Drag-reducing aero mods for mk6 golf/gti?
 

Hello everyone. I have a mk6 GTI, and my goal is to improve high-speed MPG. I get fairly good mileage below about 65 mph, but above that speed the efficiency drops off dramatically.

I've noticed that the Cd of the mk6 is often given as 0.33-0.35 depending on source, but the newer mk7's Cd is 0.27. I often only get 27-29 MPG at 70mph, but I know people with mk7 GTIs getting in the mid to upper 30s at that speed or higher (both cars have similar powertrain). The mk6 and mk7 look fairly similar, I believe I should be able to improve my Cd, although I know I probably won't reach the 0.27 the newer design has without killing the aesthetics.

I've been watching some Julian Edgar videos and I think I could reduce the vortex drag on the rear end with some sharper edges to induce flow separation - think things like rear glass side fins, and a ridge on the lower bumper where it starts to curve inward. I also plan to clean up the underbody a bit - there is an OEM underbelly pan I can get for $80 that covers about the front third of the car, OEM rear wheel suspension covers for about $40, and I'd also like to fabricate a custom underbelly for the rear axle back to the bumper (there is an OEM part for that too, but its been out of stock for years).

Does anyone have advice or opinions on these specific mods (rear window side fins, rear bumper separation edge, underbody cleanup in the front and rear)? I'm brand new to thinking about aero at all, but appreciate any advice, even if that advice is "don't bother" or "focus on something else".

I plan to make mockups with coroplast and masking tape, and do some wool tuft tests. I would make the upper body pieces with clear acrylic sheets molded with heat and cut with a router. I'm not sure what I would make the underbody pieces out of yet, or really how to test them.

Thanks!

VW know where the low hanging fruit is. How long to Return On Investment for the $120 you mention?

One step beyond the tearing edges would be a boxed cavity.

My best estimate, after using the online tool in the sticky thread here plus some best guessing, if I can get my Cd from 0.33 to 0.30, it'll save $0.90 per 100 miles@70mph, or $90 per 10k miles, I'd break even on $120 in just under a year, assuming those underbody improvements can change the Cd that much (realistically I don't know if they will or not).

Sounds reasonable. You could dilute the sunk cost with some lo-buck modifications.

Wheelwell spats and Moon disks.

Welcome nexus. I have done some aero mods to my Mk4 Golf. I did not really want to change the look of the car so I have done a full belly pan with an open section for the catalytic converter. The exterior of the car from VW is pretty good (except the back end).

https://ecomodder.com/forum/606678-post5.html

This shows a CFD of the Golf so unless you do some serious boattailing, the underside is the way to go, and perhaps rear wheel skirts. You can try smooth wheel covers but I have not noticed much difference with or without.

https://ecomodder.com/forum/member-c...4-img-0146.jpg

Your gasoline exhaust will be much hotter than the diesel so you will need to leave more open areas.

https://www.motorverso.com/vw-golf-gti-mk6-review/

I just read this pleasant review of your model MK6 GTI. Looks like a fun, sporty little hatchback. Nice choice. I did notice the claimed combined MPG was stated to be around 38. That’s a good bit more than what you are seeing at 70mph. Perhaps the stated mileage was only considering lower highway speeds.

At just shy of 3000lbs, I’d think that little turbo 2.0 would offer you better numbers at 70 mph.

As a comparison, my 2016 Jeep Grand Cherokee SRT is 5200 lbs, all wheel drive, running a 475hp 6.2 hemi with wide performance tires. At 70mph I get between 25 and 26 mpg(while running in Eco mode (4cyl mode) CD is reportedly .371.

It’s absolutely dead factory stock. I would expect your GTI to get noticeably better mileage than my Jeep muscle car SUV.

Is it possible your car is out of tune somehow? Tired spark plugs, etc. It would be great if a minor tune up service could unleash hidden mpg for you.

George

I don't know how much you've looked around the site, but one thing you might consider is user Vekke's posts, archived here: https://ecomodder.com/forum/search.php?searchid=4610501

He has a company called Tuneco and works primarily with ID-3s Audi, etc. but he focuses a lot on wheels and wheelwell spats.

$80 for a panzer pan?

Mileage sounds a bit low at 70. On the beer run from Reno to Seattle/Vancouver we were averaging close to 49 (53 if you believe the onboard indicator) running with traffic on I 5 which wasn't the speed limit. Was much better before the software change as we needed to refuel just before Seattle where before we could almost make the border. Could be in the 700 mile/tank club if I wanted to drain the tank dry.

'13 neutered, Manual trans, performance/handling tires, no mods, no drafting. Was running the lower grill block. Can't believe the slush box is costing that much efficiency.

Watching this because I'm going to steal what works if it's not intrusive to the wife particularly if it fixes the #@&%€ dirty as heck rear window. Lusting on adding a front splitter kit with added gurney flaps since they are only $80.

