STAGE I, MY AERODYNAMIC THEORYS

Knowing that the 86-87 SE has a C/D of 0.350 and an 86-88 GT has a C/d of 0.365 I had to do some brainstorming to rationalize why that would be since the GTs look slicker.

Since the only real difference between the two cars is behind the drivers head, that is where I started. I have read that BMW looks to reduce drag by starting at the back of the car first, I have made a rather pitiful diagram to hopefully illustrate what I think is going on.

Fig.1-1

As the car passes through the air it punched a hole in it. Since there is an open area just behind the rear window it causes a low pressure area that must be compensated for. This will in effect, cause a suction force. This leads me to few theories and questions.

My Theories:

1. Since the bodywork on GTs extends farther back, the low-pressure area behind the window will be deeper than a notchy's at any given speed. This is due to the extended barrier that delays side filling until further aft of the rear window. Air must come from above or below through the deck lid vents.

2. Assuming the lower pressure on the GT from theory 1, there could potentially be greater evacuation of air from below the trunk lid deck, through the vents, promoting a lower engine compartment temperature. (It would be interesting to measure the temps at highway speed for both cars given equal conditions).

3. GTs also use a different shape for the deckled vents. Whether this was intentional to take better advantage of my theoretical aerodynamic, cooling I cannot say. This leads me to another question. Is the low pressure beneath the car deeper, or is the low pressure behind the window the more intense of the two? This would determine the direction of flow through the vents and engine compartment.

4. What is the relationship between the low-pressure area behind the window, and the low behind the bumper? How do they interact? I believe that the wing prevents or diminishes the backfilling of air from below and behind the car. Therefore, if air cannot spill up and forward onto the trunk lid deck as easily, the low-pressure area directly behind the rear bumper will be diminished. This could possibly lower total drag due to a decrease in total turbulent flow. Question for non-wing drivers: If you drive on wet roads, does the rear deck , and possible rear window, get dirty with road grime? If so, how far forward does it extend? Does the grime settle evenly or does it have definite edges?

5. I am full of and should shut up.

Q. Does my theory sound plausible to explain the difference in C/D between the two cars?

Look for some experiments soon.
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87 Fiero SE, Tech IV, Auto.

[This message has been edited by RACE (edited 05-19-2004).]

my guess is exactly what you show in "Fig. 1.1"
less of a "backdraft" area behind the window

hmmm...

i notice that my rear window never gets wet on rainy hiways..i have an 85gt notchy, no wing...
i'll have to observe the deck lid patterns if any next time...

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1985 RED & SILVER GT 2.8 V6, 99.5k KM!

How much would wheel and tire combos contribute to this difference? The fatter and larger diameter tires on all the GT models might make up most of the difference that you speak of. All GTs are a little taller due to the larger diameter tires, so they are punching a bigger hole as they travel.

quote Originally posted by RickN: How much would wheel and tire combos contribute to this difference? The fatter and larger diameter tires on all the GT models might make up most of the difference that you speak of. All GTs are a little taller due to the larger diameter tires, so they are punching a bigger hole as they travel.

I was going to guess a difference in tires too, although I have no real basis to say it would or wouldn't be from the rear window/trunklid area differences between the cars. Could be a combination of both too!

1) The low pressure area behind the roof will also suck air up through the cooling vents from under the car to equalize. IMO, the extra length of the GT sail panel doesn't affect the amount of pressure back there, since they're pretty much in line with the vehicle's direction and the silhouette (looking fromt he rear) is almost the same. Mind you, the GT sails are sloped back more, creating less negative pressure above them than the notchies. That would direct the air rearward better than the notchie.

2) I agree.

3) In addition, don't the FB vents sit higher than the NB? If so, there's a more straight line for the air to come up and around the battery/air cleaner and through the vents. Better flow=more volume as long as the pressure is there.

4) The low pressure area behind the window leaves the air in a turbulent state so when it comes down the back of the car, it doesn't flow as smoothly. Secondly, I think the more gradual slope of the notchy taillight area is going to have a lower drag coefficient than the more upright taillight area of the FB. BUT! I think there coiuld be some air flow around the taillights themselves from under the car, and this would also reduce drag slightly.

5) I'm full of it, too, but this is an interesting topic.

im thinking kameo kid probably has the lowest amount of drag with that venty thing

STAGE II – AERODYNAMIC TESTING

Having been a science geek from birth, I like to form theories and then perform experiments to test them. This is summary of the events that took place during this afternoon's pointless test. Btw, don’t tell my wife this is how I squandered my afternoon.

Ambient Conditions:
Wind out of the SE 12 gusting to 15 mph, 7 miles visibility, overcast skies with bases at 1,500 feet above the ground and a temperature of 21 C, 70 F.

