I don't think this really is the issue, because basically every turbocharger goes supersonic with its outer areas at full throttle.subcritical71 wrote: ↑Mon Nov 18, 2019 3:26 pmSomething to consider when discussing compressor sizing, and this is reading between the lines on my side. Generally a larger diameter compressor would give you more flow. But there are some limits other than speed/size to consider. If the Merc have a larger diameter compressor than Honda they very well may not be able to reach the max allowable RPM due to speed of sound limitations at the compressor tips (exceeding mach 1 in a compressor means an entirely different design). If the Honda have a smaller compressor they may be able to reach a higher RPM (closer to max allowed RPM) at high altitude density tracks and compensate for the density difference where the Merc or anyone with a larger diameter compressor cannot.

We can calculate what would be the allowable diameter at 125000 rpm to stay subsonic, it's fairly easy. At least i think i got it right or its gonna be really embarassing...

So a maximum diameter of basically 5cm would be possible...I don't think any F1 manufacturer runs a turbo that small.

It's a common misconception that the blades of the compressor or fan needs to stay subsonic. If you look up the max rpm values for the fan of modern turbofan engines for instance, you will see they go far into the supersonic speed regime with their outer circumferential speed.

What is importent though is that the airflow stays subsonic, but the speed of the airflow does not perfectly corrolate with the speed of the blades.