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Discussion Starter #1
Hello everyone,

I apologize if this has already been discussed in various other threads.

First the published aerodynamic specs for the AW11 MK1:
The drag area (CdA) on a 1986 MR2 is 5.8 square feet according to Wikipedia http://en.wikipedia.org/wiki/Automobile_drag_coefficient
The drag coefficient (Cd) on a 1984 (1985?) MR2 is .34 according to Carfolio http://www.carfolio.com/specifications/models/car/?car=31768

The .34 Cd surprised me, as I thought it would have been higher.

My question is this. What would be the most sensible and cost-effective modifications to implement (and definitely in order of priority) to improve both the CdA drag figure as well as the down-force on a MK1 AW11?

I understand that CdA and down-force will oppose each other in almost every instance, but I do believe that they are not necessarily in opposition during the early stages of road vehicle aerodynamic refinement. For example both of these aerodynamic factors are improved when either an air dam is fitted to the nose or a smooth under-pan is retrofitted to the undercarriage.

The way I see it (and I'm certainly not well versed in road vehicle aerodynamics), it is a waste of time to try to improve the aerodynamics aft of about the top of the windshield along the roof & on to the tail as well as aft of the side mirrors & on to the tail. I say this because it looks like there is already severe flow separation occurring at the top windshield/roof juncture as well as at the A pillars. It seems to me that only VERY extensive (and expensive) reworking of areas at and aft of these points would produce any notable improvements and it would be far more sensible to just purchase a different car rather than attempt mods to that level.

I am aware of the plastic rear window fastback redesign on Toyota's past 222D Group S/Group B rally car AW11 http://www.flickr.com/photos/[email protected]/4060555984/ but I don't see how flow separation, turbulence, and drag would not result from the use of a rear window that is not very carefully designed (I think the down-slope angle is excessive). Again, IMO, most if not all the effort should be devoted to the front of the car.

Correct me if I'm wrong, but I think three of the most important initial aerodynamic improvements we should consider for the AW11 is to:
1-Lower the car.
2-Rake the car for a slight nose-down attitude.
3-Install a deep & effective air dam.

Thanks for any opinions on this....
 

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hm.. how much work do you want to put into this?

i've got access to solidworks and comsol. i know how to use solidworks but i have no clue how to use comsol. (i could learn though, i've been meaning to)

solidworks (or any cad software...) can be used to build a model of the car. comsol will analyze fluid flow over that model.

if we model the car as-is, we could study airflow and design new ways to improve it. we could then test them out on the compy with zero fab cost. how good are you at modeling things in cad?

maybe it's overkill, but we could pull it off and have definitive answers to mk1 aero questions at the end of the day.

EDIT: actually, check this out http://mr2oc.com/showthread.php?t=250308
 

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Discussion Starter #3
tamirtoad said:
hm.. how much work do you want to put into this?

i've got access to solidworks and comsol. i know how to use solidworks but i have no clue how to use comsol. (i could learn though, i've been meaning to)

solidworks (or any cad software...) can be used to build a model of the car. comsol will analyze fluid flow over that model.

if we model the car as-is, we could study airflow and design new ways to improve it. we could then test them out on the compy with zero fab cost. how good are you at modeling things in cad?

maybe it's overkill, but we could pull it off and have definitive answers to mk1 aero questions at the end of the day.
Thank you tamirtoad, but as far as I know, a lot of MK1 aerodynamic virtual modelling (I guess that would be the correct term) work has already been done at this thread: http://www.mr2oc.com/showthread.php?t=250308

Does that thread not already provide all the answers that could be provided from a virtual modelling perspective? Does the thread omit the changes that would result from the addition of supplemental aerodynamic devices? After a lengthy read through it, I think it does and that is what I'm trying to get to the bottom of here.

My problem is that I am no good at computers and don't have access to a CAD program anyway. I can build things if they are in my budget and do a lot of modelling in my head, but that is about the limit of my talents.

The MK1 aerodynamic upgrade program I am trying to achieve is intended for road use at freeway speeds. A common freeway speed limit in open areas in the US is currently 75MPH, and as we all know, you get run over when going the speed limit.

Texas, as of August 30th 2012, has increased their speed limit to 85MPH on a stretch of toll road between Austin and San Antonio. It is predicted that other states will follow suit and we might be seeing predominate traffic flows in some areas in the 90MPH bracket sometime soon. I know that I have personally seen predominate traffic flow speeds of about 85MPH (and even higher!) on some sections of I25 in Colorado.

The point I'm trying to make is this, at the speeds discussed above, and the short-term speed increases necessary for passing, I think it is safe to say that we would want our MK1s to be respectably stable and aerodynamically efficient to safely and competently deal with this speed environment. Hence the purpose of this thread.

