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?