Under car aerodynamics

Joined
Jan 30, 2022
Location
Pennsylvania
TDI
Looking for an education so I can buy smart.
Thinking about putting something under the car to smooth it out as best as I can.

I can get my hands on corrugated tin roofing and a very large roll of plastic. The tin is in 3.17'x8' sheets. The roll of plastic is 17" wide and 8' long.

My dad and I both have ALH, 5 speed MK4 Jettas. He brought up the tin idea. We were curious about the corrugated tin directing airflow under the car being beneficial or the plastic being smooth.

Long term plan is to lower the car an inch or so, fab some fender skirts, pizza dish hub caps and a front fender exit exhaust

Some aerodynamics, but not going full on with a boat tail.

Thanks guys and gals
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
Corrugated tin will add more surface area under the car, potentially increasing the drag somewhat. Off the top of my head I can't quantify how much, but probably not a lot. It will also be stiffer and less likely to deform at speed, but will be harder to shape smoothly to the contours of the undercarriage.

Incidentally, what's the reasoning behind the fender exit exhaust? Routing the exhaust into the slipstream behind the car will do more for aero than anywhere else.
 

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
don't waste your time.
the best bang for your buck is to buy a scangauge II and learn to drive better in relation to real time MPG and GPH.

if you want to hyper mile the best thing is to add some vortex creators on the rear bumper, its NOT the air under you, its the drag behind you from the air leaving from under you and over and on the sides!
60% of the resistance of the aerodynamics is from you just bruit forcing your way though the air. 30% is from the drag from the air rolling off you as it leaves behind you, and 10% is from the stickiness of your car to the air its self. Roll the car in bondo and roll a golf ball over it. thats 14% better.
you can gain about 2% if you just tape up all the seams there are on the outside, but all this does not add up to MPG better, just better aerodynamics meaning that its about 10% that to translate to mpg. SO even if you added as much as you possibly can and increased the efficiency of your aerodynamics by 20% and thas extreme, you would see 2% back in mpg and thats not even factoring cost.
learning how to maximize your fuel use IS however a great return.
 

hskrdu

Top Post Dawg
Joined
Oct 17, 2003
Location
Maryland and New England
TDI
2003 Golf GLS 4D 5M, 2015 GSW SE 6M
VW makes use of extensive underbody panels in models (gas, diesel, and electric) designed for high fuel efficiency. With the Mk7, these are modular, and you can see the coverage of underbody panels increase as any one model rises in FE (for a variety of reasons). Although the belly pan on the MkIV was designed to absorb engine noise and keep the area clean, it was likely also to improve Cd. If you wanted to copy the underbody of the Mk7 E-Golf, or BlueMotion TDIs, I would start with those panels and cut them to fit your MkIVs. This would play a small role in improving Cd, but you'll need additional marginal gains. We have members that have used pizza pan hubcaps, as well as eliminated pax side view mirrors, taped up seams, or modified the front end. Take a look at the nose of a BlueMotion TDI, for examples.

Lowering the car has trade-offs, and if the goal is FE, may not be the desired route. Ernie Rogers spent considerable time on this, and found taller, narrower LRR tires (and all the benefits they afford) were a more important factor in FE than was any reduced drag from lowering his car. There are extensive posts about all these factors, so my suggestion would be to read previous posts. Many/Most of our MkIV experts are gone, and our best conversations are in the past.

Obviously your goal is reduced Cd, and aero improvements, and you didn't tell us your end goals, if any. If you are looking to improve FE, the formula is well known. Right foot, slow down, lose weight, LRR tires, engine in peak condition, etc.
 
Joined
Jan 30, 2022
Location
Pennsylvania
TDI
Looking for an education so I can buy smart.
Ernie Rogers
Gents I greatly appreciate your suggestions. I will read on Ernie Rogers.

I was thinkingba smooth underside would help a great deal. Thank you for teaching me that other areas will have more affect.

Roll the car in bondo and roll a golf ball over it. thats 14% better.
I hadn't thought about the gold ball idea. Can you help me understand that better?


End goal is FE. I am at 44mpg now, I believe 52mpg is obtainable. Id like to see in the 60s.

