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GT Rear Deck Aerodynamics by gem1138
Started on: 11-05-2013 11:42 AM
Replies: 20 (695 views)
Last post by: Arns85GT on 11-07-2013 03:19 PM
gem1138
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Report this Post11-05-2013 11:42 AM Click Here to See the Profile for gem1138Click Here to Email gem1138Send a Private Message to gem1138Edit/Delete MessageReply w/QuoteDirect Link to This Post
For reasons unrelated to this thread, I had cut a hole in my catalytic converter and went for a test drive. As a few minutes into the drive I saw smoke coming from the rear deck lid vents at high velocity. That means that the pressure above the deck lid is low. That's good for engine compartment ventilation but bad for the cause of promoting down force. I long ago removed my rear wing, feeling that it is pretty well out of the flow and to close to the deck lid to function as a wing. It may function somewhat as an air dam and thus reattach the airflow to the deck. If so, this should increase the pressure on the rear deck lid. That in turn should decrease the pressure difference between the engine compartment and the space above the deck lid, thus reducing the desirable cooling airflow through the engine compartment onto the deck lid.

I have seen taller aftermarket wing pedestals and they may raise the wing enough to allow it to act as a wing and not an air dam.

Unless you do the track day thing, down force is of no importance. I have never experienced over steer that I would attribute to aerodynamic lift at street semi-legal speeds. If you drive well over the speed limit, you might want to consider the taller pedestals to produce down force while not reducing the engine compartment cooling.

I have been through ignition coils and modules, whose failure I attribute to heat buildup after one stops. The Indy Fiero had a scoop above the roof as we all know, but it is my understanding and belief that it worked as much or more as a chimney as a scoop, creating convection currents to cool the engine compartment when the stopped after a high speed run as a pace car. In the stock car the hot air is trapped.

I don't see why all of these comments shouldn't be as applicable to the notchies as the GT's except that the rear deck grill on the notchies is better located to allow the hot air to escape when the car is stopped. FYI: Heat does not rise as is often said. Hot air rises (if it is not trapped) because it is less dense than cooler air. Heat radiates in all directions, immune to the effects of gravity.
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Report this Post11-05-2013 12:58 PM Click Here to See the Profile for onesexyfieroClick Here to Email onesexyfieroSend a Private Message to onesexyfieroEdit/Delete MessageReply w/QuoteDirect Link to This Post
By far the biggest aerodynamic challenge with our cars in the build up of a pocket of air from the radiator, which can be solved by venting the hood.

I read that when developing the Fiero, the engineers tried directing air flow from under the car, up through the engine bay, but abandoned the idea because it was picking up too much dirt.

ICM failure can be pretty well controlled by ensuring the crossover pipe heat shields are intact, fans are working (for pre-88's), and the ICM is properly greased.
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Report this Post11-05-2013 01:47 PM Click Here to See the Profile for Francis TClick Here to visit Francis T's HomePageClick Here to Email Francis TSend a Private Message to Francis TEdit/Delete MessageReply w/QuoteDirect Link to This Post
For the street I would not bother, but if you're building a chump race car then you may want to make some type plexy cover over the rear to make it more of a fastback. Such will also require some type extractor scoop or venting holes and a higher wing. Checkout our MR2 chump racer whereas the MR2s have the same problem. BTW that car has 3 1st place wins and a few 2nd and 3rd place finishes.
http://trueleo.com/RaceCar.htm

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Report this Post11-05-2013 04:36 PM Click Here to See the Profile for 82-T/A [At Work]Send a Private Message to 82-T/A [At Work]Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by gem1138:

For reasons unrelated to this thread, I had cut a hole in my catalytic converter and went for a test drive. As a few minutes into the drive I saw smoke coming from the rear deck lid vents at high velocity. That means that the pressure above the deck lid is low. That's good for engine compartment ventilation but bad for the cause of promoting down force. I long ago removed my rear wing, feeling that it is pretty well out of the flow and to close to the deck lid to function as a wing. It may function somewhat as an air dam and thus reattach the airflow to the deck. If so, this should increase the pressure on the rear deck lid. That in turn should decrease the pressure difference between the engine compartment and the space above the deck lid, thus reducing the desirable cooling airflow through the engine compartment onto the deck lid.

I have seen taller aftermarket wing pedestals and they may raise the wing enough to allow it to act as a wing and not an air dam.

Unless you do the track day thing, down force is of no importance. I have never experienced over steer that I would attribute to aerodynamic lift at street semi-legal speeds. If you drive well over the speed limit, you might want to consider the taller pedestals to produce down force while not reducing the engine compartment cooling.

