i passed by an RTA approved engineer in an '83 prelude with a JDM B16A runing a 2in exhaust and a straight through Genie Turbo muffler with 2.5in tip...

i got between 88.7 and 89.1dB readings...

i doubt the tests are different,

we did the test in a car park next to a large cricket oval, so there were no walls around at all...