Eclipse and myself conducted some aerodynamic research today and the results are very interesting.

I asked Eclipse to help me on this because of his extensive applied knowledge of aerodynamics.

The basis for the research was that my GT was modded to include a scoop with fans to ventilate the engine cavity. In town, the engine temp would stay down to about 160* but on the highway, the engine would heat up and cause vapour lock. This is a malady of some carbureted high performance engines.

We applied a mixture of strips of cotton, 1" wide and 16" long and 1"x8". We placed 18" strips on the trailing edge of the cabin, out the trunk lid, on the quarter panels and one off the scoop. The 8" strips were placed on the spoiler, on the scoop, and on the engine vents.

The car was run at 60 - 65 mph and pics were taken at speed.

Here is the car at approximately 65 mph.

You will see that for all intents, the air over the deck lid is a complete turbulent mess. The vent strips lay flat on the vents, the strips coming out of the trunk were mostly sucked under the spoiler, and the strips on the roof simply pasted themselves to the rear window. In short, there was virtually no air escaping the engine bay.

It is our belief that the result will be similar if the stock wing is put back on either in the stock location or raised. I actually removed the raised wing for the test, believing that the Shadow spoiler would do a better job at preventing backwash than the raised wing. It apparently doesn't do any better. BTW, I like the look better with the Shadow spoiler so the spoiler will likely stay.

This was a bit of a surprise for me, but the pic says it all. I am now working on an alternative to ventilate the engine bay at highway speeds. There are a few options. I'll keep you posted.

Arn

Are your engine vents descreened??

Here is a closer pic. Notice how the side wind on the car actually caught the side mounted strips.

Notice the strips in the middle, plastered to the spoiler, and the strips on the scoop apparently caught in a swirl.

Arn

Good job.
I will save these pics. Lot of good visual info here.
Thank you.

what, no wind tunnel?

nice job.

It really was interesting to see how it played out. It's like the aerodynamics guys designed the car, the shape and look, and then gave it to the engineers to put the guts in. The engineers stuck a battery in front of the vent, and a license plate in from of the main cooling intake and a few other things that contradicted the original intent. It's like they didn't consult at all :} .

The fun part was driving straight, shifting, and trying to take pictures :}

Frankly this doesn't surprise me...

I have a tree that loves to dump leave and seeds (Sweet Maple) on the car. I've seen speed points where air rolls off the roof, hits the deck in the middle and pushed the leaves towards the rear glass.

Air off the back of both the coupe and fastback is fairly wacky. I'd love to see the smokes for this car in a wind tunnel as it slowly increases speed.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

Intresting. An undercar deflector of some sort might force enough flow thru the compartment at hiway speed to overcome the current defeciency, but there isn't much room for that and it would look like crap. Here's an old test I had bookmarked.
https://www.fiero.nl/forum/Archives/Archive-000002/HTML/20041015-1-041694.html

Hey ARNS air definantly escapes the engine compartment through the vents. When I had the dukie in my beater I had a head gasket go and it allowed oil to leak out onto the exhaust manifold. This resulted in a plume of white smoke coming out of the passengerside vent. This would only occur at around 55mph so I'm not sure if it was speed or RPM related. I also have the vent screens removed.

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"but when equipped with NOS, sometimes I feel invincible..."
"but when I race from a roll it's hard to hit 2nd"

quote Originally posted by Arns85GT: Here is a closer pic. Notice how the side wind on the car actually caught the side mounted strips. Notice the strips in the middle, plastered to the spoiler, and the strips on the scoop apparently caught in a swirl. Arn

So vents to the sides appear to be the only way to get an extrator system to work? That would look hideous in my mind's eye.

Anyone got a good picture from underneath the rear of the car?

Yeah, the decklid is air hell. They look stupid, but a giant 9" tall aluminum wing will really help. I have only done the car wash test myself, no smoke or strips like you did. One thing about your test that is a little flawed is the material. you should use plastic strips, not fabric.

By car wash test I mean:

Stock with stock spoiler= Water just sits on the deck at 100 Mph and even rolls foreward a bit.
Giant ecector set wing= Almost all the decklid water has sheeted off the back by 80 Mph

In stock form, the back of the fiero is a brick in the wind. You could probably achive the same results with tall wing stands and a stock spoiler. If you do use a rice wing, give it very little angle. I had mine a little too steep and it was really screwing up my mpg. Those wing can also be dangerous with too much pitch at high speeds. If you have any rear suspension issues, your car will be all over the place. I'm not saying those wings look good. I know it looks like **** .

