How many feet do they have to be from the pipes when they do the test....:Shrug:...
They test at 20 inches at a 45* angle and on the same plain at the exhaust. Also, there are many other variables come into play when they test such as traffic, buildings, vehicles, dirt, grass etc. I found a PDF that sets the guidelines for testing. Oh, the equipment also needs to be calibrated and certified. Official needs to be trained and certified to use equipment.