quote Originally posted by Blacktree: Just keep in mind that an accurate model will need to include the decklid vents and engine compartment. The vents play an important role in the Fiero's aerodynamics. They help to fill the void behind the rear window, which in turn sucks air out from under the car, which creates downforce under the floorpan.

The low pressure behind the rear window pulls air up from underneath... not necessarily creating downforce, but helping to reduce the effects of the higher pressure underneath the car.

The reason behind me measuring down force was that I was going to install a belly pan and diffuser to see what could be done for the high speed instability. Step 1 in creating down force is to make sure no air gets underneath the car to begin with. Step 2 is to give any air that does get under there a quick exit through an increasing area (either through rake angle or belly pan design). The front of a stock car offers a lot of room for improvement.

Short of a "ricer" wing, or possibly a roof wing, you aren't going to gain much from altering the top side of a fiero. I like the belly pan and diffuser idea specifically because it doesn't alter the appearance of a fiero. The diffuser could simply be an extension from the bottom of the bumper to the cradle.

quote Originally posted by ryan.hess: The low pressure behind the rear window pulls air up from underneath... not necessarily creating downforce, but helping to reduce the effects of the higher pressure underneath the car.

I say "to-may-to", you say "to-mah-to". Mathematically speaking, less lift = more downforce.

quote Originally posted by ryan.hess: The low pressure behind the rear window pulls air up from underneath... not necessarily creating downforce, but helping to reduce the effects of the higher pressure underneath the car.

Am I wrong in my understanding that by eliminating preasure under the car that the effect is more downforce? If you start with 50 pounds of preasure below and 50 pounds above. Then you vent out 20 pounds from below, effectively you are gaining 20 pounds of added downforce. The numbers are made up and I have no idea what actual would be, but I am pretty sure that eliminating preasure from under the car will add to the total downforce on the car.

quote Originally posted by ryan.hess: The low pressure behind the rear window pulls air up from underneath... not necessarily creating downforce, but helping to reduce the effects of the higher pressure underneath the car. The reason behind me measuring down force was that I was going to install a belly pan and diffuser to see what could be done for the high speed instability. Step 1 in creating down force is to make sure no air gets underneath the car to begin with. Step 2 is to give any air that does get under there a quick exit through an increasing area (either through rake angle or belly pan design). The front of a stock car offers a lot of room for improvement. Short of a "ricer" wing, or possibly a roof wing, you aren't going to gain much from altering the top side of a fiero. I like the belly pan and diffuser idea specifically because it doesn't alter the appearance of a fiero. The diffuser could simply be an extension from the bottom of the bumper to the cradle.

You are talking like a race car driver. Absolutely, ground effects to create an under car suction does more for down force than a wing. The problem is with the design of the engine compartment. Air comes up from under the car into that compartment to provide ventilation, and exits out the vents. Unfortunately, this does not work particularly well due to the location of the vents. If you check your threads on the vent covers you will see a really poor level of air movement.

The air moving from the back edge of the deck forward is actually pretty strong. As you can see in my test with my Dodge Shadow spoiler, the air really wants to move toward my scoop. It is significant. It probably delivers more air into the mouth of the scoop than what comes up from the foward firewall to the vents.

If you were to introduce ground skirts on the front and sides, combined with either side vents or the top scoop, you might have a formidable down force effect and still cool the car. This would take some careful engineering to ensure your air through the radiator was not impeded. Maybe a hood vent to make that 'cool'.

Side vents combined with ground skirts makes some interesting air effects for me.

Arn

Interesting discussion, and I'm enjoying the exchange of info/ideas..............but I don't much care. I figure my spoiler mostly just looks kinda cool. I don't need additional downforce because I don't make it a practice of driving exceedingly fast on the street. Matter of fact.....I've been considering going w/o spoiler because since I've lowered my '87 GT, the spoiler is right in the middle of my rear vision .....mostly @ night. Can't see people behind me half the time.

Was this done on a fastback or notchback? Since the spoiler was originally designed for the notchback, that would be the proper car to do it on.

