Best approach for aero improvement on sedan roof?
 

Hey guys, I've got my first question... I've got a 2007 Chevy Malibu, and I've been getting into eco-modding it, and the easiest gains at the moment are aerodynamic... Generally speaking, is it better to keep flow attached to the back glass of a sedan through use of a vortex generator(like EVO 8/9), and use a lip spoiler like a lot of new cars are using, or is it better to build a partial kamm-back"sunshade" to extend the roofline?

Given the steep angle of the back glass, I highly doubt that any sort of attached airflow is present...

Unless you have the knowledge and equipment to tune the position of the vortex generator very precisely, the vortex generators will only help if you get extremely lucky with position. The EVO VGs were very precisely tuned. We can't really hope to duplicate that in our garages. That's something like a consensus on VGs among the most engaged and experienced folks on this forum, from what I have seen.

Also a consensus, a kamm is better than a wing, and a boat tail is better than a kamm. I partial kamm off the top of the window and a flat trunk lid spoiler might be a good combination. An airdam, grill blocking, and wheel skirts are also typical recommendations. They seem to work very well in most scenarios.

http://o.aolcdn.com/dims-global/dims...st2400_037.jpg

My plan for tuning the VG was to make a full width version and experiment with its fore/aft position in relation to the upper back glass seal until tuft testing showed attached flow down the back glass... However, given that profile picture, it appears that I should be building a partial kamm-back... I already have done an air dam extension and rear tire spats, but I don't have hubcaps, just the plain steel wheels... Future plans are lowering at same time as strut replacement(by means of cutting small amount from spring), adding at least a partial undertray, rear wheel skirts, grille blocks, and the kamm-back on the roof...

The end goal is to have a car that is still aesthetically appealing, and can achieve 40mpg highway with cruise set and A/C on if applicable... Given that I set my cruise at 80mph, that's a tall order, but nonetheless it's my goal

I think you can buy replica EVO VG arrays ready to stick on. The air flow over that part of the roof of any modern car is going to be very similar so I wouldn't spend too much time re-inventing the wheel.

40mpg at 80mph is a tall order indeed. My Civic Hybrid, which is fairly aero for a stock sedan at ~0.28cd and has an incredibly efficient engine, gets between 60 and 70mpg at ~55, but is under 40mpg at 80mph.

But the VGs on the EVO were designed to direct air to that high spoiler for the sake of downforce, and he will not have one. So he'll simply be adding drag without tue downforce benfit to performance at the high speeds.

I'll keep the VGs in kind for use on my Saturn, maybe(it has a OEM wing)...

But after hearing some opinions, I think that a partial Kamm would be best for the roof, and maybe a lip spoiler on the trunk lid, if I can make a decent one...

I wish I hadn't thrown away the three OEM hubcaps that come with the car, given the easy ways to make smoothies out of them...

I also understand that the 07-up L61 Ecotec has HPTuners support, which opens me up to tune the powertrain for better mileage and power...

I believe a small spoiler at the lip of your trunklid will help you. Like an open truck bed, it will help form a stable rolling bubble of air in the vacuum behind your backlight, and that acts as a sort of Kammback for the rest of the air flowing over your roof. It's far from perfect but better than nothing - which is what you have now.

Obviously some testing would be needed.

I'll second Ecky: I think 40 at 80 is going to be a really, really hard goal to meet. I'm softpedaling pretty hard just to break 50 at 60 (also a HCH1 like Ecky's), and our cars are both smaller and lighter than yours. But if you can do it, you are The Man.

Aero is your friend. Find every advantage you can take, and take them all. I drove a Malibu from Roanoke VA to Boston MA and had a blast. The trunk is like a Las Vegas magic act - all my stuff AND the wife's AND the kids AND the strollers fit comfortably inside, with plenty of room to spare. As a long-distance cruiser goes, it was one of the nicest trips I've ever taken. And as American cars went at the time, it was super thrifty. But still, I never got better than mid 30s. If you can crack that lofty goal I will be very impressed indeed.

Well, as the car sits, I can get right at 30mpg at 80mph, cruise set and A/C on, here in WV... I have an air dam extension, good for around 1.8" lower than stock, and rear tire spats that are about two inches below my rocker panels, but I ended up getting a 215/60-15 tire instead of the OEM 205/65 due to availability... I think I lost just a little highway mpg, but gained a little on my way to work due to my being able to pitch it into corners a little faster...

Yes, but Autospeed's tuft testing showed attached airflow on the rear window of a sedan after installing VG's.

It is amazing how often I forget about Autospeed. They always have good stuff!

Yeah, that is true. BUT this is the conclusion of that study: "On the basis of the scientific evidence that is available, and the results of this test, we very much doubt whether vortex generators fitted to the trailing edges of vehicles will reduce drag."

