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If the wastegate is closed and you blocked it off then it should make boost and build pressure. 5 10 15 pounds easy. When you rev it up. So either the compressor wheel or the turnbine wheel either one or both could be an issue or it could be a shaft, make sure you have no exhaust leaks between the manifold and the turbo. Any even the smallest exhaust leak between the exhaust manifold and the turbo charger will greatly reduce boost.