Thanks....
 

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MassOut,

The CdA is not really able to be modified since it is essentially the cross sectional area of the car. The Cd how ever is where improvements can be made. From a smooth underbody, to wing size/location, front airdamn, vented fenders, venting the radiator through the front hood (this will also slightly affect the CdA if done right, but you would loose use of the frunk), and to the rear window/deck area.

Norm Floyd used to race a Mk1 in the SCCA ITA class and had probably one of if not the best prepared ITA MR2's in the country and had very few(partially because of the rules, but means it was still fairly stock areo wise) found that the rear wing does help improve the Cd of the car and he would run it on the higher speed tracks, but would remove it on the slower tracks due to the weight. Going through the Mk1 CFD thread you can see that for a wing/spoiler to be most effective it needs to hang off the back of the car to at least the rear most part of the bumper and be raised a couple of inches at a minimum.

With regards to the rear window cover, on the 222D its primary purpose was not so much to cleanup the airflow (even though it did because it cuts down on the low pressure area between the buttresses significantly), it was meant as the engines air feed because the underbody work on a car like that is non existant. It needed far more air to be used than a standard MR2 and as such extra (clean was also a big thing as even the side vents would gather a lot of dust on a gravel stage) air was fed from the top of the car.

If you are simply refering to highway speed stability, there are little things that can be done, however in the time I've spent on track with my car at 100+mph the car is not unstable at highway speeds, even at 100+ it remains stable but a little light at the frond end.
 

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Discussion Starter #5 (Edited)
Thanks tlmotorsport.

I understand that venting the radiator's outlet air through the top of the roof is very common on race cars (and for good reason), but it seems to me that this hot air going over the roof and into the cabin air pickup point at the base of the windshield would detrimentally impact comfort in the cab on hot days.

I don't know if this presumption works out in practice though, and it probably depends on the particular car's aerodynamic design as well, and that is why I'm very interested in hearing from those that have specific aero experience with the MK1. Nevertheless, despite the advantages of a front hood radiator exhaust venting concept, I'll probably pass due to the concerns stated above.

What I would really love to see here in this thread are complete explanations as to why certain competently executed changes to the MK1's bodywork result in aerodynamic improvements, not just that so and so aerodynamic alteration or device results in an improvement.

For example:
What exactly is happening to the airflow and why is it happening when a specially designed rear window cover like the one used in the 222D AW11 rally car is retrofitted?

What exactly is happening to the airflow and why is it happening when you ventilate the front wheel wells via topside louvers in the fenders and why are the wheel wells building pressure in the first place?

What other fairly easy to implement aerodynamic devices (not full body kits) benefit the MK1 and why?

What about a professional application of aerodynamic fences to certain parts of the bodywork such as to the front hood as used on the Porsche 935 K3? Could they also be used at the A pillar?

In the meantime, I found this simple info on the web: http://www.popularmechanics.com/cars/how-to/4271181 Basic, but a fun quick read.

This is also a great newfound resource: http://ecomodder.com/forum/aerodynamics.html



Thanks a bunch everyone....
 

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Discussion Starter #6
Salt flats racers etc...

Is there anyone who has run a MK1 AW11 at the salt flats or any other land speed event and modified the body aerodynamically for lower drag?

If so, it would be great to hear from them here or at least read about what they have done to their car via a hyperlink. I would also like to know where I could find a complete historical list of land speed records broken down by the make & model of car, or even just a complete list of some sort.

I am interested in land speed record information because it seems to me that the typical road racing car's aerodynamic design is dictated first by the ventilation requirements of the various heat generating components, control of high pressure and low pressure regions, and down-force. Aerodynamic streamlining seems to be of secondary importance.
 

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I realize this is old, but I figured it's the best place to post a response. I'm going to assume for general modding, nothing like decreasing the rake of the windshield to match the hood, removing road needed items [wipers] or anything. I, sadly, don't have a MkI yet, or even the funding to do these modifications. All is based on research.

First is trying to minimize the amount of surface area [Ca]. The only thing you can really do is get smaller side mirrors or move them to inside the cabin. Folding them in is simple enough though. Some smooth out the entire car [tape/putty all the gaps, eliminate recess' like turn signals, etc].

There are many ways to go about decreasing Cd. With only accounting for directional flow, in most-lease cost effective order::
{Fundamentals of Vehicle Dynamics: Thomas D. Gillespie: 1-56091-199-9
p89}
1970s Vehicle Example
1)Afterbody [engine bay on back]::0.14
2)Wheel wells::0.09
3)Underbody::0.06
4)Forebody [windshield on forward]::0.05
5)Other
Total Cd:: 0.42

Two dimensional analysis, of the MkI, is of the center line of the car. I'll start from the front and work back.