GTI
 

In 2012, Coventry University was able to reduce the drag of VW/ Audi's A2, from Cd 0.288, to Cd 0.204, without adding a millimeter of length to the car ( however, Cd 0.204 required a mirror-delete ).
They employed the modifications you're considering, along with those mentioned by members.
This research has been the focus of a dedicated thread here at EcoModder if you can find it.
And it should be an easy online search.
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The mechanical efficiency of the GTI engine may not be as high as the lower performance Golf. Don't know.
And the tire width of the GTI, if wider than the non-GTI, could also be an aerodynamic liability.
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That said, even with an engine and tire width liability, it's highly probable that you could achieve your mpg target with extreme attention to details.

Thanks for the comparison, that is very helpful! I believe my mk6 and your mk4 have similar stock Cds (.34 on the mk4 vs .32 on the mk). Does that CFD include your underbody mods as shown in your picture, or is it for stock? How did you get the CFD analysis done?

Also, I'm a little confused about something. To reduce the low pressure behind the car, I should be looking at box cavity/flow separator kinds of shapes, right? Or should I be looking more at getting flow to wrap around via rear diffuser, slotted spoiler, etc? Or does the rear airflow really not matter at all until I get the underbody cleaned up?

It is a quite fun little car. I weighed it with myself and a full tank of gas not too long ago, its 3200lbs. I'm not sure where the 38mpg comes from in that review, the EPA rating for a 2012 GTI is 24/33mpg, 27 combined (the auto is a DSG, it gets better mileage than the manual trans). I am due for a little bit of maintenance, spark plugs have 56k miles on them (60k change interval), and the intake valves/manifold I'm sure are ready for a carbon cleaning: I'm not getting any CELs yet, but I'm at 96k total miles and its never been done.

Interestingly, my car, with the gen 1 EA888 engine and 6-speed, is rated for 27mpg combined, while the 2021 GTI with the gen 3 EA888 and a newer 7-speed version of the same transmission, is also only rated at 27 combined (24/32mpg city/highway), although I know people with that car routinely getting 35mpg or better on the highway. As for me, on flat ground I get about 30mpg at 65mph and about 28mpg at 70-72mph. I've manually checked the mileage on every fillup for 5 years, and found the built-in mpg calculation to be consistently within 1MPG of actual usage.

Not a slushbox, its the DSG, should have at least as good as a traditional manual.

Anyhow, yeah, $80 is starting to sound a bit high for the bigger pan to me too: I could get a piece of coroplast and some selftapping plastic screws for $10 at Lowe's and add on to the piece I already have, they aren't that different.

What did you use as a lower grill block, and did you have any temp issues? I don't drive my car hard often, but I'm leery of blocking off too much cooling and cooking the turbo: sometimes I get in to way more idling in traffic that I'd like.

Thanks, that is a great resource, I am surprised that on their A2, simply blocking off front vents had the biggest effect of any of their mods.

I just copy and pasted from this thread. I don't know where Jorge found it.
https://ecomodder.com/forum/showthre...mk4-37836.html

IMHO you will not see the mpg gains you want unless you smooth the underbody, fill gaps and openings and add a big tail but every bit helps.

You would probably get higher mpg numbers if you swapped the tires and rims for skinny LRR tires and slowed down 10 mph, but that's not why you bought a GTI.

P.S. If you can find a Panzer Plate for $80, grab it. That is very cheap insurance for our low and easy to crack oil pans. (Edit. I just re-read your OP and you mentioned an OE cheap plastic pan for $80)

Best would be to 'fill the wake' with a boat tail that reduces the cross section smoothly to a point or vertical line. Next would be a flat truncation. Then the boxed cavity. The Cobra Coupe is a brilliant example of the line between the two. A true boxed cavity uses the low pressure of the wake to suck the boundary around a corner, reducing it's cross section.

A 1930s Dymaxion/Porsche rounded tail doesn't have the Fashenfeld tearing edge the boxed cavity (or late models of Prii) has. So the separation line can be buffeted around. Correction to this would be (purple glowing) plasma actuators or Coanda nozzles.

https://ecomodder.com/forum/member-f...25-7-36-55.png

In the example pic, the Coanda nozzle is that lateral line forward of the smoke wand.

https://ecomodder.com/forum/member-f...70-imagesa.jpg

You may find Page 1 of this thread useful before it veers off into the weeds.

https://ecomodder.com/forum/showthre...olf-28737.html

I like this one:

http://ecomodder.com/forum/emgarage-...0d4592c07d.jpg

Someone here (cowmeat?) had something similar on a Civic. The curve would hold the shape in a semi-flexible material. I've got your 'in the weeds' right here, since you opened that door:

https://ecomodder.com/forum/member-f...w12-650-16.jpg

Don't think the wife will go for those since we have clearance issues with drainage gutters and parking lot speed bumps currently. She's also not a fan of conveyor belting even if it doubled the current MPG.