Originally Posted by ResIpsa

One trick to test the functionality of a rear diffuser is to wait for a day with heavy rain and have a friend drive your car and you follow in another vehicle. As you follow, take a look at other cars on the road and notice how rain is being dispersed behind them (better yet, have another 370Z without a diffuser drive along side of the 370Z with a diffuser). You will see most cars on the road have sprays emitting from the front and rear wheel wells (not very efficient). Also take a look at how rain flows from the rear of the cars.

A functional diffuser will create a substantial rooster tail and should completely eliminate rain escaping from the rear wheel wells. My diffuser creates such a rooster tail that most cars cannot drive behind me in the rain



I created it out of aluminum flashing, aluminum plate, fiberglass and lots of pop rivets.



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