Boost to rpm relationship?

[lukelubinskyj]

Stupid question is there a relationship between rpm and boost?

[lukelubinskyj]

I don't know if I worded that correctly... for argument sake is 5lbs boost approximately 25000 rpm and 20lb 45000?
Or is it so dramatically different with each engine... reason I ask is to get a rough idea what rpm I'm turning?

[finiteparts]

The boost to rotational speed is dependent on many factors....if you can find a compressor map, you can estimate your rotational speed based on the boost pressure and a guess-timate of the compressor efficiency.

[racket]

Yep , theres a rough relationship between rpm, or more correctly comp wheel tip speed, and boost/P2 pressure , it roughly increases as the square of the tip speed , that is , double the tip speed and there'll be 4 times the pressure and is the reason why we "idle" at ~30-40% of max rpm even though idle pressures are relatively low.

Using P2 pressure as a guide to rpm is a risky business because if the turb stage is a bit oversized compared to the comp stage and the comp is flowing in the high flow choke region you can overspeed at relatively modest boost/P2 pressures .

If in doubt about your flow conditions its best to keep to modest boost/P2s of ~20 psi .

Cheers
John

[lukelubinskyj]

Thanks I didn't have a clue what was a safe operating range...

[racket]

You need to keep max tip speed of the biggest wheel to below 1450ft/sec for an old turbo of unknown heritage , a high quality modern new turbo can be taken higher , also keep temps out of the turbine wheel to below ~1200 degrees F or 650 deg C for an old turbo