1) Front end drag can be minimize with a splitter, smoothing out the bumper area, etc. You want to get the 'point' of the front close to the splitter.
0.0 < Zs / Zv < 0.1 [sweet spot is around 0.07]
Zs= distance from 'point' to splitter location
Zv= vehicle hood height
{Gillespie p 90}

2) Large build-up of pressure build up just under the lip of the front, which is why venting the hood is such a great option {Gillespie p94}. Good rule of thumb is, if you do this DIY, is to try and keep the incline of the recess to be under 28 degrees [straight ahead and angle down] {Gillespie p89}. Any more of an incline increases drag [basically a front wing]. This also has even more benefits of decreasing surface friction well past the hood. The short info of it is it helps decrease the skin coefficient of the car [aka:: the wind won't 'stick' to the car as much]. This is basic heat transfer in relation to the boundary layer. A blacked out hood may also aid in this process. This, in turn, may increase pressure build-up around the windshield/wiper area. Only way to confirm is through wind tunnel testing or simulations.

3) The one thing that is brought up, in the AW11 aero discussion, is the area just coming off the windshield. It is a low pressure area, so possible wind cap to stabilize the flow is an option.

4) The engine lid on back is the biggest area to improve. Simplest thing to do is cover it. For a hatch-like design on the MkI, you want the incline to never exceed 30 degree drop [18~ degrees is the sweet spot] {Gillespie p89} Just running a cover over the recess will add a lot of improvement.

5) The rear wing is covered enough on other discussions on here already.

Ground clearance is a double win, seeing as you upgrade to stiffer springs and lower the pressure under the car at the same time. This, again, makes the engine lid very difficult in design for venting. I would like to entertain the possibility of an actuated scoop that makes air to be pulled from the engine lid and open during overheating/fan operating.
 

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I don't have access to CFD, so i can't test my two theories. Here's a ~$100 aero/lift list.
-Plumley Rear Wing Mod
-Front End RSm Spoiler Spreader Mod [http://www.mr2oc.com/showthread.php?t=152187]
-Garden Edge Lip Mod [http://www.mr2oc.com/showthread.php?t=232226]
-Reverse Side Scoop?
-Sunroof Wind Deflector?

From the CFD posts, there is turbulence in the engine bay and by the sounds of popping the sunroof shows upwards flow on the lid. Reversing the side scoop to vent, rather than intake, forces the engine lid to be the only intake for air, in theory.
There is a low pressure build-up just coming off the windshield. A sunroof wind deflector will make the transition from windshield to roof line more gradual, thus increasing pressure in the area.

I'm really interested in seeing if anyone can help prove/bust my two ideas. One seems free and the other is a $20-$40 add on.
 

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What about air entering the engine bay from underneath the car? Looking at the theories posted on the forum, that's the general consensus.
Driving down the freeway at speed, if I leave the engine lid unlatched, it will rise up to about 45 degrees from horizontal at speeds around 75-80 mph suggesting that airflow moves from the bottom to the top of the engine compartment.

I really don't think the side vent does much unless you're in stop and go traffic with the vent fan running. I also think it's pretty clever too, as air drawn in from the vent during stop and go driving is a bit higher off the ground and hasn't traveled through the exhaust manifold area, delivering negligibly cooler air.

I have no proof of this theory, but I invite you to try driving with your engine lid unlatched to see what happens. I've also heard you must have the clear rear visor to encounter the lid lifting at speed, but who knows...
 

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Depends on how you measure it. If you take height of the top of the roof off the ground multiplied by the maximum width it's optimistic. If you do an integration of x and y dims, it's perfectly believable. Keep in mind that the height from the bottom of the rocker to the top of the roof is almost exactly one meter, and the width across the roof is only a bit more than one meter.

Of course it's all largely irrelevant because it's just a reference to scale. If you remember your similitude lessons, it's clear the Cd is only good when taken for a single specific reference area (hell we could have based it on trunk lid area if we wanted to, and then Cd would have been higher). But in the end, drag is drag. That's why CdA has become the preferred number to report.

The mk1 mr2's are tiny and have reasonably modern aerodynamics and as a result have low CdA values. In fact, most references have it among the lowest CdA's values of any production car ever made.

O
 

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Haha. I vaguely recall the significance of similitude, but anyway you cut it, it does seem remarkable if the drag is really that impressive. CdA is still just a combined factor, which is my guess as to why it is a popular reporting figure. All ya really want is the factor needed to multiply the variable with.

So assuming a 1.6m^2 frontal area, and CD of .34, drag limited speed is about 160mph with 220chp/190whp. I find that pretty awesome.

I also vaguely recall that certain features have drag "penalties" that equate to larger than actual projected frontal areas. Mirrors for example.
 
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