Drag-reducing aero mods for mk6 golf/gti?

[nexus_2006]

Originally Posted by 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).

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.

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?

[nexus_2006]

Originally Posted by Gschuld

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

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.

[nexus_2006]

Originally Posted by 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.

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.

[nexus_2006]

Originally Posted by 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.

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.

[COcyclist]

Originally Posted by nexus_2006

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?

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)

[freebeard]

Originally Posted by nexus_2006

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?

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.



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

[COcyclist]

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

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

[freebeard]

I like this one:



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:

[Piotrsko]

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.