The compressor upgrade or hybrid upgrade is a very popular choice for a turbo upgrade.  The stock turbine and center housings are kept, so installation is simple.  Most times the stock turbine wheel and center shaft is kept, but it is possible to even change that part.

The compressor wheel is upgraded to a larger unit, and this is where the increased power is made.  With the larger wheel, this will require machining both the backplate and compressor housing or replacing both with larger units.  Since Garrett parts are easy interchanged with the stock Hitachi stuff, most popular comp upgrades are from Garrett series turbos.  Most popular are the V-trim, H-trim, 60-1, TS04, and T04E "60" wheels.

The V-trim wheel is the smallest size listed, but power potential can be up to 300hp at 15psi of boost.  Boost response will be like stock, even with the significantly larger compressor wheel.  The V-trim compressor wheel is small enough that a machined stock Hitachi back plate and compressor housing can be used; this makes for a good economical upgrade as you only need to purchase the V-trim wheel and machine the stock pieces.

The H-trim is the next size up, and power potential jumps up to 350hp.  The H-trim does a little lag to spool-up, but most people won't notice the difference versus the stock turbo.  We've seen H-trim compressor upgrades use machined Hitachi pieces or Garrett H-trim back plate + compressor housing.

At the largest end, the 60-1, TS04, T04E "60" all produce around the same amount of horsepower, as compressor wheel sizes are similar.  Power potential will start to edge close to 400hp!  Due to the sheer size of these compressor wheels, a larger back plate and compressor housing is mandatory.  The 60-1 compressor housing is so large, that on a Zenki engine, you might need to grind down part of the compressor housing to clear the lower intake manifold.

It is possible to go larger (compressor wheels), but fitment will be an issue.

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