here's the email i got from TAKEDA as to why they dyno with the hood open and also explaining their dyno process using a MUSTANG DYNO:
The Mustang Dyno actually adds electrical resistance to simulate wind resistance as the vehicles speed increases. We have to input weight and HP@50 mph for each vehicle.
As far as testing with the hood open, we are not trying to simulate wind. We need to keep the motor cool so that it does not overheat. If we had to test actual wind, we would need a wind tunnel. That wind tunnel would have to be able to simulate wind as you built up speed and actually blow as fast as your vehicles speed and then some, depending on the direction of the wind. We hear time and time again "Why do they test with the hood open?" I hope this answers the question. We could never simulate 70mph winds you would get on a highway with the fans we are using. For those that feel very strongly about the hood open, I ask them about the vehicle manufactures dyno numbers. They are giving a horsepower reading with the motor outside of any vehicle in a test lab with fans. That is crank HP, not wheel horsepower. We are giving horsepower results to the actual wheels.
What I can tell you is that we test one way for almost all vehicles. We do our best to keep all variables the same except for the product we are testing. If we test stock with the hood open, we are going to test our part with the hood open as well. We key in temperature, humidity, and barometric pressure. The dyno does the correction factor at the end of the runs.
We have done baseline tests on the 370Z. We have sent the driver away with the intake so that the vehicle computer could adjust to the new intake. After a few hundred miles, we will bring the vehicle back in, input temps, and test the same way we did the stock runs.
We are dropping off our 370Z this morning so they can do the dyno results. They will be doing a video of the dyno session, results and process.