Holset / HT-18 hybrid
 

Pretty simple question here. And I am guessing yes. But besides machining the turbine housing for the bigger wheel, would a holset chra bolt to the s4 or s5 housing?

It should be something easily search-able but I am not coming up with an answer.

I'm not sure it's ever been attempted. Most people use Garrett parts in their hybrids.

only way to know is to try, but i would assume that it is nowhere near going to fit the holset CHRA because it is a MUCH bigger turbo than the standard HT-18. most hyrids utilize the stock CHRA and simply replace everything on the cold side in front of it(garrett T3 turbos such as typical T04B turbos are direct bolt ons to the HT-18 CHRA). BNR, for example, also machines and installs larger turbine wheels but they are still fitted to the stock CHRA, where the turbine housing is machined to accept the larger turbine wheel.

i think you're going to find that the holset is only going to work on your car only as a standard upgrade, requiring a new exhaust manifold and external wastegate. it also is going to have massive issues with boost creep unless you highly modify your stock wastegate. trying to take a sledge hammer to an already good working turbo in order to make it into a hybrid is like taking 2 steps backwards, part of the uniqueness and design of the newer holset turbo is the variable vanes that help the turbo spool at various speeds.