Hey Arn,

Material doesn't actually matter that much, what does matter is size and shape of the test strips being used. I've done wind tunnel testing using everything from wool yarn to advanced dye flow visualizations (I'm an Aerodynamics/Thermodynamics Engineer working on military aircraft). There are a few possibilities that could help explain why your test results were a mess besides the already correct statement that the area behind a body in a flow usually is a 'turbulent mess.' Some of the strips in very odd locations (twisted, turned backward,etc) are most likely a result of being too long. Usually the test apparatus in the tunnel uses strips mounted at equidistant points in 2 dimensions over the surface(s) and the length of the strip relative to the test apparatus is much smaller than what you have because interference between strips is not desierable. The other important thing is that the <a style='text-decoration: none; border-bottom: 3px double;' href="http://www.serverlogic3.com/lm/rtl3.asp?si=31&k=test%20strip" onmouseover="window.status='test strip'; return true;" onmouseout="window.status=''; return true;">test strip</a> be equivalent in size in the length and width dimension. That's why I said earlier yarn works pretty well because it's essentially a tube, it looks like you were using some sort of plastic/fabric strip that was thin as paper but maybe a thumb wide? I see that many of your strips have become entangled which is undesireable because once they do you'll never know exactly what the flow is doing as any fluctuation is now affecting multiple strips as well as each individually. Your test will definatley yield you some good results, but your test method needs a couple minor tweaks, just use something like yarn and make sure they are spaced such that they cannot come in contact with one another.

As for your dilemma to prevent such recirculation, (what you refered to as backwash) a much higher wing would help tremendously as HellYes alluded to. It will essentially act like a high lift device on aircraft wings (flaps, slats) where the air will stay attached to the lifting body longer, essentially 'tricking' the incoming air to accept a cleaner line from the back of the roof to the wing surface and then begin recirculation, you will still have a pressure gradient behind the cabin but it will not be as severe. This is a big reason why placing a large 'ricey' wing on the back end of non-aerodynamically shaped vehicles can help the areodynamics by delaying and decreasing the formation of the recirculation zone. Again, there are other issues though at speed to consider especially downforce and handling like HellYes mentioned. A possible way around this would be to mount an uncambered airfoil (think of an oval that's sharp instead of round at either short end, at a 0° angle of attack to level ground. This would provide the benefit of removing the recirculation zone behind the cabin and at the same time not add much in the way of downforce (should theoretically yield none provided the freestream to the body is at a 0° incidence angle) to mess up handling. Cool that you actually went out and tested this, I would be interested in further results.

[This message has been edited by Raze (edited 07-03-2005).]

Thanks for all the observations and discussion folks. Going to shorter strips is a definitely good plan for a second try, but, for my purposes, I think I understand what I am dealing with for cooling my carb and I may not do further tests.

Actually, the problem of heating up at freeway speed occured with the raised wing, using 5.5" stands. This was the reason I tested using the Shadow wing. I was hoping for some improvement. The 5.5" raised wing would not at all influence air moving approximately 2" off the surface of the bumper and deck lid but it would effect the overall slip stream.

What the pics do not show clearly, is that the strips taped to the top of the spoiler wrapped themselves back and down under the spoiler vents, getting caught in the air flow up the rear bumper. The air coming up the bumper grabbed them and pulled them down.

It was pretty evident that the air was moving strongly up the bumper and over the trunk. Oh for a real wind tunnel.

What you can't see in the pics is the orientation of the vent strips. They were at 60* pointing toward the sides of the car.Looking in the rear vision mirror, I could see they did not appear to get more than an inch off the surface. Yes some air was coming out, but less, the faster you went. The large scoop was likely affecting this.

I am of the opinion that it doesn't much matter which wing or spoiler you use. You either need very large vent area like the Ferrari's or you need a powered vent. I am currently inclined to put 10" fans inset into both engine vents blowing upward.

The old archive was also using long yarn and I think 6" plastic strips might be better as suggested, but I don't have any yellow plastic currently. Maybe somebody can do the test with a stock deck?

Arn

perhaps changing your decklid scoop to face the other way would help better with cooling, then it would no longer be fighting any airflow flowing against it?

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1986 Pontiac Fiero GT
Auto, soon to be 5 spd Getrag from 88 z24, Best 1/4 = 16.1 at 83mph, mods, wires, CRX intake, and power pulley. Planning Turbo 2.8 swap for a little more umph!!!!

Bought for 2500$ Canadian.

I don't know that there is any need for diflectors etc under the car... air under there is anything but clean flowing.

The only thing I'd be worried about if someone tried that... crap getting hung up in it. There are amazing amounts of trash on the roads.

I was going to mention something about the size of the strips, but Raze already covered it.

