Turbo overspeed on free flow or block

[ShaferDiesel]

I have always thought that any impeller type pump would freewheel when the flow was capped and pressure was the greatest, and be under maximum load at maximum flow low pressure. This would suggest to me that a cold side pipe blowing off would NOT overspeed the turbo. I was just reading the other thread about BOV's and most of the comments there say that a BOV in the cold side could cause an overspeed. Am I all wet, or is everybody else wrong?

 

[Scooter's Roofing]

A BOV releasing boost at zero load isn't going to overspeed the turbo.

 

[jgsturbo]

Blow the cold side at max load and you'll get an over-speed.

People are generally misinformed when a BOV is supposed to open...

BOV are designed only to keep the charger from flowing too little air at the charger speed prior to fuel being removed. Since the bov should only open when fuel is removed (which drops turbine drive pressure) there isn't anything to drive the turbo faster.

The only thing keeping the turbo moving (when dp drops) is its own inertia. That pushing against a decelerating engine w/ no drive pressure to balance the turbine to compressor wheel load is what causes surge and destroys thrust bearings , etc.