1999 Acura EL or Civic EX sedan - dropping Cd from 0.33 to below 0,20

[ACEL]

Every year we do a road trip across a hilly country area. When I compare the mileage of the recent 2020 edition to the 2019 drive in similar conditions, I get 6,13 L/100 km in 2020 compared to 6,69 L/100 Km with a major difference.

The 2020 gas tank fill up, before and after the road trip, was till gas almost flowed out. In the 2019, the before and after fill up were to the 3rd gas pump stop leaving some play because I cannot ascertain the gas pump was the same before and after.

Still, a potential gain of 0,56 L/100 Km is interesting. That is 3,18 MPG US and 3,89 MPG Imperial. That is a 8,4 % less fuel consumed with the modifications since the 2019 trip. If I blindly apply the following :

"“The Effect of Aerodynamic Drag on Fuel Economy”
Aerodynamic drag is proportional to the square of velocity, and hence the power needed to overcome drag is proportional to the cube of velocity.
if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%."

Then applying this the reverse way, with a 8,4 % increase in fuel economy, I may claim at least a 16% drag reduction. That means from a Cd=0,34 to Cd=0,28. If Cd is the same as the 1999 Civic EX at Cd=0.32 than my actual Cd is 0,27 on this 1999 Acura EL.

Anyway, I am not running after Cd record figures, but after fuel economy. So any improvement in fuel economy is welcomed. The rest is for around campfire boasting stories.

[California98Civic]

Congrats on a great result! And congrats for campfires and campfire boasting stories, too. What are the new mods on the car this year, beside the wheel covers?

[ACEL]

The actual aerodynamics modifications that are on the car right now:

Tires overinflated to 45 psi over the 32 psi,
Front air deflector - splitter,
Complete belly pan or under tray if you prefer,
Front wipers deflector,
Upper grille block,
Front and rear wheels deflectors,
Side skirts,
Vortex generators,
Rear wheels boat tails,
Air flow separation edges

The wheel covers are not on the car yet.

[ACEL]

I read somewhere :

“The Effect of Aerodynamic Drag on Fuel Economy”
Aerodynamic drag is proportional to the square of velocity, and hence the power needed to overcome drag is proportional to the cube of velocity.
if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%.

If I take this upside-down, it would read like:

If your highway fuel economy improves by 5%, that is because you made a modification that brought a 10% reduction in aerodynamic drag.

So that means sort of one can compute the drag coefficient inferring from the fuel economy improvement like you get a percentage in drag reduction that is double to the percentage in fuel economy.

So if I go creasy about this hypothesis:

Right now, I get 27,3% fuel economy over EPA, then did I get a 54,6% reduction in Cd? From the original 1999 Acura EL Cd=0,34 or Cd=0,32, the actual Cd would be around 0,15? That seems very optimistic.

I revised the data, mod by mod, and the results are in the attached pic file. The drag reduction computed would rather be 36% in average in the speed range of 116-125 km/h for an actual Cd in the range of 0,20-0,22.

Boy, it seems that car had an aerodynamics improvement potential from the start. There are some other mods I want to try, before I am done:

- Front wheel air curtains,
- Front wheel "aero-corners" - separation edges,
- Reduce front wheel arches,
- Rear wheel skirts

With some luck and budget, I will go back to the mechanical side of the Force and replace the 195-55R15 tires by some 195-65R15 with will allow to go from about 2700 RPM at 100 km/h to 2550 RPM. Then, to improve the exhaust flow, I may replace the catalytic converter and muffler. I do not want a noisier car but a better breathing engine, better torque and better fuel economy.

After all, a better fuel economy, a quieter car that looks good is all that I work for.

[California98Civic]

I am familiar with the rule of thumb about fuel economy and aero drag. I think aerohead has posted it up a couple times. I do not know what it is based upon. Nonetheless, 0.15 is certainly not the Cd of your car, though I can easily believe that 0.27 figure, plausibly. Factors like ambient temp, humidity, wind, tire wear condition, and drivetrain and charging system conditions can all affect our results, so as you onow we take results as provisional possibility only unless the testing is very rigorous. I like you project a lot!