It's been my experience that the airflow across the decklid gets more turbulent the faster you go. At a slow roll, the air flows neatly over the roof, curls down to the decklid, and slides off the back. I've noticed this effect when rainwater dripping into the hood vent is vaporized by the hot radiator. But at fast highway speeds (75-80MPH), the airflow in the back is a turbulent mess. On cold days (with no rear spoiler), I've actually seen the exhaust vapors come up onto the decklid and swirl around.

My Fiero used to have a stock spoiler with 7" tall stands. The "high-rise" spoiler, along with open vent grates, seemed to help straighten out the airflow over the rear deck. I clearly remember one instance when I had an oil leak that was dripping onto the exhaust pipe. Driving down the highway at about 70MPH, I saw a column of smoke come out of the passenger side vent and stream straight back across the deck (passing under the spoiler).

I was actually thinking about doing what Arns85GT did, but with shorter strips. I have some body mods planned, and would like to see first-hand how they effect the airflow. But there are other things that have higher priority for now and the near future. If/when it happens, I will make sure to post photos.

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A note on the Dodge Shadow spoiler: That spoiler was designed to be practically non-functional. Most spoilers on production cars are just cosmetic. In contrast, the IMSA Fiero spoiler is quite functional, even though it resembles the Shadow spoiler. There are 2 reasons why:

1) The IMSA spoiler is not vented. The purpose of the IMSA spoiler is to keep air from curling up onto the decklid. The vents in the Shadow spoiler defeat that purpose.
2) The IMSA spoiler is much taller. So it redirects more air away from the decklid.

[This message has been edited by Blacktree (edited 07-03-2005).]

quote Originally posted by Blacktree: 1) The IMSA spoiler is not vented. The purpose of the IMSA spoiler is to keep air from curling up onto the decklid. The vents in the Shadow spoiler defeat that purpose. 2) The IMSA spoiler is much taller. So it redirects more air away from the decklid.

The IMSA spoiler is also wider as well. This aids in scooping air off the sides trailing the sail panels.

quote Originally posted by theogre: I don't know that there is any need for diflectors etc under the car... air under there is anything but clean flowing. The only thing I'd be worried about if someone tried that... crap getting hung up in it. There are amazing amounts of trash on the roads.

Yeah, I've heard about those big dead possums on the Delaware Turnpike.

I really don't care how 'clean' the air is under the car, as long as it can be 'scooped up' and pushed into the engine compartment. That is the normal flow path for air isn't it-from the bottom-up thru the engcompartment and out the vents? Or, as this test shows-the lack of flow.

quote I am of the opinion that it doesn't much matter which wing or spoiler you use. You either need very large vent area like the Ferrari's or you need a powered vent. I am currently inclined to put 10" fans inset into both engine vents blowing upward.

Perhaps, just for the heck of it, I'll try this test in one of our 84s, since they both have vents all the way across the back.

Wonder what effect a choptop has on the air on the decklid area?
jscott has an interesting mod on the lower rear fascis of his choptop, it may also yeild different results regarding air coming back over the back of the car.

MaryJane, that would be a great idea.

Learning from my mistakes, 6" lengths of yarn or thin plastic are likely with way to do it.

The answer is easy regarding how hot it will be. Just go to Walmart and pick up an electronic meat thermometer and when you pull over, take a reading off your valve covers or lower plenum. The electronic meat thermometer is cheap and effective. I'll bet it will be well over 200*

Arn

I'll try it tomorrow Arn. My 84s are 4 cyls tho-no lower plenum. I do have an oven thermometer I can just stick down thru the center vent, Not sure how accurate it is in air tho. I'll talk Jane into taking pics standing out the sunroof.

I can tell you this. My back window in both cars is so hot on the inside you can't hold your hand on itfor more than a couple of seconds, so I know some heat is escapng from that center vent.Whether it is from air flow or the natural "heat rises" phenomena, I can't say.

[This message has been edited by maryjane (edited 07-03-2005).]

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[This message has been edited by Tina (edited 07-05-2005).]

Arns,

Sounds good, I didn't notice the second picture until today, I can now say i'm not suprised that your exhaust vents are not doing their job because as you've noted the flow will essentially 'curl' around off the roof and cause a low pressure region behind the cabin, and since it's lower pressure than the surrounding air, all that air will try to move into that region effectivley preventing any flow out of your vents. Your solutions sound good, though I have seen some TT MR2s that used 2 radiator fans mounted above the engine to help exhaust, They also had side ducting much like your vehicle that was channeled up to the sides of the engine to help create a pressure gradient favorable for the fans to operate (i.e. not tax them to not only exhaust but overcome the pressure differential). Considering none of us has a windtunnel to really do good areodynamic ducting & cooling result run, or the time to use a privately available CFD program to model the flow path I think your fan solution will be the easiest to implement and attain measureable results.

