Modifying a Honda Civic for maximum MPG

[WTF]

I started this fuel economy project after the Hurricane Katrina-inspired gas price run up, as I have a long commute and, except for a few hybrids, the car companies were busy introducing models with ever poorer gas mileage.

Overview:

•    Background - the car & aero influences
     Research & construction
     Coastdown testing to calculate Cd
     Update: lean burn engine installed for more MPG
     What fuel economy does it get?
     What's it like to drive?
     Compared to "regular" cars...
     Aero observations: bugs, leaves & deer
     The aerodynamic modifications in detail
     Cost
     Aerocivic media coverage & links
     Other aeromodded vehicles
  

Update: October, 2011 - $5288 in savings

As of October 2011, my improved mileage has saved me $5288 in gasoline over what it would have cost me to travel the 184,955 miles at my original mileage (pre-modified). This is over a 6 year period. Not a bad return for a $400 investment in aeromods.
Background - the car & aero influences

My car is a 1992 Honda Civic CX which I bought new, has an EPA fuel consumption rating of 47 mpg (US) highway / 39 city (5.0/6.0 L/100 km ... 20.0/16.6 km/L ... 56.4/46.8 mpg (Imp.)), and averaged mid 40's when driven "normally".

Before Katrina, I had been using hypermiling techniques to get me up to an average in the high 50's, but I had to keep the speed below 55 mph (89 km/h) to achieve it, which can be inconvenient to do on busy highways. About this time I had read Phil Knox's article "Free Fuel Riding on the Wind" in EV World where he talked about using commonplace materials from the local hardware store to lower his truck's drag coefficient (Cd) and improve its gas mileage at highway speeds. Also, having lived in Germany, I was familiar with many of the ultra-low Cd cars designed in Germany during the 1930's.

Since it didn't look like the car companies were planning to do anything in the foreseeable future about introducing a small car that would get good fuel economy on the highway, I decided to see what I could do towards improving my existing car's gas mileage.
Research & construction

1930's aerodynamic test car First, I studied the 1930's German cars and found one whose shape roughly matched my own car's. This car was a streamlined 1939 Maybach designed for The Fulda Tire Company for high speed tire testing on the autobahn and could cruise at over 150 mph with a 150 hp engine. I also researched some of the more recent low Cd concept cars that used recently developed aerodynamic techniques to clean up the airflow around the wheels.

Initially I built the aero mods in aluminum bar, Coroplast, and clear vinyl and then as they proved themselves, rebuilt them in the stronger and more permanent aluminum monocoque and Lexan, using construction techniques common on homebuilt aircraft. Since I was building this car for use as a commuter, not a show car, or as a "no holds barred" extreme high mileage vehicle, everything I did to the car had to pay for itself in fuel savings within two year's time. Since it was a 15 year old car with over 350,000 miles (563,270 km) on the odometer at the time that I started this project, I had no concerns with resale values.

The end result is a car with such low aerodynamic drag that it is down in concept car territory and well below the 0.25 Cd (coefficient of drag) of a Toyota Prius (3rd generation).
Coastdown testing to calculate Cd
The results of coast down testing are linear out to 90 mph / 145 km/h (it coasts almost as well at 80 mph / 129 km/h as it does at 50 mph / 80 km/h) and the Cd tested out to be 0.17.
Update (Feb 2010): lean burn engine installed for more MPG

In the late summer of 2009 at 512,000 miles my engine and transmission were wearing out, so I had a lightly used transmission and lean burn (VX) engine installed in my car.

So far, the new engine is averaging 7 mpg better than the old engine. This engine doesn't get the same mileage over a broad speed range that I saw with the previous engine, but instead shows the steady decrease of mileage with increasing highway speed typical of most cars. In the summer on a level road the new engine gets 100 mpg at a steady 60 mph and 120 mpg at 40 mph. It has tank averages in the upper 60's in the winter and upper 70's in the summer. I have had it remain in lean burn on a level highway up through 91 mph and estimate that it should be able to remain in lean burn up to speeds slightly above 100 mph.
What fuel economy does it get?

