50 mpg at 80 mph (project 2013 Golf TDI aero mods)

[renault_megane_dci]

Important question : how do you measure your average mpg and average distance and above all, on which distance ?
Or is it purely academic Cd boost and relevant mpg estimated through this site tool ?

[cfd_guy]

Originally Posted by aerohead

VW has already optimized everything you see on the Golf.I'm pretty certain that the car's visible ' look' will have to change in order to achieve your goal.

This is not true

[cfd_guy]

Originally Posted by renault_megane_dci

Important question : how do you measure your average mpg and average distance and above all, on which distance ?
Or is it purely academic Cd boost and relevant mpg estimated through this site tool ?

Purely academic with Cd as the means of improvement. I will test when I drive on the interstate for 100 miles. No way to really test on the street accurately since the conditions change so much.

[COcyclist]

cfd, I wish you luck on achieving your goal with your VW. I am working toward my personal goal of 70@70 but I am not there yet. I have done some extensive stealthy mods but I think I am about at my max unless I do more visible mods such as fender skirts and a boat tail. I have a full belly pan, inner and outer side skirts and a homemade diffuser underneath. I am closing in on 60 mpg but at about 60 mph.

Also, my wife's 2011 A3 TDI with DSG does not get as good mpg as my older TDI with the 5 speed manual transmission. I will be eager to see your mods. Good luck.

[cfd_guy]

Originally Posted by COcyclist

cfd, I wish you luck on achieving your goal with your VW. I am working toward my personal goal of 70@70 but I am not there yet. I have done some extensive stealthy mods but I think I am about at my max unless I do more visible mods such as fender skirts and a boat tail. I have a full belly pan, inner and outer side skirts and a homemade diffuser underneath. I am closing in on 60 mpg but at about 60 mph.

Also, my wife's 2011 A3 TDI with DSG does not get as good mpg as my older TDI with the 5 speed manual transmission. I will be eager to see your mods. Good luck.

Thanks for your post. Most of my work will be the diffuser and bits to feed the diffuser with more air. I feel like that will be the biggest gain.

[bitmap]

Aerodynamic drag caused by cooling package and rear axle alone is HUGE on these cars.
I have a Skoda clone of this Golf and grill block + rear axle coved made really big difference in aero drag.

I have 2.0 16V PD engine and currently achieve 60MPG at 68MPH indicated.
Good luck with your goal and I will definitely follow this thread

[Sam Powell]

I am eager to follow progress. Thanks for sharing. This should be enlightening.

Sam

[COcyclist]

Originally Posted by bitmap

Aerodynamic drag caused by cooling package and rear axle alone is HUGE on these cars.

I forgot to mention that I have the upper grill blocked year round, the lower grill partially blocked depending on outside temperatures and the lower left grill flush covered. These diesels run pretty cool, especially when hypermiling so you don't need as much grill opening.

[aerohead]

Originally Posted by cfd_guy

This is not true

Okay,you're the boss of this one,so I'll wait for your images.
Just bear in mind that Volkswagen 'invented' aerodynamic optimization in the 1970s.
I'm just having difficulty imagining things which will 'show' aerodynamically,which don't 'show' visually.

[aerohead]

I messed around on the calculator to get a rough feel for the Golf.
Loose Specifications:
*Cd 0.30
*Projected frontal area 20.7 sq-ft
*Curb weight 2,800 pounds
*Coefficient of rolling force 0.015
*47 mpg at a steady 70 mph
*1.4893 gallons/hour
*10.173 pounds diesel/hour
*Aerodynamic Road Load @ 70 mph 77.882 pounds/14.539 hp
*Friction and tire losses @ 70 mph 7.84 hp
*Total Road Load @ 70 mph 22.379 hp
*Required engine power @ 70 mph 24.325 bhp
*BSFC 0.4182 lbs/bhp-hr
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If I leave the car alone,but increase the velocity to 80 mph the engine power requirement rises to 33.328 bhp.
*Rolling resistance tractive power is up to 8.96 hp.
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To achieve 50 mpg @ 80 mph requires:
*1.6 gallons/hour
*10.9286 pounds/hour
*dividing by BSFC = 26.133 bhp
*= 24.042 hp
*minus 8.96 hp rolling resistance leaves 15.082 hp for aerodynamic drag.
*Solving for the Cd using the standard aerodynamic power equation yields Cd 0.208 necessary to produce 50 mpg @ 80 mph.
-------------------------------------------------------------------------
*Cd 0.208 is 30.5% lower than Cd 0.30.
*A 'typical' belly pan would yield about a 12% drag reduction and 7.2% mpg improvement @ 70 mph.
*Even the 1982 Audi 100 III with Cd 0.30 yielded only Cd 0.23 with a full belly pan with diffuser.
*The Golf has a smoother underside than the 1982 Audi to begin with and the best underside would yield less of an improvement than what Buchheim et. al. achieved with the Audi after $1.5 million in wind tunnel time (1982-Dollars).