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If you've installed a smaller diameter exhaust pipe to the turbo, which I understand from your post, the turbo should spin faster than stock. Your ECM could be sending extra gas to the engine while the blowoff valve is preventing extra air from entering. That would explain the black smoke. I'd look for info on the RPM of the turbo vs. the engine RPM for a stock motor and compare what you're experiencing. Hope this helps.
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