Here is a muffler/exhaust thread and summary I created years ago to distil into one place what seemed the best thinking on this site about such mods. I think it will help you, maybe:
https://ecomodder.com/forum/showthre...tml#post289125

[ACEL]

“The Effect of Aerodynamic Drag on Fuel Economy”

"For passenger cars this means that aerodynamics is responsible for a much higher proportion of the fuel used in the highway cycle than the city cycle: 50% for highway; versus 20% for city. This means that if you make a 10% reduction in aerodynamic drag your highway fuel economy will improve by approximately 5%, and your city fuel economy by approximately 2%."

This is copied from url:

The Effect of Aerodynamic Drag on Fuel Economy | ARC

The company that claims the above seems very serious, rigorous and knowledgeable:

"Auto Research Center is a specialized research facility, which sells time in its various test rigs (such as our wind tunnel and seven post rig) and provides design and consultancy services to government/military, commercial vehicle, automotive and racing clientele. ARC prides itself on remaining a competent tier one supplier to multiple OEMs throughout these industries. We also offer scale model development, vehicle aerodynamics and vehicle dynamics capabilities and expertise, computer aided engineering software, vehicle dynamics software and computer modeling programs for the purpose of trackside simulation. ARC is a worldwide company that is committed to developing innovative solutions in the ever-changing field of transportation."

With private clients and OEM intellectual property jealous safe keeping, this company and its personnel do not write scientific papers freely or low cost accessible. They may publish on SAE with permission from the client. Reading these articles is not for free.

Anyway, with such credentials claims, I thing the above general rule they publish on their web site is credible. They offer some white papers under the "media" site thumb that are well supported by a solid bibliography.

The site offers some history notes :

"ARC was opened in 1998, as the North American headquarters for Reynard Motorsports. Reynard Motorsports was based at Reynard Park, Brackley, England and at that time was the largest race car designer and manufacture in the world, producing cars for both open wheel and sportscar racing. In the USA, the primary effort was on open wheel in Indycar/CART."

Yes, I think these guys know what they are talking about.

[California98Civic]

Yes, I think it is credible too. One can easily find graphs of speed/FE relationship online to suggest further corroboration. Too, we know the drag force magnitude increases faster than velocity until it becomes a major factor in determining a car's top speed. I'm using laypeople lingo for it, but you get the principle.

[ACEL]

In a number of posts on this thread, I have posted Cd figures attained according to my way of doing the throttle stop before and after modification comparison and Cd difference computation.

I can nor say that I was wrong or right. I can only say as in engineering teaching (https://www.engr.colostate.edu/~alla.../page8f.html):

"How much power does it take to go 60 mph? 80 mph? The weight of the car and its speed all are factors that affect the amount of power required. The force balance of a vehicle is

Drag force= Fd = Cr .m.g + 1/2 Cd. Ro. A V^2

where

Cr = coefficient of rolling resistance
Cd = drag coefficient
m = mass of vehicle [kg]
A = frontal surface area [m2]
g = 9.8 m/s
Ro = density of air, 1.2 kg/m3 @STP

First, to really compare before and after modification Cd, one has to take into account the Ro air density parameter. Air density varies with temperature and atmospheric pressure.

Then the above Ro definition relates to STP : Standard Temperature and Pressure

"Since 1982, STP is defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 10E5 Pa (100 kPa, 1 bar)" (https://en.wikipedia.org/wiki/Standa...e_and_pressure)

Even in advanced education there can be mistaken information. I doubt that 0 °C, 32 °F, is a standard temperature in a wind tunnel.

So, from the same wikipedia article, there is a more realistic "standard conditions" definition that would suit wind tunnel drag measures :

"NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP)."

So, to precisely compare drag force, thus Cd, before and after modification to the OEM published Cd, one would have to trial mods in the same "normal temperature and pressure (abbreviated as NTP)" as the car manufacturer. This would hold true only if the manufacturer used this exact set of conditions. Which one can also doubt as these are not published data. Marketing claims sell cars better than real technical and scientific data.

On the other hand, EPA and other countries government agencies that require the car manufacturers to provide the information for the car window tag that you see when visiting the car dealership, do not measure the drag force or evaluate the Cd. Since 2007, the EPA, Canada and some others use the 5 cycles fuel economy tests. (https://www.fueleconomy.gov/feg/fe_test_schedules.shtml). For the highway speed test, the lab temperature may be anywhere between 20 and 35 Celsius (68 to 86 F). So, no reliable and accurate data to compare.