I have been through ignition coils and modules, whose failure I attribute to heat buildup after one stops. The Indy Fiero had a scoop above the roof as we all know, but it is my understanding and belief that it worked as much or more as a chimney as a scoop, creating convection currents to cool the engine compartment when the stopped after a high speed run as a pace car. In the stock car the hot air is trapped.

I don't see why all of these comments shouldn't be as applicable to the notchies as the GT's except that the rear deck grill on the notchies is better located to allow the hot air to escape when the car is stopped. FYI: Heat does not rise as is often said. Hot air rises (if it is not trapped) because it is less dense than cooler air. Heat radiates in all directions, immune to the effects of gravity.



I had a spoiler on my 1985 Fiero GT 4-Speed back in the day... (notchback obviously). In around-town driving... water that was sitting on the spoiler would basically sit there, but then when I reached speeds at over 80-85 miles an hour, I felt a difference. Now, I added the factory spoiler afterwards, and after installing it, it "felt" like the vehicle sucked to the ground a bit more at irresponsible speeds where as before it felt a bit more twitchy. Could just be placebo effect, but I have no way to verify. It's worth mentioning too that the water would sit still on the spoiler until I actually reached speeds of about ~45... then it would start to drip away. By 60, the spoiler was bone-dry.
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Report this Post11-05-2013 05:55 PM Click Here to See the Profile for gem1138Click Here to Email gem1138Send a Private Message to gem1138Edit/Delete MessageReply w/QuoteDirect Link to This Post
I have been driving on the Interstate at 75-80 and had it start raining. The rear deck stayed bone dry till I slowed down. Now though, I understand that the air passing through the deck grills likely contributed to that phenomenon. Nonetheless, the stock wing set up, at least on the gt, appears to be ineffective.

Also, my gt is an 88 without to ICM cooling fan. Years ago, after being left walking several times, I installed a 3" diameter inline fan (from Pegasus Racing) in the trunk and ducted it onto the ignition coil and module with PVC pipe. It is activated by a thermostat attached to the EGR heat shield set to 150 degrees Fahrenheit. I have looked at a pre 88 fan and I concluded that GM decided it was ineffective. By then they’d had years to compare the life span of components with and without the fan and likely observed no difference. I like to think that the one I installed is more effective. It blows like a hair dryer with the heat off and I had no problems till debris broke a fan blade unbeknownst to me. All is well now.
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Report this Post11-06-2013 07:19 AM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
Some info you might find interesting from Road & Track November 1984:

"For those aerophiles among our readers, the following will be of more than passing interest. The original Fiero with 13-in wheels and tires has a Coefficient of Drag (Cx) of 0.377; add the 14-in wheels and tires and that figure climbs to 0.406. With the pace car nose and 14-in wheels and tires the coefficient of drag drops to 0.372 and falls still further to 0.350 with the addition of the rear wing."

And from Car & Driver Feb 1986 about the fastback:

"The drag coefficient has been lowered a point to 0.34 with the optional wing..." [and with the new fastback]

[This message has been edited by Bloozberry (edited 11-06-2013).]

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Report this Post11-06-2013 09:56 AM Click Here to See the Profile for BoostdreamerClick Here to Email BoostdreamerSend a Private Message to BoostdreamerEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Bloozberry:

The original Fiero with 13-in wheels and tires has a Coefficient of Drag (Cx) of 0.377; add the 14-in wheels and tires and that figure climbs to 0.406. With the pace car nose and 14-in wheels and tires the coefficient of drag drops to 0.372 and falls still further to 0.350 with the addition of the rear wing."



I don't understand those numbers. Shouldn't a rear wing create MORE drag if it is functioning correctly by increasing down force?

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Report this Post11-06-2013 02:40 PM Click Here to See the Profile for 84fiero123Click Here to Email 84fiero123Send a Private Message to 84fiero123Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Boostdreamer:


I don't understand those numbers. Shouldn't a rear wing create MORE drag if it is functioning correctly by increasing down force?




Maybe this will help.

Rear spoiler[edit]

A rear spoiler usually comes standard in most sports vehicles and resembles the shape of a raised wing in the rear of the vehicle. The main purpose to the inclusion of a rear spoiler in a vehicles design is to reduce lift, therefore increasing stability at higher speeds. In order to achieve the lowest possible drag, air must flow around the streamlined body of the vehicle without coming into contact with any areas of possible turbulence. A rear spoiler design that stands off the rear deck lid will increase downforce, reducing lift at high speeds while incurring a drag penalty. Flat spoilers, possibly angled slightly downward may reduce turbulence and thereby reduces the coefficient of drag.[10] Some cars now feature automatically adjustable rear spoilers, so at lower speed the impact on drag is reduced when the benefits of reduced lift are not required.

http://en.wikipedia.org/wik...ile_drag_coefficient

They did, by the way for the younger crowd here do wind tunnel tests on these cars during the testing and prototype builds to get the best of both worlds out of the design of them. you can't have a best of both worlds without some compromises.