The Test Vehicle was outfitted with very sensitive aerodynamic indicating equipment, lengths of yarn from Wal-Mart, $1.45 and secured with the cheapest clear tape that I could find, $.045. This tape later proved to be a problem for the indicator on the wing pylon since it stuck the string in one position rendering it useless. The surface condition of the car was very slick, since it had just been waxed. This eliminated drag for the yarn and gave the truest indication of what is happening in the airflow. The test was conducted with the windows up and the roof closed.

The First Test Run was made with the wind. This photo was taken as I accelerated through an airspeed of approx 25 mph. Almost immediately the vents began exhausting air from under the deck lid as indicated by a tissue paper that I attached to one of the vent blades. Notice the string attached to the glass hangs almost straight down. The string just behind the vent wound itself up, suggesting a small vortex that was spinning there. It later pointed rearward and slightly to the center at higher speeds.

Interestingly, the indicator in the center of the trunk always pointed forward. It did move a little side to side during gusts . The string attached to the front and center of the wing pointed forward for most of the test.

The Second Test Run was made into the wind and yielded an airspeed of approximately 70 to 75 mph. You can see the string midway up the roof pillar curve inward at the end. This was typical at higher speed. The backside of the vent seems to have air moving upwards at a steep angle. This indicated to me that the center of low pressure was moving rearward.

The pylon string became tangled and was useless. Notice the edge of the wing, the string has moved slightly inward and shows a slight curl possibly indicating the production of negative lift.

At approximately 75 mph of airspeed you can clearly see the strings on the front of the wing are pointing forward as well as the string in the center of the decklid. Even the stuck indicator on the pylon seems to be pointing forward.

This final photo shows a length of yarn that I secured to the center of the roof. It would have indicated more if I had not secured the tail to the backside of the spoiler. However, I believe the area of slack, in the center of the string, shows where a shear zone exists. I believe it is where rearward-moving air from the roof encounters forward flow from the back of the car.

There were also strings attached to the rear of the spoiler that were never clearly visible during the test.

Conclusions:

-Air was exhausted upward through the vents at all speeds in the test although the center of pressure seemed to move rearward.

-Airflow moved forward in the center of the decklid during the entire test.

-There seems to be enough of a Low created by the rear window area to drawl air forward toward the rear roofline.

-Air at the tips of the wings always moved rearward and then slightly in and up at high speed.

-Since air on the outer portions of the deck initially swirled, then become more laminar and move rearward, there must be one or more shear zones where the air reverses direction.

-I do not believe the swirl patterns are as tight as I had originally thought.

-I need professional psychological help for even conceiving of the idea to drive down the road with all of that crap taped to my car.

Disclaimer: No Fieros were harmed during the production of this web page.

[This message has been edited by RACE (edited 05-19-2004).]

Proposal 1.0a for Airflow ExperimentA

Locate source of exhaust leaks on one GT and one Notchback (NB)
Determine positive findings of aerosol combusted oil particles from furthest point of exhasut system.
Route airborne particles through heatprotective conduit of decreasing diameter.
Conduit will branch and endpoints will be located at key aerodynamic locations on vehicle.
Secure optical imagery recording device to obtain visual observations of flow of airborne particles.

Well, i dont think my GT smokes QUITE enough to use it to check the cars airstream, but the general ieda might work... Use that stuff people use to water small gardens, with tiny spouts. Pipe smoke thru as your driving, and record with a vid cam. of course, a wind tunnel would be best, but a calm day would work too... Anyone have any weird ideas?

by the way, you're not the only born science geek out there... I have a thing for lasers

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DON'T FORGET TO VOTE FOR THE FIERO!

Black '87 GT Auto - Daily Driver & Autocrosser
Eibach springs, Koni shocks, Kosei K1s, Kumho Ecsta V700s, poly bushings, rear swaybar, MSD ignition, shift-kit, custom intake scoop

Excellent scientific method...formulate a thery, then test to see if it's confirmed. The only sugesstion I have is that you should have picked a different color for your yarn to be more visible. And motion video would be helpful here.

The more sleeker look of the fastback is an illusion. The roof line is identical and I agree that the sail panels, if anything, contribute to the negative pressure area behind the window. The air leaving the roof is going to be turbulent and the sail panels are going to delay the stream reattaching to the car increasing the drag over the notchback.

The only way to decrease drag is to have a hatchback window, like the finale, that covers the area. Good work!

Very nice looking car, Race! Kudos on your investigative prowess. I would still challenge that although your investigation does an excellant job of defining air movement around the car. I don't believe that any statements can be made about drag which was your original discussion.

[This message has been edited by RickN (edited 05-19-2004).]

come on, I know one of you is hiding a wind tunnel in their back yard

RickN, thanks for your comments. I believe that the higher sides on the GT deepen the low and produce more drag. Future experiments should shed some light. My theory is that the forward moving air over the decklid will exist farther out to the sides than on the SE therefore widening, and increasing the size of the low pressure area.

