended for outlets, but given the constraints in THIS application, they are about as good as is practicable. John Stricker

I under stand that it's not being used in it's intended purpose. But, if you look at how they are installed in it's true application, it goes from the small "inlet" and gets wider as it dips into the duct, creating a bit of a vacuum. That's all I was looking at. I was thinking that with it going from a wide start to a narrow exit, you were bottle-necking the flow out from the radiator. Hope that clears up what I was thinking.

quote Originally posted by AquaHusky: ... in it's true application, it goes from the small "inlet" and gets wider as it dips into the duct, creating a bit of a vacuum.

It's counter-intuitive, but that's where you're wrong; it doesn't create a vacuum. The whole purpose of good inlet and diffuser design is to recover some of the free-stream dynamic pressure, while minimizing drag, such that near its design point the static pressure at the throat of the NACA inlet is significantly higher than the ambient static pressure (e.g. at the narrow entrance to the ramp), not less. From the reference cited, an optimized NACA submerged inlet operating at or near its design point can recover about 40% of the dynamic pressure present in the free air stream.

[This message has been edited by Marvin McInnis (edited 12-08-2008).]

quote Originally posted by Marvin McInnis: It's counter-intuitive, but that's where you're wrong; it doesn't create a vacuum. The whole purpose of good inlet and diffuser design is to recover some of the free-stream dynamic pressure, while minimizing drag, such that near its design point the static pressure at the throat of the NACA inlet is significantly higher than the ambient static pressure (e.g. at the narrow entrance to the ramp), not less. From the reference cited, an optimized NACA submerged inlet operating at or near its design point can recover about 40% of the dynamic pressure present in the free air stream.

Makes me wonder how they come up with this stuff.

Necessity, compounded by the demands of WW-II, was the mother of a LOT of inventions. The "NACA Inlet" design was just one result of the search for efficient, low drag airflow management for the first generation of jet airplanes that were being developed during WW-II.

Designs like this don't just spring up fully formed out of some genius mind. They are usually the result of a lot of theoretical analysis, as well as many, many hours of tedious testing in the lab, before finally going to flight test. The technical report I cited is only one of eight or ten reports generated over about a five year period of research on this single design idea alone. After all that time and effort, as far as I know, the NACA Inlet was never actually used on any production military aircraft. Even today, its use on aircraft and race cars is more often cosmetic than functional.

[This message has been edited by Marvin McInnis (edited 12-08-2008).]

You see them Marvin, but not as primary air inlets (although that may be what you were getting at with the "cosmetic" mention). I've seen them used for air inlets for apu's, sometimes for cockpit ventilation, oil coolers, things like that. Nearly always secondary systems and not as a primary air feed.

John Stricker

quote Originally posted by Marvin McInnis: Necessity, compounded by the demands of WW-II, was the mother of a LOT of inventions. The "NACA Inlet" design was just one result of the search for efficient, low drag airflow management for the first generation of jet airplanes that were being developed during WW-II. Designs like this don't just spring up fully formed out of some genius mind. They are usually the result of a lot of theoretical analysis, as well as many, many hours of tedious testing in the lab, before finally going to flight test. The technical report I cited is only one of eight or ten reports generated over about a five year period of research on this single design idea alone. After all that time and effort, as far as I know, the NACA Inlet was never actually used on any production military aircraft. Even today, its use on aircraft and race cars is more often cosmetic than functional.

lose the fan .at high speeds it blocks more air than it moves ,running or not .i think your extra ducting will do the trick .if it hasnt been mentioned before , i think adding the rubber under car scoop from the gt would help as well .

Not an option. We autocross the car as well and rarely have speeds over 70 mph. Mostly half that. Not to mention sitting at the starting line and having to cool it down after a run. The fan has to stay.

The rubber under the car that the stock aero nose uses is useless for this car. First of all, I now have that ducted tightly and directly up to the radiator. Second, the bottom of the rubber is about 2 " higher than the bottom of the air dam.

John Stricker

quote Originally posted by wftb: lose the fan .at high speeds it blocks more air than it moves ,running or not .i think your extra ducting will do the trick .if it hasnt been mentioned before , i think adding the rubber under car scoop from the gt would help as well .

Only thing I can think of doing is devise a way to get a bit of vacuum behind the rad to pull air through it somehow. Or start looking into thermal dynamics.

I road race an 86. If you do not have an opening on your "chin" to scoop up air you will over heat. If you lose the rubber insulation around under the hood and raise the hood up a little in the back by the windshield, you will be able to lose all the
air you need with out cutting a hole in your hood. I like what you have done, but there is an easier way. The best, fastest, probably the cheapest in the long run is to just buy a "thicker" radiator, if you go thick enouch the fan will not kick on. And for road racing you "do" want a vacumn under the car if at all possible, it will cause the car to stick to the track. Most racing groups will not allow "ground effects" that will create a "vacumn" under the car.
Keep up the development, never know where it will lead you.

You may actually be putting too much air to the rad and stalling the air flow at high speeds. There is a maximum volume of air a rad can pass, once that limit is reached, the air in the duct will "stall" and act as a deflector virtually closing the duct off. F1 cars tape up their brake and rad ducts for the high speed tracks for that reason.
Try closing the ducts up on a "test" day. It makes sense if you are not over heating after 5 hours on the dyno.
If you had the money, you could also try a open fin rad - I have no idea where to get one, or how much, but that would allow more volume to pass through the rad.

thats probably whats happening. the mouth on your nose its angled like a scoop, from all ive read about making frontal scoops for radiators this type of design looks the best but functions the worst. at high speeds, even measurable at low speeds, the air hitting these panels gets pushed out and in turn causes a restriction on the airflow flowing straight in. if the air becomes restricted like it is before it hits the radiator the fins will have an easy time pushing the air away instead of allowing it through, regardless of the opening behind it. if you change the inlet to the radiator from a scoop like this > to one like this < you will actually have more airflow due to the venturi effect.
also since i think all fieros were designed as bottom feeders you may want to realign the radiator since the big chin spoiler will prevent this from happening.