A cutaway drawing shows many small blades on the exhaust side. I would think 80 to 100 blades would be a good estimate, as opposed to Trainiac's speculation of 12. This would throw the math, based on frequencies, way off.
It certainly would! I've done a little more searching, though, and it's the impeller (on the input side, not the exhaust side) that causes the whistle (I'm only just learning about the internals of turbochargers
). The sound created by 80 to 100 blades at 20,000 rpm would be far above the range of hearing. I found a cutaway view of an EMD turbocharger, and the impeller appears to have 16 blades--which coincides precisely with the actual turbo sound (the blade whistle is 16 times the frequency of the impeller whine--four octaves higher). http://www.epowerrail.com/images%20prd/ ... 0view3.jpg
I still don't know how many blades are on ALCO turbo impellers, but if it is 16, then the turbo would be spinning at about 22,000 rpm (given a 6,000 Hz blade whistle). The figure of 12 was just based on a 30,000 rpm speed, which I had heard from other people.