The hourglass like shaped block that gets installed with the downpipes to separate the waistgate and exhaust sides of the turbine outlet is not a perfect seal. Even when you were idling around not on boost (i.e. waistgate is closed) I bet you would hear exhaust leaking past that block and out the cut off tube.
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90 300zx TT 12.731@114.00 94 Camaro Z28 13.751@101.83 03 WRX 13.973@94.88 02 Mazda MPV; four kids and a wife don't fit in the Z. 90 240sx (sold) 16.9@87