Finally, from my old days in engineering training, the car drag force coefficients analogically relate to the "safety coefficient". Before 1980s, this parameter was introduced in constraints computation by engineers to size posts and beams in buildings, machine components and whatever to take all unknown and extra loads into account. So the Cr, rolling resistance coefficient, and the Cd, the drag coefficient are just some compound value of various "unknowns" that the measuring conditions are kept from us car drivers mere mortals.

In the end, the only true measure of all car eco-modifications is the actual fuel economy realized.

However, I will keep on measuring modification with the throttle stop procedure to decide if I stop the improvement path experiment with a satisfactory result or should I discard the mod and trial something else until I get satisfied.

The path to enlightenment is a tortuous road frequently giving the impression of the necessity to begin the journey all over again. Wow! did I just write that?

[California98Civic]

I would guess you have already seen this EM wiki, but just in case you have not seen it... it has a number of 1990s era Acuras on it: https://ecomodder.com/wiki/Vehicle_C...t_of_Drag_List

Originally Posted by ACEL

In a number of posts on this thread, I have posted Cd figures attained according to my way of doing the throttle stop before and after modification comparison and Cd difference computation.

I can nor say that I was wrong or right. I can only say as in engineering teaching (https://www.engr.colostate.edu/~alla.../page8f.html):

"How much power does it take to go 60 mph? 80 mph? The weight of the car and its speed all are factors that affect the amount of power required. The force balance of a vehicle is

Drag force= Fd = Cr .m.g + 1/2 Cd. Ro. A V^2

where

Cr = coefficient of rolling resistance
Cd = drag coefficient
m = mass of vehicle [kg]
A = frontal surface area [m2]
g = 9.8 m/s
Ro = density of air, 1.2 kg/m3 @STP

First, to really compare before and after modification Cd, one has to take into account the Ro air density parameter. Air density varies with temperature and atmospheric pressure.

Then the above Ro definition relates to STP : Standard Temperature and Pressure

"Since 1982, STP is defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 10E5 Pa (100 kPa, 1 bar)" (https://en.wikipedia.org/wiki/Standa...e_and_pressure)

Even in advanced education there can be mistaken information. I doubt that 0 °C, 32 °F, is a standard temperature in a wind tunnel.

So, from the same wikipedia article, there is a more realistic "standard conditions" definition that would suit wind tunnel drag measures :

"NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP)."

So, to precisely compare drag force, thus Cd, before and after modification to the OEM published Cd, one would have to trial mods in the same "normal temperature and pressure (abbreviated as NTP)" as the car manufacturer. This would hold true only if the manufacturer used this exact set of conditions. Which one can also doubt as these are not published data. Marketing claims sell cars better than real technical and scientific data.

On the other hand, EPA and other countries government agencies that require the car manufacturers to provide the information for the car window tag that you see when visiting the car dealership, do not measure the drag force or evaluate the Cd. Since 2007, the EPA, Canada and some others use the 5 cycles fuel economy tests. (https://www.fueleconomy.gov/feg/fe_test_schedules.shtml). For the highway speed test, the lab temperature may be anywhere between 20 and 35 Celsius (68 to 86 F). So, no reliable and accurate data to compare.

Finally, from my old days in engineering training, the car drag force coefficients analogically relate to the "safety coefficient". Before 1980s, this parameter was introduced in constraints computation by engineers to size posts and beams in buildings, machine components and whatever to take all unknown and extra loads into account. So the Cr, rolling resistance coefficient, and the Cd, the drag coefficient are just some compound value of various "unknowns" that the measuring conditions are kept from us car drivers mere mortals.

In the end, the only true measure of all car eco-modifications is the actual fuel economy realized.

However, I will keep on measuring modification with the throttle stop procedure to decide if I stop the improvement path experiment with a satisfactory result or should I discard the mod and trial something else until I get satisfied.

The path to enlightenment is a tortuous road frequently giving the impression of the necessity to begin the journey all over again. Wow! did I just write that?

[ACEL]

California98Civic,

Yes, I have seen this thread. Since Acura EL has been only built and offered in Canada, its Cd is probably the same as Civic sedan or in between the latter and Civic SI same year at 0,34.

Unfortunately, there is no better data than Cd = 0,34 for Honda Domani estimated by car-catalog https://www.automobile-catalog.com/c...automatic.html

Cheers!