Quote:
Originally Posted by Christ
The pressure there [back of car] is lower than that anywhere else around the vehicle...
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This isn't quite accurate
Quote:
Originally Posted by Christ
I'm not sure what, exactly, the results of the water test would prove, other than (for my car, specifically) how much of a difference there between each end of the vehicle.
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Just about everything is car specific... What applies to your car probably won't apply to another car
Aero "rules" generally are ratios depending on geometry (such as width versus curvature). When doing that test, don't compare to the front of the car, compare to inside your car (which should be "0" - atmospheric as long as you have the a/c blower off). You don't care so much about the difference between each end as much as the difference from atmospheric (plus it's easier to setup that test).
That said, it's not so much what the results would prove - it's to free your mind from your own intuition. Intuition doesn't really work here unless you've actually observed what's going on (which technically is more observation less intuition).
Here's a pressure plot
It's all just colors - blue is lower, red is higher... Green is about 14.7 lb/in^2 (aka, atmospheric). Low pressure happens (drum roll), on top of the car! Where, as you may have guessed, flow velocity as at it's maximum. The pressure differential between max (front of car) and minimum (top of car) is ~14.9-14.3 ln/in^2 - .6psi (the validity of those numbers should be tested).
Now, I'm not saying direct exhaust on top of your car,=... That'd be bad... The idea behind putting exhaust behind your car is to keep the flow moving back, away from the car rather than swirling about. The exhaust has the capability to push some of that out of the way.