The approximate mileage on a level road burning non-ethanol gasoline at 85 degrees F ambient (29.4 degrees C) is:

   95 mpg (US) at 65 mph
   (2.5 L/100 km at 105 km/h ... 40.4 km/L ... 114 mpg (Imp.))
   85 mpg (US) at 70 mph
   (2.8 L/100 km at 113 km/h ... 36.1 km/L ... 102.1 mpg (Imp.))
   65 mpg (US) at 80 mph
   (3.6 L/100 km at 129 km/h ... 27.6 km/L ... 78.1 mpg (Imp.))
   50 mpg (US) at 90 mph
   (4.7 L/100 km at 145 km/h ... 21.3 km/L ... 60.1 mpg (Imp.))

Using E10 (10% ethanol/gasoline blend) drops these numbers by about 5 mpg. It was averaging mileage in the low 70's until E10 was introduced and is now averaging high 60's (see current fuel economy log). The flip side of the improved mileage is the increased range and I have no problems getting over 800 miles (1,287 km) from my 11 US gallon (41.6 liters / 9.2 Imperial gallon) tank in the summer.
What's it like to drive?

I have to get above 90 mph before I start to feel any wind load and it has a calculated top speed of about 140 mph with its 102 hp engine (the OEM max speed was 95 mph). The wind load I now have at 80 mph is close to what I used to have at 60 mph. Wind noise is practically non-existent. It's a joy to drive on the highway since it doesn't feel any wind load at normal highway speeds and loses speed MUCH more gradually than a normal car when coasting.
Compared to a "normal" car...

This car coasts so well that when I switch from driving this car to a "normal" car, it feels as if I am driving on a road covered with molasses, and that, like a powerboat, you have to keep pouring on the power to maintain headway. By contrast, with this car on a level road it only takes a light touch on the accelerator to maintain speed and it takes only the slightests of downhills to maintain speed in a coast. Even a Prius now feels "draggy" at highway speeds compared to this car.

Needless to say, the car gets plenty of strange looks out on the highway and one of my biggest problem while driving out on the highway are the rubberneckers cueing up to stare and take pictures.
Aero observations: bugs, leaves & deer

A fringe benefit of the aeromods are that the bugs in the summer and salt spray in the winter pass over my car without impacting. So I don't have to clean dried bugs or road salt from my windshield and rarely need to wash the car to remove the dirt and salt that accumulate on the surfaces of most cars.

When driving through snow, the snow flakes ahead of the car smoothly lift, pass over the top of the car, and continue their fall undisturbed, with no zone of turbulence such as you see behind "normal" cars. The air also moves smoothly under the car so leaves and other debris on the road move only slightly as I pass over them.

Another fringe benefit is protection from damage when encountering deer on the road, the low sloped front of the car scoops them up and over, with minimal damage to both deer and car. It has happened to the Aerocivic. Sound implausible? Watch this video of a deer colliding with the low sloped front end of a race car.
The aero modifications in detail

Boat tail

Aerocivic boat tail The biggest one, both in strange looks received and on its effect in improving my gas mileage is the boat tail. This is the tapered back end of my car that resembles the back of an aircraft fuselage. This eliminates the recirculating, low-pressure eddy that forms behind "normal" cars and that act to slow them down. This eddy is also responsible for the dirt that accumulates on a car's backside and allows a closely following car to improve its mileage by drafting. As a result of my boat tail, the back end of my car remains clean and anybody drafting me would gain no mileage benefits.

Expanded storage

Aerocivic expanded storage I took advantage when installing the boat tail of extending the interior of my car into the boat tail, so I now have an additional two and half feet of cargo space behind the back seat. The rear glazing still functions as a hatch for access to cargo. The boat tail also provides an additional three feet of crush space in the event I am rear ended and proved itself when I was rear ended by a high speed car. The tail lights are completely LED and the backup lights are two tractor work lights mounted in the underside of the boat tail.