My Fiero is an "aero notchie". Specifically, it's an '87 SE.

[This message has been edited by Blacktree (edited 06-01-2008).]

The only published data I have ever seen that relates to actual functionality of the wing/spoiler on the Fiero says it is a DRAG REDUCTION device. The most aerodynamically efficient Fiero is the Notchback Aero Bodystyle WITH THE WING/SPOILER. Without the wing/spoiler, the drag coefficient is higher. This suggests that the Fiero wing/spoiler is a simple airfoil that reduces drag a little but mostly just looks "cool." Due to air flow patterns over a vehicle, this wing/spoiler is not high enough into the air flow to create downforce.

-Gary


Fiero Aero Simulation

https://www.fiero.nl/forum/Forum2/HTML/088990.html

Front end lift:

https://www.fiero.nl/forum/Forum2/HTML/089032.html

[This message has been edited by FieroFanatic13 (edited 06-02-2008).]

quote Originally posted by FieroFanatic13: This suggests that the Fiero wing/spoiler is a simple airfoil that reduces drag a little but mostly just looks "cool."

Specifically, the Fiero spoiler reduces drag by obstructing the back-to-front airflow pattern that generates drag. Unfortunately, it isn't very good in that respect, because air can flow underneath it. This is why the IMSA people went with the whale-tail type spoilers. They block the back-to-front airflow pattern. This is also why I'm experimenting with spoiler modifications.

[This message has been edited by Blacktree (edited 06-02-2008).]

quote Originally posted by FieroFanatic13: Front end lift: REPORTED FRONT LIFT FIGURES WERE ABOUT 120 LB AT 100 MPH

I'm guessing since this was 83, it was a bumperpad notchie? Like I said, the GT is 160# at 90mph. It would be close to 200# at 100mph.

quote Originally posted by ryan.hess: I'm guessing since this was 83, it was a bumperpad notchie? Like I said, the GT is 160# at 90mph. It would be close to 200# at 100mph.

Yes, it was a figure for the original '84's. But where did you get this 160# of lift at 90mph for the GT's front end out of curiosity? The "aero" nose and additional parts improved aerodynamics, and yet you're suggesting that the GT nose is WORSE than the upturned wedge nose on the 84-86 coupes which doesn't make sense based on every magazine article I've read.

Do you have a source for this information? I would be curious to see it!

[This message has been edited by FieroFanatic13 (edited 06-02-2008).]

See page 1. I measured it myself. No guarantees to accuracy, it could be off by +/- 25% or more. It was an average of multiple high speed runs. I wouldn't call it fair to compare a test on a 20 year old car to a test on a new car 20 years ago. Let alone differences in road surface, winds if any, etc. Maybe they had their head lights up.

But 200# is quite significant and only gets worse with speed.

quote Originally posted by ryan.hess: See page 1. I measured it myself. No guarantees to accuracy, it could be off by +/- 25% or more. It was an average of multiple high speed runs. I wouldn't call it fair to compare a test on a 20 year old car to a test on a new car 20 years ago. Let alone differences in road surface, winds if any, etc. Maybe they had their head lights up. But 200# is quite significant and only gets worse with speed.

I was hoping you'd gotten information from a source other than the fuzzy math we have to come up with on our own I guess! Thanks for the reply and input!

It's not fuzzy math or a guesstimate. It was probably measured the same way the magazine measured it - specifically by measuring suspension travel in real time vs speed. Springs deflect a certain amount based on the force acting upon them. If you know the travel, you know the force.

I drive an 85 SE notchie with wing (my favorite Fiero model year) and have added a chin spoiler to shark nose (not a fan of aero noses) and an extraction scoop to the hood with non-scientific but practical improvement at high speeds. Car is stable and cools well.

Arn mentioned on page one how inefficent the cooling vents are. I disagree. My a/c compressor locked up going down the highway & the belt started smoking. I was amazed how much smoke came up out of the passenger side vent and impressed that it was moving such a volume of air. Great discussion Mike! I'm going to be replacing the winged decklid on my GT soon with a wingless version. Would be nice to duplicate your tests to see what the difference will be.

