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Originally Posted by Phase
Then what’s the best way to test?
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Throttle-stop testing is just one method for measuring drag changes--a very good one, but one that may not work on all cars (e.g. electronic throttle controls). Other methods include measuring fuel economy over long distances/repeated long trips, and coastdown testing. You have to be careful with coastdowns; you can find my attempt to improve coastdown testing on the road
here.
But drag changes are just one aspect of aerodynamic measurement. Tuft testing and pressure measurement (of both body panels and heat exchanger flows) will tell you a lot. Ride height sensors will show you whether your car is creating lift or downforce. If you buy Julian's book, you'll see new techniques for measuring lift and trailing vortex strength/direction.
Approach aerodynamic modification like an investigation: Your job is to figure out what happens to airflow as you change the shape of your car. Same as if you were diagnosing a fault code to figure out what's wrong with an engine.
For example, I bought a cheap Hellcat spoiler on Amazon to test. I didn't just use one method because that wouldn't give me a complete picture of what this spoiler does on my car. I taped it on and measured pressures on the rear window and hatch; I tufted the window and spoiler and videorecorded it; I did several coastdown tests and ran a statistical test to analyze the data. Even having done all that, I still don't have a complete picture, as I don't know what that spoiler does to the trailing vortices, for instance (so I'm planning on finding out soon).
Unfortunately, what you'll find on this site and a lot of others is a belief that guessing is better. It isn't.