^ turbine wheel listed on the Garrett site is "68 mm diameter" and 84 trim, so I presume that is ~Q trim (see below).


There's been extensive discussion about compressor maps and compressor trim, as well as turbine A/R. But I feel like I don't read much about turbine wheels themselves, beyond a general recommendation to get a P trim or an O trim or whatever wheel. Is that because it's not all that important? Or is it because it is a sort of black art of building a custom turbo setup?

So can someone talk about how turbine trim affects spool? Turbine trim numbers are a lot higher than compressor trim numbers. And I know that with compressor trim, a higher trim # correlates with less efficiency at higher boost (which has its own set of advantages and disadvantages) to an extent. So what about on a turbine wheel?

And what's a good number (% value) for turbine efficiency? How do we know what kind of lb/min we need for a turbine wheel? Can someone describe the interplay between the sizing of the wheel and the sizing of the A/R for the turbine housing? How does manifold design play into this?

These are all issues that I hope will spark some fruitful discussion.

Other common Garrett turbine wheels (courtesy FC3Spro)

trim exducer major
N 52mm 74.22
O 58.32 74.22
P 64.62 74.22
Q 68.40 78.99

 

Just one more thing to think about-

Exhaust wheel shape.

The GTQ series of exhaust turbine wheels was redesigned w/ computer modeling for use in the GT series turbos. You can bet they were optimized for modern smooth pulse equal length manifolds.

Mazda designed its own exhaust turbine wheel shape with Hitachi for the rotary,

They found there were two forces that acted upon the exhaust turbine, impact and reaction The impact force with a rotary was very strong so they designed a special flat bladed turbine that produced better turbo response, less lag and more power.

Consequently, exhaust manifold runner length will play a large part in this as well as an impact wheel will really work well on a short runner manifold and the GT series respond much better to a long runner manifold.