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Old 11-27-2018, 06:02 PM   #20
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Join Date: Dec 2009
Location: Torrance, CA

Originally Posted by 2turbotoys View Post
Using no bypass or dump such as crogthomas says he does leads to premature turbo bearing failure, especially if running a decent amount of boost. When letting off the throttle under boost the air pressure backs up in the intercooler tubes, pushing back on your turbos impeller, trying to spin it backwards, its not made for that stress.
Almost but not quite. Surge won't try to spin the wheel backwards; and even if there is some torque reversal at the wheel it's not the main concern.

Compressor surge essentially creates a rapid oscillating axial thrust load, pulling and pushing the shaft in and out. With a journal bearing turbo that can quickly damage the thrust bearing since it will likely get overloaded with more thrust load than a hydrodynamic film can support. With a ball bearing turbo that has angular contact paths, there is no separate thrust bearing, and they can take a lot more abuse via surge without any real damage.

The reason the wheel and rotor group thrusts in and out in surge is due to the pressure differential from the front of the wheel to the backdisc. You always have a net positive force pushing the wheel out of the turbo on boost, because the cavity behind the wheel "fills up" and reaches close to full boost pressure. The cavity in front of the wheel is the intake tract of course, and will be slightly below atmospheric. So there's always a net force pulling the wheel outward. Hard compressor surge is a pressure oscillation and even some flow reversal, but the main impact is that the static pressure behind the wheel will vary rapidly, which beats the crap out of the thrust bearing (again, in a JB turbo with a separate thrust bearing).

The dorifto guys running no BOV at all and getting away with it are likely also using ball bearing turbos.
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