anyone use a rear diffuser?

str8t six

Veteran Member
Joined
Sep 7, 2012
Location
louisiana
TDI
2001 GLS TDI
i was looking over one ecomodder and some of those guys run full underbelly pans. they also mentioned a diffuser to smooth out the air coming from under the back of the car.

i googled one and this popped up on ebay. http://www.ebay.com/itm/Universal-R...ncreased-down-force-reduce-drag-/290660214214 i was thinking about maybe making my own for my car. i drive %90 highway so anything might help.





im also going to block off the front lower grill to see if that helps.
 
Last edited:

Scoutx

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Jun 4, 2012
Location
Virginia
TDI
2012 Jetta (6MT) - 1000 Mile Club (retired)___ 2015 Jetta SEL (6MT)
I think the fins are a waste of time, material and weight OTHER than to provide stiffening for the plate to prevent 'flutter'. Otherwise, the smoother you can make the underside, the better your aerodynamics and thus better fuel economy. Please note...Do not block off the exhaust system, that air provides cooling and otherwise the retained heat can do bad things....like potential fire type problems to lesser ones.
Maintain access to towing points, and break it up into sections so you can remove parts to get to what you need to work on without having to take down the whole thing.
 

Diesl

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Joined
Dec 3, 2012
Location
Chicago
TDI
'78 Golf Diesel (long gone); 2012 Jetta Sportwagen TDI w/ DSG
Those fins might help break up the boundary layer and fill in the low pressure hole behind the car. This might actually be better overall (creating down force instead of lift as secondary effect) than doing the same thing at the end of the roof. Downside: more dirt is moved from under the car to the rear.
 

DieselBruce

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Joined
Aug 20, 2013
Location
New Jersey
TDI
2001 jetta tdi
I think the fins are a waste of time, material and weight OTHER than to provide stiffening for the plate to prevent 'flutter'. Otherwise, the smoother you can make the underside, the better your aerodynamics and thus better fuel economy. Please note...Do not block off the exhaust system, that air provides cooling and otherwise the retained heat can do bad things....like potential fire type problems to lesser ones.
Maintain access to towing points, and break it up into sections so you can remove parts to get to what you need to work on without having to take down the whole thing.
Smoother does not necessarily = better aerodynamics.
 

goodysgotacuda

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Joined
Oct 22, 2011
Location
Denton, TX
TDI
'12 Goft TDI/6spd & Jetta TDI/DSG
I think the fins are a waste of time, material and weight OTHER than to provide stiffening for the plate to prevent 'flutter'.
You may want to let most-every high-speed race team on the planet know of your findings. I'm sure they'd be surprised to hear that.
 

str8t six

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Joined
Sep 7, 2012
Location
louisiana
TDI
2001 GLS TDI
well no update on the diffuser yet, but I aired my tires up to the max of 50 psi and gained 1 mpg so far!

i just made a front grill block and am waiting to see if that helps too. so far according to the scanguage my GPH decreased about .08 while cruising!
 

goodysgotacuda

Veteran Member
Joined
Oct 22, 2011
Location
Denton, TX
TDI
'12 Goft TDI/6spd & Jetta TDI/DSG
i just made a front grill block and am waiting to see if that helps too. so far according to the scanguage my GPH decreased about .08 while cruising!

I've considered doing a lower-portion grille block off on mine. Not only for the drag improvement, but hopefully reduce warm-up times in the winter as well.
 

str8t six

Veteran Member
Joined
Sep 7, 2012
Location
louisiana
TDI
2001 GLS TDI
yeah im no stranger to grille blocks, I used to do it when i lived in michigan. i would block the whole top and bottom
grille and 2/3 of the radiator with plastic board! i even used pipe insulation on the coolant hoses too lol
 

shizzler

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Joined
Jun 25, 2005
Location
Ann Arbor MI
TDI
05 BEW Wagon
Hey that's my car! haha.

Yeah, the diffuser was a terrific mod. Very worth the hour or two of fabrication time. I believe I gained a full mpg hwy from that one.

It's simply a sheet of abs plastic from the trunk pan (spare tire well) back up into the rear valence at a calculated anlge of around 10 degrees. I read that more than that and you get airflow separation (worse aerodynamics). The fins on the side are for stiffening/aesthetics/aero. I figured putting extra fins in the middle was dumb and would only add drag.



