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I got to thinking about the 1 bar limit for R32/R33 ceramic (turbine) turbos. Surely the maximum safe boost depends on the exhaust being used. To attain a certain level of boost, you need a certain flow and pressure gradient across the turbines. Hence, if you have higher back-pressure (e.g. using stock exhaust, downpipes and cat) you need higher pressure on the inlet to attain 1 bar and the blades are more at risk of shearing off. Using a 90-100mm decat turbo-back system, there is next to no back-pressure and hence the pressure on the inlet is less, meaning less risk to blades.

Is this reasoning sound?
 

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Its shaft torque that breaks ceramic wheels-off (assuming a good map with no det). As temp increases the torque limit does come down as well.

More inlet boost = higher compressor tip-speed. Higher compressor tip-speed = more torque from the turbine and = higher shaft torque.
 

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If it's shaft speed/torque that breaks them, then low exhaust back pressure will exacerbate this by increasing airflow, and hence compressor speed required to develop any given boost pressure, particularly as compressor efficiency drops at higher flow rates.
 
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