For those who missed it, here is my high speed test :
http://ecomodder.com/forum/showthrea...mph-27313.html
Forgot to mention it, I installed a throttle block at the end of the cable. It prevents me to go further then about 80% load. I hope it helps during acceleration phases and P&G.
Today, I made some coastdown tests. The conditions were good, almost no wind. Flat, dual lane road. Paid attention not to be in a vehicle's wake.
Temp : 17°C
Humidity : 95%
Pressure : 1020 hPa
Air density : 1,233
For the Cd, I coasted from 110 to 80 km/h, I recorded the whole test on GPS and calculated out of the excel sheet. About 10 coastdowns were made in each direction. Each direction has about the same values, meaning that there's is no grade and no wind. On average, the car needed 21,207 seconds to coast from 110 to 80 km/h.
For the Cr, I coasted from 17,1 km/h to a complete stop. 5 runs in each direction gave 65,74 seconds average.
The results are :
Cd : 0,395 (with A = 1,696 m²)
Cr : 0,0071
Thoughts :
The Cd seems very high. The car official's Cd is 0.31, I can understand that the value is optimistic but to get as high as 0.395... And my car is even aeromodded... I don't think my tests have any inconsistencies. I'll take the value as it is. The good news is that my engine has one hell of an efficiency with a 0,67 CdA. About 230 g/kWh at 2900 rpm and 80% load.
The Cr seems too good. It is very hard to match the segments in each direction : if I accelerate too much, I end up coasting a bit further (be it 10 m) and very small gradients can alter the test. I cannot say that the average gradient of "E direction" = average gradient of "W direction".
Other explanation : accuracy of initial speed. I drove at 20 speedo for a while and took the GPS average. The problem is that the counter seems to have some inertia at these low speeds, it only moves for a noticeable speed change but not very small changes that the GPS sees. My GPS average was 17.1 km/h and I started the time counter when the needle passed 20 km/h. But I'm not 100% sure the speed is 17.1 km/h.
I hade made 2 other tests, not exactly at the same place but flat and same road quality:
Michelin Energy at 2.2 bar : 0,0109
Michelin Energy at 3.0 bar : 0,0095
Yokohama Bluearth 3.5 bar on front + Energy 3.0 bar on rear : 0,0071
That would mean that Yokohama's allover at 3.5 bar would have 0,0047... not possible.
Starting from a stop and roll down some very light decline might be more precise.