They argue that VGs energize the boundry layer and creates more attached flow, but they do not conclude that VGs reduce drag at the steeper angle that the trailing edge offered. And the car they used, a GEN 1 Prius, arguably had a much better angle on the rear glass than this 2007 Malibu. Considering the steeper edge, I'd suggest avoiding the VG tuing for this application.

AutoSpeed - Blowing the Vortex, Part 4

your going to REALLY need to lower the rpm to get 40 at 80. there is a new guy here with the old big body caprice getting some crazy numbers but hes running a 2. 7 (??) rear ration and taller tires.
Since you just bought tires, you'll have to wait to go up size.
And thinking that your 'cutting the corners faster' is gaining anything is false.
how often are your 'turning'??? maybe 10% of the time?
if you're turning more than that, you'll NEVER get better mpg.
you would be better off with taller tires because if you are on the freeway, it's all about low rpms.
Do taller tires and nylon bushings on the sway bar and suspension points if you want to race thru corners.

Well, my car has the 4T45E transaxle(still despise it's being an automatic but no manual was offered on this car) with the 3.63 final drive ratio, and I turn around 2600 at an indicated 80mph... Other final drives were offered (the 4T45 behind the 3.5V6 was 3.08, and the Malibu Maxx had like a 3.23 ratio to compensate for weight), but I don't know how well my L61 (2.2 L4)PCM would play with a taller transmission... Not that it has power to turn taller gears anyway... The main issues with getting great highway mpg is the interstate on which I do much of my highway driving(most of it is two lane at 55-60mph), it has lots of 5 and 6% grades, on which the engine struggles, and the trans kicks down, and then I'm turning 4000+RPM getting an instant reading of about 12... Figuring out how to get the trans to stay in OD and still maintain speed is just a matter of reducing resistances(drag, rolling resistance, weight), but I'm just getting started...

... And I read the whole article on the VGs, and they claimed that they achieved a .007Cd reduction on the Evo they developed them on, and the result of installing them was a smaller wake(in the second part of the above posted article by AutoSpeed), even with the wing considered... The makers of the AirTabs claim as much as a 7% fuel savings at a constant 120km/h... That alone, if I gained the full claimed benefit, would get me up to 32.8mpg at 80mph(30x1.07)...

Test the manufacturer's own claims for FE at the risk of your own $$. It might be fun, and perhaps there is an off chance that it'll work.

The AutoSpeed story does claim a small reduction in drag for the AirTabs. I see that. But 0.006 is not going to produce a 7% gain. A kamm or boattail is going to be better. I just think those are your better proven options. But if you are curious, then experiment. Share your tuft testing here.

On third thought, maybe your Malibu's rear window is close enough to the angle of the Gen 1 Prius that AutoSpeed tested, though the interaction with the trunklid is harsher:

http://o.aolcdn.com/dims-global/dims...st2400_037.jpg

http://www.fleetsandfuels.com/wp-con...Prius-1111.jpg

Good luck.

I don't plan on spending anything to make the VG array... I can get scraps of coroplast(trapezoid shape 19x15) for free at work, I plan on using that to make the VGs... All I need are nine 1"x2" right triangles and a 1" wide strip the width of the roof, and tape to hold it on...

I do need to figure out how to photograph my tuft tests, as I usually take photos with my iPad, and I'm not about to hang that outside the car for photography...

Anyway, the roof is one of the last places I'm gonna look for a gain, because that is harder to decide on a path... I need some sheets of coroplast to make a belly pan and to make wheel skirts, and maybe buy some cheap hubcaps to smooth out...

Try something like this?

http://forums.pelicanparts.com/uploa...1467033147.jpg

Aerodynamics by George Kachadoorian | Photobucket
http://i184.photobucket.com/albums/x...psxv4ya4md.jpg

Malibu roofline
 

Hucho states emphatically,that the lowest drag is always achieved with the fastback roofline.And it needs to follow closely to the Mair/Buchheim contour.
http://i1271.photobucket.com/albums/...ad2/scan43.jpg
Here,GM has used it on their Citation concept,which has returned Cds below 0.14.
http://i1271.photobucket.com/albums/...titled6_29.jpg
A recent Buick Riviera concept
http://i1271.photobucket.com/albums/...itled14_12.jpg
Here is Mair
http://i1271.photobucket.com/albums/...titled2_31.jpg
Here is Buchheim
http://i1271.photobucket.com/albums/...itled18_16.jpg
The Aerodynamic Streamlining tool at the top of the forum page respects Mair's contour.If you wanted to consider a Kamm extension,that's what it's for.

Don't leading edge air slots/slats alter the story line?

Wing Design - Level 3 - OTHER WING ADDITIONS
http://www.allstar.fiu.edu/aerojava/images/fig22.gif

NB1 BMW ACS Style Roof Spoiler - BMW E90 3 Series Coupe
http://www.pyspeed.com/v/vspfiles/ph...R-328335-3.jpg

Porsche
http://s1.cdn.autoevolution.com/imag...ke-86246-7.jpg

Just trying to be practical here.

