More danger possible to turbos at higher elevations?

[losfinch]

At higher elevations, where the air is thinner and hence less resistance for the turbo to work against, is it possible to overspeed a turbo before actually seeing dangerously high boost numbers?

For example, would the boost numbers for a given turbo RPM be the same at sea level as they are at 8,000 feet above sea level?

What I'm thinking is that at higher elevations one would need to be much more conservative with the boost numbers that they allow since you could over-speed the turbo (to the point of damage) before actually seeing dangerous levels of boost. And, if this is true, how does one know what the safe boost levels are for different elevations?

Anybody know more about this?

[klhansen]

Just making a partially educated guess here, but I don't think it's the RPM that's the limiting factor on the turbo, but the pressure across it and the thrust that it develops along the shaft. I believe the thrust bearings are usually what goes bad.

So the altitude effect would be fairly small.

Sea level 14.7 psia intake to 15 psig boost is 29.7 psi pressure difference.
5000 ft ~ 12.2 psia intake to 15 psig boost is 27.2 psi pressure difference, or about 8% LESS.

Maybe a turbo rebuilder will weigh in.