Blacktree,

The result of more turbulent air the faster you go is due to the separation of flow from the body due to an increased adverse pressure gradient that increases the boundary layer thickness to a point where the flow 'separates' from the surface of the body so you're absolutely correct that that's exactly what you can expect. Most wind tunnel testing is done to counteract flow separation and shock formation over the body, wing, control surfaces and on engine inlets on aircraft. You won't need to worry about shock formation since it only occurs at transonic speeds, but flow separation and areas of extreme pressure gradients will form behind the very 'unsleek' rearend of the Fiero.

[This message has been edited by Raze (edited 07-03-2005).]

quote The result of more turbulent air the faster you go is due to the separation of flow from the body due to an increased adverse pressure gradient that increases the boundary layer thickness to a point where the flow 'separates' from the surface of the body

Whut he say???

quote Originally posted by Raze: The result of more turbulent air the faster you go is due to the separation of flow from the body due to an increased adverse pressure gradient that increases the boundary layer thickness to a point where the flow 'separates' from the surface of the body

Elementary aerodynamics. Theoretically, the velocity of an air molecule at the surface is zero. As you go further from the surface, friction from molecules closer to the surface slow down the molecules above them. Think of layers only a .001" thick, stacked above each other. Eventually the velocity matches the airstream velocity of travel. The gradient from zero to airstream velocity is called the "boundary layer".

Unfortunately, that big rear window, and the sudden drop from the roof to the decklid is a killer. Low pressure zone big time.

Some airplanes have little "turbulence inducers" mounted like little "wing stubs" on the trailing edge of their wings. The purpose is to put more energy into the boundary layer, so that it will not separate.

The ideal at automotive speed, however, would be to have so called "laminar" flow throughout. But once again, unfortunately, this does not look "cool".

Welcome to the forum Raze.

I'd like to see a test with those 80's louvers on the back, see if they make any difference.
Too bad they're so rare.

Well I think I have the process worked out.

Rather than fight nature, I'll go with it. I'll let the air move up the bumper and across the deck. I'll use my rear facing scoop to catch that air. I'll then install 10" fans inset in the vents, using the 84 solid vents as mounting platforms.

The air will be forced upward @ 650 cfm out of the vents and be drawn in the scoop, right past my air cleaner, as well as from the bottom of the engine bay.

What say? Will it work?

Arn

quote Originally posted by zardoz: Some airplanes have little "turbulence inducers" mounted like little "wing stubs" on the trailing edge of their wings. The purpose is to put more energy into the boundary layer, so that it will not separate. Welcome to the forum Raze.

These are called Vortex Generators. The new Evo 8 comes with them.

I know very little about the aerodynamics of a Fiero, but what effect would a roof wing have? It would look better (in my own opinion) than a foot-tall erector set wing.

i'm surprised at the stillness of the strips over the vents because of what i noticed when i was putting in fans on one of my cars.

i had cut circles in a set of '84 engine side covers and put 10" fan under each of them. before i got them covered and wired up i noticed at highway speed (at least 60 or so) the fans would almost spin like i had them wired up. not like fully powered but enough that the blades were a blur. they got to moving pretty good.

I finally got the proof of the air flow without a smoke generator or wind tunnel.

I had to take the car out this morning with a good coating of dew on it. The trip included a drive down a dirt road.

PIP is malfunctioning today with a runtime '13' code so the pic can be seen here.

http://img.photobucket.com/albums/v701/Arns85GT/Airflow.jpg

It shows that the air came up over the back of the car, and under the spoiler and into the engine cowling. It could not have been much clearer.

I noticed I did not get any mud on my vent fans, but a very noticable spread in front of (or behind) my scoop.

Arn

So I guess the best spolier for our cars is on that stretches out instead of strteching up? Drag spoiler vs a hi rise spoiler?

And as far as the roof wing/spoiler idea goes what about a lip spoiler like off of a trans am or the like the kind on the PPG fiero's decklid?

i just realized the aerodynamics you were tlking about was dealing with cooling not downforce.

[This message has been edited by goatnipples2002 (edited 09-28-2005).]

For downforce you need "clean" air flow.

That means that the Fiero wing has no chance much below 8" above the deck so far as I can see, and probably needs 12".

Arn

Would NO wing be better? Or worse?

I like the stock wing particularly on the Notchy. But, the stock unit does not seem to do much other than to interrupt the air flow over the deck from the rear. If I had the 86 or newer GT I'd be losing the wing. I think they look a little better without. IMHO

I had the stock wing raised for a while and it looks ok and seems to work, but I very much doubt significant aerodynamic advantage at street speeds or even highway, other than to smooth the air flow turbulence at the back.

Arn

Edit - no wing is 10 lbs lighter

[This message has been edited by Arns85GT (edited 09-28-2005).]