Tons of excellent info in this thread!TypeC said:There is also a line show compressor rpm. The higher the compressor rpm, the hotter the air will be and the harder the turbo will be working (read: longivity).
This particular point is, in my opinion, an overgeneralization... the islands of efficiency have MUCH more bearing on outlet temperatures than the compressor speed. If a compressor has 78% efficiency at 200,000 RPM, the outlet temperature will be no higher than another compressor that makes the same pressure ratio at 78% efficiency at 120,000 RPM. In fact, below a certain speed it becomes very difficult to attain efficiencies as high as 78%, which is why compressor design is leaning toward smaller and faster compressors. Watch your efficiencies above all else. Chances are, if your compressor has peak efficiency at 150krpm, you WILL be making too much heat at 200krpm, because you will be operating at a reduced efficiency. But if your peak efficiency is at 200krpm, don't let that scare you off. And if the turbo is designed around a peak efficiency point up at 200krpm, that speed is certainly not going to harm your turbo's longevity. These things are made to go fast! As long as your oil and cooling system is adequate, the bearings should not take any more wear and tear regardless of speed. The real problem is, if the turbo is designed for a PEAK speed of 200krpm, it's likely that the shaft has a vibration problem somewhere a little way above that speed. If the shaft starts shaking, you WILL potentially get metal-metal contact at the bearings or at the compressor wheel/shroud interface and then UGLY things start to happen.
I'm not talking completely out of my ass here, I am a mechanical engineer working in centrifugal compressor design for the past year. Not a ton of experience but I've learned a lot, and took coursework specializing in turbomachinery and bearing design when I was finishing up my degree. I've also got a few very useful excel worksheets I've put together that I'll see if I can upload and share if they'll be of use to anyone else. Turbos are FUN! This thread remains the best summary of turbocharger info I've seen on any auto forum!