Cd change for irreversible mods

[Piwoslaw]

A-B-A-B testing is a good way to measure the Cd change of aeromods that can be taken off/put on on the road without too much fuss, but how does one go about quantifying improvements of irreversible mods, like bellypans, lowered suspension, roof chopping, etc.?

My guess would be to do a bunch of "A" tests before any mods. Each test would be 3-5 pairs of runs (1 pair=both directions), with weather details recorded. The next tests would be on differnet days, but on the same stretch of road with similar weather. After a certain number of tests on different days, the average from all runs in all tests is less sensitive to slight weather differences. This is the "old" Cd. After finishing the mods, new tests over many days will reveal the "new" or "B" Cd. The multiple tests over many days compensate for not being able to do a second "A" run after the "B".

Question: how many tests are enough to get a good Cd value? 5, 10, 15? Enough so that each next test effects the grand average by less than 5%? 1%?

Anything important to pay attention to that I didn't mention?

[vtec-e]

Build up a good average, perhaps using the fuel log here as a reference. Then do the modification. If there is an improvement then it will show up in the fuel log.

ollie

[bestclimb]

I would think that 4 or 5 sets of consistent data for each mod should be plenty, any more and you start getting diminishing returns on how accurate you are.

[aerohead]

If you have a really good baseline on the car,and after modifying you experience a change in fuel economy,you can calculate your new Cd through the back door.
At 55-mph,a 1% improvement in mpg equates to a 2% drag reduction.
At 70-mph,a 10% drag reduction will net you a 6% improvement in mpg.( apologize for mixed units,don't have calculator with me ).
Hope that helps.