The next test will involve more targeted areas instead of everything at once. The most intriguing area for me right now is the wing and the rear deck lid. Flow around the roofline will come later. Shorter strings that cannot touch each other will be used in larger numbers to gain a more complete picture of the airflow at various speeds.

I hope to use video for my next tests but will need someone to host it for me. Video will allow me to make comments as the test is being conducted. I would also like to have somebody drive the car for me so that I can concentrate on the test and run the camera. A chase car would also make for some interesting video but would require an additional person.

Once I have reliable consistent results I would like to repeat some of the tests with a fastback to see any differences. If you have an 86-88 GT, live in central Iowa, and would like to kill some time on a completely pointless experiment, send me a pm.

quote Originally posted by Spyhunter: Proposal 1.0a for Airflow ExperimentA Locate source of exhaust leaks on one GT and one Notchback (NB) Determine positive findings of aerosol combusted oil particles from furthest point of exhasut system. Route airborne particles through heatprotective conduit of decreasing diameter. Conduit will branch and endpoints will be located at key aerodynamic locations on vehicle. Secure optical imagery recording device to obtain visual observations of flow of airborne particles. Well, i dont think my GT smokes QUITE enough to use it to check the cars airstream, but the general ieda might work... Use that stuff people use to water small gardens, with tiny spouts. Pipe smoke thru as your driving, and record with a vid cam. of course, a wind tunnel would be best, but a calm day would work too... Anyone have any weird ideas? by the way, you're not the only born science geek out there... I have a thing for lasers

What the heck did you just say ?!?

lol it took me a second

What your experiment does prove in a way, is what I've been saying all along: The design of the rear wing is to manage "back-draft" air flow coming up off the back of the car, which screws up the air flow rearward of the back glass ( in other words in minimizes the flow disturbance that would occur if the wing wasn't present ).

The only true way to see everything that's happening back there is in a wind tunnel with a smoke jet nozzle. The wing does assist down-force, same as the IMSA type spoilers, but they do it differently than most people think (because they can't see everything that's going on back there).

As far as your theory between notch-back and fastback......I won't go there, except to say it can get very complicated.

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0 to 75 Audio http://www.cartsys.com/fiero/cal-engine.wav

[This message has been edited by California Kid (edited 05-19-2004).]

RACE, kudos on doing this test! (and using the string and posting the pics of it all!)

My comments are based in part on information from the book "Fiero" by Gary Witzenburg. On the bottom right of page 74 he writes "...The '85 GT... parted the air with a Cd of .35. Compare this with the 1984 Sport Coupe's .38 and the fat-tired 1984 SE's .41..." If I am correct there is no difference in the bodies of the '84 SC and the '84 SE, just the difference in the width of the tires contributed to an increase in Cd of .03. The original discussion was about a difference in Cd of only .015. Since the bodies of the'85GT & '86-'87 SE look the same to the on coming air as the '86-'88 GT, I would think that the extra .50 inch of ride height due to the difference between 14" tires (24" OD) Vs 15" tires (25" OD) would be the first place that I would look for the extra .015 Cd. If anything I might speculate that without the extra sail panel length, the difference in Cd might have been greater than it is. After all .50" times the full width of the car is a lot of extra area, much greater than the extra width of 4 tires.

quote Originally posted by California Kid: ... The wing does assist down-force, same as the IMSA type spoilers, but they do it differently...

An IMSA wing would be another interesting experiment. It would like to see what effect it would have.

[This message has been edited by RACE (edited 05-19-2004).]

mabey you could try it with and with out the wing then we can see what the wings effect is btw the strings from a pom pom might work well

Thanks for this scientific work. Here is my situation.

My 85 Notchy now has a 5" scoop facing rearward on the deck. It is about 2' wide similar to Tina's. My wing is now on 5" stantions.

I assumed the air would flow out of the scoop but, the indicated shear tells me that air may actually flow under the wing from the tailight panel forward, and INTO my scoop.

I think I need to buy some yarn.

Arn

RACE....

Very interesting. You sound just like me. Always interested in experimanting. Example, I designed an anomometer, built it and now it is on my roof calibrated to the nearest 1mph (up to 55mph). I like your string idea. Infact, that is the way the racers used to do it years ago but they used smaller string and many more of them. If I talk to John C. in the near future, I'll ask about the wind tunnel testing they did with the IMSA wing. I know the trick was to get enough down force while minimizing drag. One big problem with the IMSA (race spoiler, not the street one) is that you can't see out the back. You may have noticed that an additional clear plexiglass "lip" was placed on later cars for just this reason... to see. My car is a prototype body, so you may not get exactly the readings the race team did, but if you catch me in the right mood at WheatStock, we could try taping some string to the rear of my car and turn the video camera around (if the intake is not in the way) and watch it at some real speeds (130mph +). I an almost certain that John tested the IMSA bodies to 160mph or so.

Great testing!

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Paul