Steve

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Report this Post11-06-2013 02:49 PM Click Here to See the Profile for 84fiero123Click Here to Email 84fiero123Send a Private Message to 84fiero123Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by gem1138:

I have been driving on the Interstate at 75-80 and had it start raining. The rear deck stayed bone dry till I slowed down. Now though, I understand that the air passing through the deck grills likely contributed to that phenomenon. Nonetheless, the stock wing set up, at least on the gt, appears to be ineffective.

Also, my gt is an 88 without to ICM cooling fan. Years ago, after being left walking several times, I installed a 3" diameter inline fan (from Pegasus Racing) in the trunk and ducted it onto the ignition coil and module with PVC pipe. It is activated by a thermostat attached to the EGR heat shield set to 150 degrees Fahrenheit. I have looked at a pre 88 fan and I concluded that GM decided it was ineffective. By then they’d had years to compare the life span of components with and without the fan and likely observed no difference. I like to think that the one I installed is more effective. It blows like a hair dryer with the heat off and I had no problems till debris broke a fan blade unbeknownst to me. All is well now.


Earlier models with the V6 had a fan that blew cold air on the alternator to prolong its life, but as with all of GMs cars the bean counters come into play as always so that may have had something to do with them eliminating the blower in the truck. if the alternators were eliminated it could have been just a cost savings thing because they would last far beyond the warranty period. They don't care about building a long lasting car, just a car that will last till the warranty expirers. after that its the owners problem. just like how they will engineer things to wear out after a few years so that they can sell you a new car.

so it may not be because it didn't work but because it did, ever think of that.

Steve
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Report this Post11-06-2013 05:55 PM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Boostdreamer:
I don't understand those numbers. Shouldn't a rear wing create MORE drag if it is functioning correctly by increasing down force?


I don't believe the rear wing was positioned to function as a down force generator, too much of it is simply masked from clean airflow by the vertical rear window. I suspect it was primarily for aesthetics and a small side benefit was that it reduced some of the turbulence generated by the abrupt end of the roof line. Interestingly, the numbers show that from an aerodynamic perspective, all Fiero's are essentially notchbacks.
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Report this Post11-06-2013 07:49 PM Click Here to See the Profile for BoostdreamerClick Here to Email BoostdreamerSend a Private Message to BoostdreamerEdit/Delete MessageReply w/QuoteDirect Link to This Post
So long story, short: If the wing creates down force, it will also increase the drag. No free lunch.

Jonathan
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Report this Post11-06-2013 07:57 PM Click Here to See the Profile for BlackEmraldClick Here to Email BlackEmraldSend a Private Message to BlackEmraldEdit/Delete MessageReply w/QuoteDirect Link to This Post
A member here did a large aerodynamic study about the effectiveness of the wing and the air coming through the vents. there are several videos, and I will see if I can find the thread. The basic conclusion was that the wing smooths out the air going off the back of the car, which is good for 2-3 MPG on the highway.

Here's the thread: http://www.fiero.nl/forum/A...110502-2-092223.html

[This message has been edited by BlackEmrald (edited 11-06-2013).]

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Report this Post11-06-2013 09:05 PM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks for posting that BlackEmerald.

 
quote
Originally posted by Boostdreamer:
So long story, short: If the wing creates down force, it will also increase the drag. No free lunch.


That is correct.

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Report this Post11-06-2013 11:57 PM Click Here to See the Profile for jim94Click Here to Email jim94Send a Private Message to jim94Edit/Delete MessageReply w/QuoteDirect Link to This Post
What do you think happens to the air if you move the spoiler back and down. I can see out my back window better though. This is my Aero story. I washed my car and took it for a ride and it took till 90mph till the water left the spoiler . That happened in 1989 with my first GT and oh I received my first ticket that day too.
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Report this Post11-07-2013 04:50 AM Click Here to See the Profile for f85gtronSend a Private Message to f85gtronEdit/Delete MessageReply w/QuoteDirect Link to This Post
I've been thinking about this for a while and wonder if simply adding a roof extension, or lip, of about two or three inches to the notchie over the rear window would clean up a lot of air. in my head, this makes sense because you would be effectively creating a defined low pressure pocket which would create more vacuum for the engine vents to work correctly and the fast air coming off the car would stay organized until it reached the rear where the spoiler is..... somebody with bad valve cover gaskets and burning manifolds could test this and let us know the results?
ron
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Report this Post11-07-2013 06:44 AM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
I think to get any measurable downforce from a wing you'd need a full length hatch like some of the choptops have done. But even then, the Fiero suffers from a front end that's too light at speed (Road & Track measured a 100 lb lift at the front end at highway speeds). Adding more downforce behind the rear wheels would only exaggerate this.
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Report this Post11-07-2013 08:34 AM Click Here to See the Profile for rogergarrisonClick Here to Email rogergarrisonSend a Private Message to rogergarrisonEdit/Delete MessageReply w/QuoteDirect Link to This Post
I pretty much believe the spoiler was simply for looks. Ive had them with and without wings and notice no difference at all. Ive seen string tests that show the air actually goes from rear to front over the decklid, so its not giving any downforce. If you want it to provide any percievable downforce, you need to mount it at roof level or above. I simply dont believe any stats that say you improve gas mileage by 3 mpg with it.
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Report this Post11-07-2013 10:28 AM Click Here to See the Profile for jaskispyderSend a Private Message to jaskispyderEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rogergarrison:

I pretty much believe the spoiler was simply for looks. Ive had them with and without wings and notice no difference at all. Ive seen string tests that show the air actually goes from rear to front over the decklid, so its not giving any downforce. If you want it to provide any percievable downforce, you need to mount it at roof level or above. I simply dont believe any stats that say you improve gas mileage by 3 mpg with it.


 
quote
Originally posted by Bloozberry:

Some info you might find interesting from Road & Track November 1984:

"For those aerophiles among our readers, the following will be of more than passing interest. The original Fiero with 13-in wheels and tires has a Coefficient of Drag (Cx) of 0.377; add the 14-in wheels and tires and that figure climbs to 0.406. With the pace car nose and 14-in wheels and tires the coefficient of drag drops to 0.372 and falls still further to 0.350 with the addition of the rear wing."

And from Car & Driver Feb 1986 about the fastback:

"The drag coefficient has been lowered a point to 0.34 with the optional wing..." [and with the new fastback]



lowering drag will increase MPG.
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Report this Post11-07-2013 11:32 AM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rogergarrison:
I pretty much believe the spoiler was simply for looks.

Yup, Most OE "Wings" are decorations and do nothings or hurt MPG and do nothing.

At normal highway speed, including higher speed limit like 75MPH, all they do is add drag to the car.

Many with "pop up" wings activate at 80-100MPH, some even higher, to help MPG. Lower speed, does nothing and often hurts MPG.

Both styles use same rear aero to help vent engine bay. Many think engine bay is hot all the time is . Car moving, above a few MPH, means engine bay temp is around ambient air temp, IE local weather temp. If you saw Mythbusters pickup gate on vs off w/ model truck in water tank... Fiero rear aero is similar to trunk w/ gate on and up. Air hit middle of lid then rolls toward the window and create low air pressure to suck air from engine bay.

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[This message has been edited by theogre (edited 11-07-2013).]

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Report this Post11-07-2013 01:24 PM Click Here to See the Profile for BloozberrySend a Private Message to BloozberryEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by theogre:
At normal highway speed, including higher speed limit like 75MPH, all they do is add drag to the car.


According to the R&T numbers, the Fiero wing does reduce drag.

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Report this Post11-07-2013 03:19 PM Click Here to See the Profile for Arns85GTClick Here to Email Arns85GTSend a Private Message to Arns85GTEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by BlackEmrald:

A member here did a large aerodynamic study about the effectiveness of the wing and the air coming through the vents. there are several videos, and I will see if I can find the thread. The basic conclusion was that the wing smooths out the air going off the back of the car, which is good for 2-3 MPG on the highway.

Here's the thread: http://www.fiero.nl/forum/A...110502-2-092223.html



That thread is really well done and explains allot. The air which flows under the car, moves up the back bumper and forward over the deck. The wing interrupts the flow to a degree and it works to a degree. The problem is that the air spilling over the roof, runs into the fairly forceful forward moving air over the deck, and it tumbles right behind the window. The stock vents pick up enough of that tumbling action so the air moving out has no where to go and exits very low to the deck and off to the side. I didn't try the screening and that is really interesting.

for this reason, when I installed my scoop I installed it facing back with the fan blowing down into the engine compartment. This captures the forward moving air even when the fan isn't running.

On that front vent in the hood, it is really interesting to see the air tumbling after it clears the bumper and is headed toward the windshield. I installed a Camaro cowl similar to what other guys have done and it solves that problem, but does channel warm air toward the intake for the cabin vents. I also eliminated the pop up lights because they are just plain annoying to me. That video of the vortex created around the pop ups confirms I made the right move, albeit not for that reason.

Arn

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