Theory is great for a clean sheet design, but we are reto-fitting, right?

http://zenithch801buildlog.blogspot....dge-slats.html
http://3.bp.blogspot.com/-Y__EvVvYKV...00/6919748.gif
http://www.homebuiltairplanes.com/fo...dge-slots.html
http://www.homebuiltairplanes.com/fo...lots-slat1.jpg

So, I wonder how much benefit of the Kamm I could gain if I terminated it about halfway down the back glass? That way, it wouldn't interfere with trunk operation...

I might still experiment with a VG array, since I don't have enough material for the kamm, but have the stuff for the VG array... I did buy some coroplast, but not a wide enough piece to do the roof(still need to find sheets at least 6' wide)

Double Post

Or this?

http://ecomodder.com/forum/showthrea...1-a-19525.html
http://4.bp.blogspot.com/_Ak_mOrsdsx...x_kammback.jpg

I assume this can be done in a transparent material.

Arerohead has posted data/diagrams on truncated Kamm's before, check out his forum gallery.

slats / slots
 

Slats and slots are for 2D-flow wings at very large angles of attack,to prevent stall during short field takeoff or landings.
A fastback car is more a half-body with 3D-flow.Since the belly of the car is flat,there is no camber,no angle of attack,and no lift.If the roofline is proper there won't be any separation,and separation-induced lift.
Spirit of EcoModder is more like the half-body with it's mods and is a zero-lift vehicle.

halfway
 

Unless you extend the Kamm addition to the rear of the car,it will have little effect.
You might even end up with higher drag.

That's what I was fearing; part of the reason I considered a vortex generator array was that it(on the evo) produced a smaller wake, which can't hurt FE

VGs
 

Here's an early investigation for VGs on a notchback by Gary Wheeler.
http://i1271.photobucket.com/albums/...itled-12_2.jpg
http://i1271.photobucket.com/albums/...itled-10_2.jpg
http://i1271.photobucket.com/albums/...itled-11_2.jpg
Ideally,you'd have a wind tunnel at your disposal and a lot of time and money.
*there are VGs like these.
*there's also a 'wishbone' variety as well.
*the kind Subaru used on the Lancer EVO
*co-rotating VGs
*counter-rotating VGs
*angled grooves
*trip-wires
*and sub-boundary-layer-thickness 'nub' VGs
Theoretically,one would have to investigate them all,and test for quantity,and placement.The best application might include a variety of different VGs used together.

I'm not sure about that detour through leading edge slats. :confused:

To me it's 'how much pain are you willing to suffer to achieve your goals?' and I doubt vortex generators will get you there. The Blackfly Metro pictured in Permalink #23 is a good start on implementing Dr. Alberto Morelli's Fluid Tail Technique (FTT); it just needs lots of work below the beltline. See the PDF at https://www.google.com/url?sa=t&rct=...9cFbTNPI2EPR0A

You mention trunk access. The best solution I've seen was done by christofoo on his notchback.

http://ecomodder.com/forum/member-ch...back-moves.jpg

The Kammback is raised by the act of opening the trunk and acts as a shade/raincover while it's open. All conforming to Morelli of course.

While I agree that a full Kamm-back is the ideal solution, I don't currently have the material to build one, while I have enough scraps lying around to build a VG array like the one used on the Evo... It just consisted of 30x60mm triangles(about the thickness of coroplast) fixed four inches apart, and four inches from (ahead of) the flow separation, and at a 15* angle compared to the airflow, eight in total(plus the antenna)...

With HPTuners you can command the transmission to shift faster (for less time spent shifting, spinning and creating friction) and cater your shift points to how you want. You can have the transmission shift at certain TPS%, load, speed and pressures. You can fine tune the TCC lockup points to command to stay on for longer or early, so you can stay in OD longer. It's not something anyone can do and it's something I've rarely messed with, but it can be done by a skilled tuner. I'm just a novice tuner at best.

Depending on ECU, you might have access to it's lean burn modes, deceleration fuel cut off points, you can command when the drive by wire throttle hits PE mode (power enrichment, which adds extra fuel to prevent lean conditions on sudden acceleration), some older models can have the O2 sensor trigger/switch points adjusted to keep the engine running leaner for better economy (doesn't seem to work with my 09 Sierra, but it did with my 02 Silverado, for example).

Even a proper adjustment to the VE and MAF tables to follow the true curve of the airflow coming into the engine can improve power delivery and efficiency, when you're commanding the perfect AFR for your driving conditions. Monitoring AFR with a wideband is definitely needed to start running lean like that, but it's all possible with a tune. You're not going to magically get 3-4MPG, but I can see a bunch of these changes squeezing 1-2MPG out of most engines, especially GM's. They are generally tuned very conservatively to make sure they can run on the worst kinds of fuels in all weather conditions in various states of disrepair. This means they run kind of rich by default, it's safer for everyone, but is not ideal for performance and economy.

