Ummm... yes it would happen on a dyno, and there is a powerful fan which is on producing cold air, which blast against the car, creating the "Same" amount of wind as when you were driving.
Trust me a Dyno, WILL tell you if the nossle facing either way, is either better or worse.
The amount of wind is a constant/controlled variable, and the facing the nossle towards or away from the guard - and measuring it will give you a good indication wheather it's better or not.
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