But yeah, I am a huge believer in aero mods. Most of mine have held up well for the last several years and undoubtedly saved me many dozens of gallons of fuel consumption over that time. My suite of aero-mods increased my hwy mpg from 47 to 56 (cruising at 70-75 mph without much care). So it's very real.
 

str8t six

Veteran Member
Joined
Sep 7, 2012
Location
louisiana
TDI
2001 GLS TDI
wow thats a huge gain!! what all did you do to decrease your drag?

how did you attach the diffuser? just nuts and bolts or self tapping screws?
 

shizzler

Veteran Member
Joined
Jun 25, 2005
Location
Ann Arbor MI
TDI
05 BEW Wagon
Let's see, off the top of my head, I've done:

- lowered the car ~2.5". This definitely helped a lot, but I couldn't stand dodging potholes anymore, so now it's lifted again.
- front grill blocks. ALH's really don't need much airflow to stay cool, but a scan gauge is recommended (dash coolant-temp gauge is mostly a dummy light)
- Sealed up some front body seams with weather-stripping
- Front skid plate (was more for engine safety back when I was lowered, but it's quite smooth
- Additional small side aero panels around the skid plate to fully flatten the bottom side of the front area
- Passenger side mirror delete (also a noticeable gain alone)
- Rear diffuser, as pictured
- rear axle cover, but it only lasted a couple years before ripping off.
- Shorty antenna
- rear vortex generators on the top of the rear window glass (no net gain observed, even though they clearly affected/improved airflow).
- Regular waxing of the exterior :)

Other non-aero mpg improvements
- lightweight (ish) wheels
- LRR tires at 40+ psi
- stripped about 150 lbs of unnecessary items out of the car

I should have noted that the increase in hwy mpg from 47 to 56 also includes my engine mods, which definitely contributed, but at 75 mph, it's mostly aero. I had further plans for a deck-lid extension and wheel-well covers, but i got lazy.

Diffuser is simply held up with some screws rammed into the trunk well. Not a real good way to do it, but it's held up.
 

str8t six

Veteran Member
Joined
Sep 7, 2012
Location
louisiana
TDI
2001 GLS TDI
i have the front skid plate, grille block, short antenna, and lowering springs. im going to add those covers to smooth out the gaps in the front and rear bumpers. i have new tires so i wont change those out.

do you have pics of the rear axle cover?

do you think a spoiler would help with air separation off the trunk? maybe in conjunction with the vortex generators?
 

dirtysouthjacket

Veteran Member
Joined
Jun 5, 2009
Location
Charlotte, NC
TDI
2012 Jetta TDI Premium 6MT, 2004 Jetta TDI GLS (sold)
Let's see, off the top of my head, I've done:

- lowered the car ~2.5". This definitely helped a lot, but I couldn't stand dodging potholes anymore, so now it's lifted again.
- front grill blocks. ALH's really don't need much airflow to stay cool, but a scan gauge is recommended (dash coolant-temp gauge is mostly a dummy light)
- Sealed up some front body seams with weather-stripping
- Front skid plate (was more for engine safety back when I was lowered, but it's quite smooth
- Additional small side aero panels around the skid plate to fully flatten the bottom side of the front area
- Passenger side mirror delete (also a noticeable gain alone)
- Rear diffuser, as pictured
- rear axle cover, but it only lasted a couple years before ripping off.
- Shorty antenna
- rear vortex generators on the top of the rear window glass (no net gain observed, even though they clearly affected/improved airflow).
- Regular waxing of the exterior :)

Other non-aero mpg improvements
- lightweight (ish) wheels
- LRR tires at 40+ psi
- stripped about 150 lbs of unnecessary items out of the car

I should have noted that the increase in hwy mpg from 47 to 56 also includes my engine mods, which definitely contributed, but at 75 mph, it's mostly aero. I had further plans for a deck-lid extension and wheel-well covers, but i got lazy.

Diffuser is simply held up with some screws rammed into the trunk well. Not a real good way to do it, but it's held up.
Shizzler, so glad you found this thread! Your "Where are the Aeromodders" thread is one of my top three favorite threads on all of TDI Club!!! I think the stuff you did to your car is awesome! Do you mind doing an update over there one day? Especially about sealing up the body gaps? I keep wanting to try that on the headlights and hood on my '12 Jetta. Seriously, awesome work. :cool:
 
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