I've been reading into tuning as well, and I do plan on doing it, but first I want to maximize results with the stock tune... There is a guy that I've been following on the Malibu forums who is using HPT to tune his 09 3.6 LTZ... So far, he has gained 2mpg and is running low 14s in the 1/4mi...

One of these is not like the other.

I see what you're getting at... I did plan on eventually doing a full Kamm-back to maximize aero benefit, but I need 6' wide sheets of material (not even sure what I want to use)to work with, and I can't even find it without ordering a bundle... I however, do have the stuff to try the VG array for now once I get some tuft testing in and determine proper placement...

My above thought was that the HPT software/hardware runs $500+, so that would be a pretty low ROI...

It just does not seem like aerodynamics can be improved with vortex generators without a wind tunnel and an engineer, but the VGs are the market would fall into two categories:

1. Designed by engineers with access to a wind tunnel.
2. Designed by advertising to separate customers from their money.

Not only are you planning on skipping #1, but I cannot imagine how small bits of coroplast will make anything more aerodynamic.

I see you have already started making proven modifications that are popular around here. Thinking out of the box is great. We need innovation. You really want to do this, so go ahead, but please tuft test and A-B-A test fuel economy.

Once moving beyond scraps you might have laying around is on the table and you are looking at new material, I suggest considering PolyMetal or equivalent.

http://ecomodder.com/forum/member-fr...1-100-0866.jpg

Here is some of the experimentation I did with scraps. It consists of a 3-6mm thermoplastic core with aluminum skin on at least one side (usually both) of .007 or .014" thickness. About the same as an aluminum can, with enamel finish in various colors; available in 4x8, 4x10, 4x12 and 5x10ft sheets. It's used for commercial signage and the big wings on dirt track race cars.

The stiffness of 5/8" plywood at 1/10th the weight. It's more expensive, but the gains in not needing finish work are immense. Rolling and shearing (by hand) worked great but braking failed and dimpling would require a hydraulic press with a bigger throat than what I have.

It falls between hand-formed and planished aluminum and coroplast, in cost and effort.

Where can I get that? Looks nearly ideal for this kind of thing...

Signage shops, possibly. The vendor local to me is Multicraft Plastics. They have three pricing levels, approach them business-to-business instead of as an individual.

PolyMetal

Grimco have MAX-metal and AlumaCorr, an aluminum skinned coroplast.

Man, there are just not many sign shops here in WV to even just buy coroplast from, let alone some AlumaCorr... I might have to stick to eBay or the like and only buy what I absolutely need...

Okay, so I've made the vortex generator array for my roof, but haven't tested it due to lack of additional people to take photos of tufts and/or run stopwatches(coast down testing)...
http://i1140.photobucket.com/albums/...sapm7o2wn.jpeg
So, I basically paraphrased the details from the Mitsubishi Lancer EVO: 8 fins(plus one in the center that was the antenna on the EVO), 5mm thick(the coro I used was 4mm), 30x60mm(I used 1"x2" because I was using a measuring tape), 100mm apart(I used 4"), at a 15* angle relative to airflow(I didn't have a protractor handy, so used 1/4"width for 1" length and it looked close), and 100mm from the point of separation... I could have tested that but instead assumed it to be the top of the back glass and went 4" forward from that... I can't tell any FE gain, but I did this at basically the same time as finishing my wheel skirts, so I might need to remove it and actually do some ABA testing, and/or tuft testing...

What I found was, that before, when the back glass was wet, the middle 50-60% would take forever to clear, without running the defroster... After installing the vortex generator array, the top half of the middle 50-60% clears much faster, but the bottom half still takes a while...

Yeah, I know, I still need to tuft test to show any attached flow on the glass, but it seems promising... However, I'm also aware that in ideal conditions, with probably millions in R&D budget, the above mentioned Mistubishi engineers only got a .006 Cd reduction, which basically amounts to nearly no fuel savings for the effort...

I think that even if I only gain 0.5% in fuel mileage due to drag reduction, the ROI will be nearly instant; I spent nothing on this at all, not even the tape to hold it to my roof was purchased...

However, I might end up building a Kamm-back extending to the end of the trunk lid if I can get some materials...

I found a shop that can sell me sheets of coroplast for $25 each, and they can order AlumaCorr for $120 a sheet(too rich for my blood)... Once I decide on whether the VGA is actually working or not, I'll see about designing a Kamm-back that hinges with the trunk(but doesn't come off in wind)... However, that'll be after the belly pan is finished, and other, simpler gains are pursued...