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87 GT Maroon 3800SC & White Formula

Dumb question. Is there a difference between the Notchie and Fastback design? What design has the advantage, if any, over the other? I would think the Notchie has better aerodynamics. Your thoughts?

Cordially,
Kevin

The differences between the two basic body styles are more cosmetic than aerodynamic. Behind the passenger cabin the airflow is a real mess with either body style.

quote Originally posted by ryan.hess: It's not fuzzy math or a guesstimate. It was probably measured the same way the magazine measured it - specifically by measuring suspension travel in real time vs speed. Springs deflect a certain amount based on the force acting upon them. If you know the travel, you know the force.

Except that your math tells you that the GT/Aero has more lift than the upturned original nose, which doesn't make sense according the point of the pace car style bodywork?

[This message has been edited by FieroFanatic13 (edited 06-03-2008).]

quote Originally posted by FieroFanatic13: Dropping drag coefficient from .410 (84 SE) to .365 ('84 Indy, '85 GT, 86-87 SE) is pretty substantial I think?

Yes, an 11% reduction in the drag coefficient would be significant, but I don't have any way to validate or refute those C_d figures. Where are they from? Were the tests all performed at the same time? ... in the same facility? ... under identical conditions? I'd like to see the raw data (or at least the original reports) before drawing any conclusions.

quote Originally posted by Marvin McInnis: Yes, an 11% reduction in the drag coefficient would be significant, but I don't have any way to validate or refute those Cd figures. Where are they from? Were the tests all performed at the same time? ... in the same facility? ... under identical conditions? I'd like to see the raw data (or at least the original reports) before drawing any conclusions.

Those are numbers published in the magazines at the time of production for various models as supplied by GM themselves, so take that for what it's worth! I would presume GM would be consistent, but that's probably a bad presumption, LOL...

quote Originally posted by Dirty Harry: Arn mentioned on page one how inefficent the cooling vents are. I disagree.

I'll have some data regarding this subject soon. I started working on it today, but ran out of time. So hopefully tomorrow, I'll be able to finish up.

quote Originally posted by FieroFanatic13: Except that your math tells you that the GT/Aero has more lift than the upturned original nose, which doesn't make sense according the point of the pace car style bodywork?

You can't compare them. C&D tested a new car. I tested a 20 year old car.

Even a little sag will change the ride height enough to affect the aerodynamics.

A couple of points:

1] Ryan Hess, did you measure your suspension deflection under power or coasting? The power required to push the car throught the air will pitch the nose up. But you're a pretty bright guy, I don't think you missed this.

2] The Fiero rear spoiler is drag reduction device. Most people seem to be missing its actual function. it gives something for the air coming off the rear of the roof to "re-attach" to at the back of the car. This simulates (poorly) a complete rear fastback roof. The bulk of the air is fooled into moving smoothly from the roof to the spoiler as iuf the rear deck area was one continuous curve. Yes, there is a lot of turbulence underneath all that, and the decklid vent air mixes it up, but that's how it works/

http://www.a2wt.com/CARS.htm

hmmm....

quote Originally posted by ryan.hess: You can't compare them. C&D tested a new car. I tested a 20 year old car. Even a little sag will change the ride height enough to affect the aerodynamics.

True...

quote Originally posted by Dirty Harry: Arn mentioned on page one how inefficent the cooling vents are. I disagree. My a/c compressor locked up going down the highway & the belt started smoking. I was amazed how much smoke came up out of the passenger side vent and impressed that it was moving such a volume of air. Great discussion Mike! I'm going to be replacing the winged decklid on my GT soon with a wingless version. Would be nice to duplicate your tests to see what the difference will be.

