Drag-reducing aero mods for mk6 golf/gti?

[nexus_2006]

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!

[freebeard]

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.

[nexus_2006]

Originally Posted by freebeard

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).

[freebeard]

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

Wheelwell spats and Moon disks.

[COcyclist]

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.



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

[Gschuld]

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

[freebeard]

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.

[Piotrsko]

$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.

[aerohead]

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.

[freebeard]

And it should be an easy online search.

ecomodder.com: Drag reduction modifications in wind tunnel: Audi A2 from Cd 0.288 to 0.204