6 month plan is the Scanguage, steelie rims with smooth hubcaps, taller tires, cover the openings in thr front bumper, lower it an inch or so, make a front air damn, make side skirts, .717 or .700 5th gear. I will do the vortex generators on the trunk lid from suggested above.
 

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
Forget all that noise.
5th gear mod is good though.
I put 764s with a big nasty 2052 turbo at 32psi and a bunch of stuff like fmic and 8x17 wheels with soft wide tires. I got 62mpg on the highway and was able to reliably get into the low 50s with driving habbit.
If I drove it like a normal same car like everyone else... about 38. When I raced it I'd get 8. I auto crossed her for 3 years solid. 22 events a season (year)
How did I get into the 50s you might ask. Driving habits
Get the scangauge and maybe upgrade your nozzles and keep everything stock.
It's not about modifying the car. It's about being efficient with the fuel you have.
I promise you will get into the 50 era for next to no work but on yourself.

What you are trying to do is go fast and get mpg.
I got 62 going strait across country drafting mostly behind a truck at 65mph for the entire tank basically. Cruiscontrol... learning to not go fast from a light... and pretending there is an egg under the gas and brake is the way to do it.
I'm telling you... you are wasting time and money for no reason with smooth hubcaps and all that noise.
If you want that.
Go buy a 1st and 2nd gen prius. They get 60mpg stock and you can get them for less than your going to spend now!
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
You should be able to easily get to 52 MPG with a car that's set up properly and driven for FE. No mods needed. I regularly get close to 50 MPG in winter and over 50 in summer in my '02 Wagon simply by keeping highway speeds under 80 and driving sensibly.
 
Joined
Jan 30, 2022
Location
Pennsylvania
TDI
Looking for an education so I can buy smart.
Under what driving conditions?
124 miles a day. Majority highway. 65mph for 84 miles. 55mph for 32 niles, 40mph for 8 miles a day. I drive responsibly and conservatively. Shippensburg PA - Ft Indiantown Gap PA.

1oz of Hot Shots EDT per tank. VNT17, Kerma 3bar tune, South Bend Stage 2 daily tune. Oil and all filters changed 3200 miles ago.

I believe something in the rear calipers is dragging. Thats this weekends maintenance plan.
 

vandermic07

Veteran Member
Joined
Jun 5, 2011
Location
West Central Pennsylvania
TDI
01 Golf 5 spd, 03 Jetta Wagon
Teach me to MPG

You should be able to hit 50 mpg all day long with that commute. Even with draggy calipers, 2" lift and clunky winter tires i get 45 mpg. Good summer tires and I hit 50-52. My golf is stock. My FE was lower when running additives (PS Desiel grey bottle) but only 1-2 mpgs. I dont know why.

Might be something with the tune, boost leak, vacuum leak, tires, Thermostat, . I get the best FE with stock 15" aluminum rims that come with the golf or Jetta. my steel rims always have winter tires on them.

better off making sure you are mechanically sound before diving down those rabbit holes.

Thank you for Serving!
 

shizzler

Veteran Member
Joined
Jun 25, 2005
Location
Ann Arbor MI
TDI
05 BEW Wagon
Blast from the past for my ALH Jetta sedan, but when I had it stock, it would get around 46 or 47 mpg highway at my typical 70-80mph speeds.

I went on a pretty long, multi-year crusade to improve engine efficiency and reduce aerodynamic drag. End result was 55-56mpg at the same highway speeds; no drafting or hypermiling.

I had:
  • front grill block and all gaps sealed w/ weatherstipping
  • passenger mirror delete (noticeable gain)
  • underbody panels wherever I could (coroplast and abs plastic sheets)
  • Car lowered ~2.5" on koni coilovers
  • home built rear diffuser; screwed right into the spare tire well and cut up into the rear bumper valence
  • Vortex generators on top of rear glass (tuft tested on highway, effective air redirection but no clear mpg gain alone)
  • short antenna
  • 16" lightweight alloys with good LRR michelins at 40 psi

Plus pp520 injectors, kerma "low-boost at cruise" tune, egr disabled, all intake, intercooler and exhaust piping upgraded. It was pretty dang fast on the highway! (any hp you dont lose to drag is available for accelerating) - outran my buddys mk4 GTI.

