Some interesting tidbits I found:
http://forums.tdiclub.com/showthread.php?t=206994
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Now that I have an EGT gage I've found that at WOT thru the first 4 gears it hit 1720 F, steadily climbing there in 4th gear to about 95 MPH.
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Measured at the turbine inlet (not the downpipe) you can run the following without time limits:
Cited directly from Garrett and KKK
GT-15: 1550F/843C
VNT-15/17 and all other series: 1650F/898C
KP39: 1904F/1040C
Go above those temperatures and you are completely on your own. Often a few seconds (5-10) and no more than 50F should not result in damage. Keep in mind that the Inconel alloy is very strong at it's maximum rated temperature, once you get above that temp range the strength and durability of the alloy plummet exponentially with time and temp.
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More than just the effect on the turbo itself (see Drivbiwire post) is the effect on other parts of the engine. For example, on my Mitsubishi, I've discovered that, with inlet temps above abt 1300 F for more than a few minutes [that equates to about 12psi boost continuously] the EXHAUST VALVES STRETCH! Then I have to go in and adjust the valves (not hydraulic lifter equipped).
Whether or not this happens on the TDI is unknown. Consider though, that if the turbine inlet temp is at 1500F, the exhaust valve edges are at least that hot. This CAN lead to cupping of the valve head due to the spring pressure and compression if the whole valve head begins to creep. The best way to think about creep phenomena is to imagine the part - in this case a valve head - made of very slowly deforming plastic.
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