The 87/88 GTs have a shrouded vent area by way of the sail panels. This allows the air to move out of the vent more easily. On my 85 GT, when I attached ribbons to the vents, the ribbons in the center section of the deck hardly moved. The ribbons on the outer edges flattened out at about 45* toward the outside of the car. At speed, I could only see evidence of about 25% of the vent actually working. At low speeds it is a different story. Convection makes them work and the tumbling of the air stream at the rear window top edge does not kick in. The ribbons nevertheless flopped around alot and did not show a steady stream even at low speeds.

Effectively the tumbling air coming over the roof completely distorts how the vents should be working on the notchy.

I expect that with the same test on the fastback body, you would find less pronounced distortion until probably 60 mph and above. The sail panels on the sides effectively shield the fastback vents so far as I can see. Different story for the notchies. Hope this explains better.

Arn

Today, I took some video of the decklid vent gratings. I compared a stock vent to a modified, free-flowing one to see if there would be a noticeable difference.

Unfortunately, I couldn't get a good camera angle from inside the car. So I had to mount the camera behind the back window.



I was concerned that heat (both from the hot Florida sun, and from the engine compartment) would damage the camera. But it survived unscathed.

You can see the video in my original post on Page 1. I decided to consolidate all the results there, so people wouldn't need to scroll through several pages of conversation to see it all.

[This message has been edited by Blacktree (edited 06-04-2008).]

Whats the overview of what was done and results? I'm confused as to what was done to your car Blacktree.

Most of the mods and upgrades I've performed on the car can be found in the Silver Freak Project thread.

[This message has been edited by Blacktree (edited 06-04-2008).]

Really good research with the modified grill. It sure does make some difference. Way to go.

With 6" string you would likely get turbulence but with the short string not so. Very good.

Now how do we do the same thing but have the grill look nice?


Arn

I imagine removing the "leaf catcher" from under the stock grille would improve airflow quite a bit. Hmm... maybe it's time for another video.

Thanks for taking the time to not only try these expeiments but video and put them on youtube as well

Did you get up to 70mph with both runs? It seemed to flow better at the higher speed?
Aside from them being a real PIA to get leaves and especially oak tree seeds (those lil one-sided propellers) out of, the stock vents work fine as long as you don't have too many engine mods. The 10" fan under my rear extractor scoop seldom turns on as my engine runs quite cool and thats with a turbo and headers, etc, etc.

Come up with something real cool looking that 's not a leaf/seed trap while youre at it. Unlike dyno time, some research can be fun whereas it don't cost you anything.

Here's an aero what-if thought: I wonder how much drag would be reduce if one were to cover the area between the roof side skirts, roof and rear with say clear plexy and duct the vents out the rear of that cover? Be a big PIA to do and probably wound not look cool, but still ya have to wonder. I think Ford did something like that back in the 60s to one of their Nascar entries?

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[IMG]



Trueleo.com/fiero.htm
RSpiderII@aol.com

quote Francis T said: Did you get up to 70mph with both runs? It seemed to flow better at the higher speed?

Here ya go:

"The first half of the video shows the airflow (or lack thereof) through the stock vent grating. The second half shows the airflow through the modified one. In both cases, I accelerated to about 70 MPH. I was going the same direction, on the same stretch of road, in the same weather conditions.

What a difference! The increased airflow through the vent gratings should help with engine compartment cooling, as well as helping to fill the void behind the rear window (i.e. less drag)."

I took some measurements earlier today. In order to see the entire decklid, the camera will have to be mounted about 3 feet above the roof. So I'll need to build a roof mount. That ought to be interesting. The plus side is that there's nothing stopping me from swinging the camera around and filming the hood.

Since this thread is rapidly growing, I decided to consolidate all the test results into my original post on Page 1. This way, people won't need to scroll through several pages just to see it all.

[This message has been edited by Blacktree (edited 06-04-2008).]

quote Originally posted by 87GT_97114: I figure there can't be a great deal of force in any direction, consider what's holding it to the deck lid. 4 6MM screws into plastic.

Well, the force was capable of pulling a couple of those screws through my wing, lol. I think there's more up force than down force. My reasoning, is that when my bolts were broke loose, it was basically lifting my wing up off the car.