Wheel/fender skirts

Aerocivic wheel skirts The next most visible mods are the wheel well covers. These streamline the sides of the car and as a fringe benefit, keep the sides of the car clean by eliminating the water spray that you see boiling out from the sides of most cars when driving on a wet road. The covers are hinged at the top for tire access and front covers are equipped with rollers that the sides of the tires push against to open it as needed when making a sharp turn. There is a spring at each lower corner that holds the cover closed when not making a turn.

Rounded nose

Aerocivic rounded nose The rounded nose lowers my stagnation point to reduce the amount of air passing under the car. It also houses my adjustable grill inlet so I can adjust my cooling air drag to the amount actually needed to cool the engine. Most cars use a one-size-fits-all approach to grill inlet design with a grill opening sized for the worst case cooling scenario. Since the underside of the engine compartment is paneled, I exhaust the radiator air out through the front wheel wells. The rounded nose starts the air moving smoothly up over the front of the hood and over the top of the car. It functions as a two mode bumper; acting as a scoop when I run into deer to redirect the collision energies upwards so the deer are propelled up and over the top of my car. But if I hit something heavier, it also provides about one foot of additional crush space as it crushes back to the car's original bumpers.

Undertray

Aerocivic undertray The car's underside is completely paneled so the wind can travel smoothly under the car. This is an area almost totally ignored on most cars (except on some exotic sports cars and luxury cars), since out of sight is out of mind. In addition to drag reduction, additional fringe benefits of the underbody paneling are a reduction in "road noise", much of which is actually generated by wind turbulence under the car, and a clean engine, since mud is no longer being splashed up from below.

"Catamaran" style flow management around wheels

Aerocivic catamaran flow management The wheels have been integrated into an airflow management system that splits the wind to either side of the front wheels, then keeps this divided flow moving smoothly down past the rear wheels where a wheel boat tail allows it to rejoin behind the car. This keeps the air moving linearly under the car with minimum turbulence and greatly reduces the amount of water spray, road salt, or dust kicked up behind my car.

Side mirrors

Aerocivic mirror Both rear view mirrors on the side of the car have been moved inside the glass to eliminate their contribution to drag and wind noise.


Wipers deflector

Aerocivic wipers deflector There is a small wind deflector in front of the windshield wipers to get them out of the wind flow.



Sealed panel gaps

Aerocivic panel gap All of the small gaps in the body around the doors and between various panels have been filled in to cut their slight contribution to wind drag.
Cost

So now, for a total materials cost of $400, I've created a car that blows away the highway mileage of any car currently in production. I wish I could buy a car with as low drag is this one, and even though the car companies are starting to come out with higher mileage models, they still have a ways to go match the low drag and highway mileage of mine.
Aerocivic media coverage & links

   For many more pages of photos & discussion of the Aerocivic, see: Aerocivic - how to drop your Cd from 0.34 to 0.17 - EcoModder.com
   Carolina Traveler | A fuel economizer named Basjoos - WCNC Television
   The Aerocivic: Ugly as Sin, But Hey - It Gets 95 MPG - WIRED Autopia
   Basjoos Tells All About His "95 MPG" Aerocivic - WIRED Autopia
   Hypermiler builds 95 mpg aero-Civic - Edmunds.com
   More about Basjoos' 95 MPG Aerocivic - EcoModder blog
   DIY AeroCivic: It's Ugly, But it Gets 95 Miles Per Gallon - TreeHugger
   Aerocivic's mods combine great aerodynamic and a, um, stunning look in 95 mpg Civic - AutoblogGreen
   Homemade Boattail Aerocivic Gets 50 Miles Per Stare - Jalopnik
   Aerocivic - green modded Honda Civic - MAKE blog
   Inexpensive MPG modding - Hack a Day
   Mod your car for better gas mileage - CNET News.com
   Aerocivic slideshow - TreeHugger