 
Joined
Jan 30, 2022
Location
Pennsylvania
TDI
Looking for an education so I can buy smart.
Incidentally, what's the reasoning behind the fender exit exhaust? Routing the exhaust into the slipstream behind the car will do more for aero than anywhere else.
I always wanted a fender exit. Think they are fun. I always heard that diesel does best with as open of exhaust as possible.
 

PradoTDI

Veteran Member
Joined
Sep 25, 2020
Location
MT
TDI
1991 Toyota LandCruiser LJ78 with ALH Swap
I always wanted a fender exit. Think they are fun. I always heard that diesel does best with as open of exhaust as possible.
Can’t argue with that! I have a 2.5” catless straight pipe with a side exit right in front of the rear wheel, it sounds great. I will say though, there is a lot of highway speed drone with that setup.
 

Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
HI,

Before mods I was pretty much 46 and 47 MPG ... afterwards (with the mods in my signature) I can average 53+ MPG highway.

I would be curious what aero mods can yield....

Belly pan , passenger side mirror delete, smaller driver side mirror , grill blocks, taped seams should be fairly cheap mods especially since you already have the materials for the belly pan.

Maybe even try vortex generators ...

FWIW - I tried the pizza pans but saw no real difference in MPGs (admittedly tho I bolted the pizza pans onto cheap plastic hubcaps on steelies and there was a ridge formed so maybe that messed up the aero of the pizza pans since the pizza pans were not flush).

I did try grill blocks and while it did make the car warm up / run hotter I don't recall seeing alot of extra mpgs ....maybe a little but nothing super significant as I recall ( a deer strike broke my blocks .... I may try making a new set since I found my original template a few days ago_).

The larger diameter tires I used when my smaller tires wore out did yield more MPGs because of the Overdrive effect ... I have heard that the 6 speed tranny can get even more MPGs (or modding the 5th speed gearing).

Good that you are a tinkerer .... I would love to see / hear about your results.

Thanks

Andrew
 

jackfolstam

Veteran Member
Joined
Oct 14, 2016
Location
CA
TDI
MkI Rabbit ALH swap
I have an ALH swapped in a Mk1 which is less aerodynamic than a Mk4. I have gone from 43-46mpg to 51-53 with two major changes; 0.66 fifth gear and keeping it 70mph or lower (was doing 75 prior to the war in the Ukraine) on the highway. Other changes: tire psi up to 32 from 29, removed roof basket, adjusted ebrake cables slack as much as possible, added cat. (No passenger mirror stock.)

I put on a cat (I like to see smog free sunsets), no muffler, and it is pretty quiet at 70. Much louder at 75 (2450rpm or so). Oh yeah it was also crazy loud without it.

I have big injectors and an 11mm, VNT 15, tune. CTN but with Mk3 wheels/tires, pretty much same engine RPM to wheel RPM as Mk4.

My thought with the under car sheeting is that I got a giant election sign that I would use. I'm thinking of gluing it on with silicone or something so I don't have to drill into the metal, and it would seal the seams. Another thing is that I would put aluminum tape or something on the upper side where the exhaust is. Another thing is putting drains in it so the car doesn't rust immediately if moisture gets in there.

I see the Nissan Leaf has under car sheeting but it has naca ducts in the rear,I don't know why. They are just behind the rear seat and I can't remember which way the face. It also has a rear splitter.

Other thoughts:
My next big aero mods are wheel aero deflectors, full under car sheeting, and maybe fix the front air dam and put it back on. There are too many crappy driveways and such that it gets banged up real good. I was thinking to hinge it so it deflects rather than breaks. It definitely makes the car accelerate down some hills in neutral when without it, it would decelerate.

Other minor aero mods I am thinking about are antenna delete, wiper delete/mono wiper, manually controlled grill blocking from the cabin (cable controlled venetian blinds or something), and radiator exhaust routing. Check out how much effort they put into cooling exhaust for airplanes, I know they are doing much higher speeds but still it would do something (Porsche did it in cars too).
 
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