Garrett G42: Specs, Fitment, and Is It Worth It

The Garrett turbocharger G42-1585 G-Series II is a super core assembly rated for up to 1,585 horsepower and represents Garrett's second-generation G-Series engineering in the compact large-single category. The G-Series II designation indicates updated aerodynamic development over the original G-Series lineup: refined compressor wheel geometry, improved efficiency mapping, and the same CHRA-only format that allows the builder to pair the center section with turbine housing and compressor cover specifications optimized for the specific application. For a built diesel engine that has outgrown a direct-fit platform upgrade and is targeting the 800-1,585 horsepower range, the G42 is positioned as the logical step between the direct-fit GT4094-class kits and the G45 at 1,600 horsepower. This guide covers G42-1585 specs, what build levels it supports, how it fits into the G-Series II lineup, and who this Garrett turbocharger is actually built for.

G42-1585 Specs and Sizing

The Garrett G42-1585 G-Series II Super Core (938339-5003S) is the CHRA assembly only -- turbine housing and compressor cover are sourced and specified separately. Key specs:

• Power rating: up to 1,585 horsepower
• Series: G-Series II -- Garrett's second-generation performance super core family with aerodynamic refinements over G-Series I
• Configuration: super core assembly (CHRA) -- turbine housing and compressor cover sold separately
• G-Series II position: above the G40 (1,300 hp) and alongside the G45 (1,600 hp) -- effectively the G-Series II equivalent covering the 1,300-1,585 hp range with updated aerodynamics
• Applications: built diesel and high-output gasoline engines where the G40 ceiling has been reached and the G55 would be oversized

The G-Series II designation distinguishes this lineup from the original G-Series assemblies. Where G-Series I established Garrett's super core product line with the G25 through G55 range, G-Series II applies updated compressor wheel aerodynamics and revised efficiency mapping to the same power steps. The practical difference for the builder is a compressor map that holds efficiency better across a wider operating range, which means less heat generation and better performance at the power levels below the rated maximum -- not just at the peak. For a diesel build that will run at varying load rather than only at maximum effort, the G-Series II efficiency improvements matter more than the maximum rated horsepower number alone.

The G42 sits in the G-Series II lineup above the G40 and at effectively the same power range as the G-Series I G45. A builder evaluating the G45 should also consider the G42 G-Series II if the application benefits from the updated aerodynamic package and the 1,585 horsepower ceiling is sufficient. Both units serve the same general build segment: engines that have outgrown direct-fit platform kits and are in the 800-1,600 horsepower power target range.

Top Picks: G42 and the Garrett G-Series II Lineup

The Garrett G42-1585 G-Series II Super Core (938339-5003S) is the right Garrett turbocharger choice for a built diesel engine targeting up to 1,585 horsepower where the G-Series II aerodynamic refinements are the priority over the G45's higher rated ceiling. The G42 gives the builder the updated efficiency mapping of the G-Series II platform in the compact super core format, paired with custom housing selection to match the specific exhaust configuration and boost target.

For a build targeting above 1,585 horsepower where the G42 ceiling would be exceeded, the Garrett G45-1600 Super Core Standard Rotation (888169-5006S) is the adjacent option at 1,600 horsepower. For a build targeting the maximum ceiling at 2,250 horsepower, the Garrett G55-2250 94mm Super Core (880547-5004S) is the top of the G-Series lineup. For platform-specific direct-fit upgrades on the 2004.5-2009 Duramax where no custom housing selection is needed, the Garrett GT4094VA Stage 2 Turbo Kit and GT3794VA Stage 1 Kit are complete bolt-on solutions. Browse the full Garrett turbocharger lineup for all G-Series II super cores, G-Series I assemblies, and platform-specific kits.

Match to the Build

The G42 G-Series II is the right Garrett turbocharger for the following situations:

• Built diesel targeting 800-1,585 hp with G-Series II efficiency: A built 5.9L or 6.7L Cummins, 6.6L Duramax, or 6.7L Power Stroke on stage 3-plus injectors and a matching fuel system is the primary application. The G-Series II compressor map is designed for sustained operation in this power range with better off-peak efficiency than the G-Series I equivalent.
• Stage 3-4 builds that have outgrown direct-fit kits: When a GT4094VA or similar platform-specific kit has been installed and pushed past its efficient operating range, the G42 is the next step. The transition from a direct-fit kit to a super core with custom housing is the point where the build has become serious enough to require that additional engineering effort.
• Applications where G-Series II efficiency mapping matters: A build that runs varying loads -- street, towing, occasional competition -- benefits from the G-Series II efficiency improvements more than a dedicated competition engine that operates only at maximum output. The G42 maintains a better efficiency island across the operating range rather than only at peak flow.
• Fuel system must be matched: The G42 moves significantly more air than a GT4094-class unit. Injector sizing, pump capacity, and the tune must be calibrated to the G42 compressor map before the upgrade produces useful gains. Installing the G42 on a fuel system sized for a smaller compressor will not produce a power gain -- the fuel delivery will be the constraint.

Signs the Current Turbo Has Reached Its Limit

Builders stepping up to the G42 are typically working off data that confirms the existing turbo is the constraint rather than the fuel system or engine:

• Boost pressure ceiling at wide-open throttle before the injectors and fuel pump have reached their delivery limit -- the compressor is at maximum flow before the fuel is
• Compressor outlet temperature rising sharply at high load -- the existing unit is working past its efficiency island and generating heat rather than pressure
• EGTs climbing faster than expected at high power output despite adequate fuel delivery
• Dyno results plateauing at a power level consistent with the existing turbo maximum, not with injector or pump limits confirmed by data
• Boost falling below the tune target at full load -- the turbo cannot maintain commanded pressure at the airflow rate the engine is requesting

Frequently Asked Questions

What is the difference between G-Series I and G-Series II?

G-Series II applies updated compressor wheel aerodynamics and revised efficiency mapping to Garrett's super core lineup. The power ratings at each step are similar, but the G-Series II compressor map maintains efficiency over a broader operating range than the G-Series I equivalent. For a build that operates at varying loads -- mixed street and towing -- the G-Series II efficiency improvement is meaningful. For a dedicated competition engine that runs only near maximum output, the practical difference is smaller.

Does the G42 include a turbine housing?

No. The G42-1585 G-Series II (938339-5003S) is the CHRA assembly only. The turbine housing and compressor cover are sourced and fitted separately by the builder. This allows the housing A/R ratio and inlet/outlet geometry to be matched to the specific engine and installation rather than accepting a fixed OEM configuration.

Is the G42 compatible with both Cummins and Duramax builds?

Yes. Because the G42 is a super core with separately selected housings, it can be adapted to any diesel platform including 5.9L and 6.7L Cummins, 6.6L Duramax, and 6.7L Power Stroke applications with appropriate housing and adapter selection. The specific turbine housing inlet size and compressor cover outlet must be matched to the exhaust configuration and charge pipe routing of the target engine.

Bottom Line

The Garrett G42-1585 G-Series II super core is the right Garrett turbocharger for built diesel engines in the 800-1,585 horsepower range where the G-Series II aerodynamic efficiency improvements are the priority alongside power output. It serves the same build segment as the G-Series I G45 but with updated compressor wheel geometry that maintains efficiency better across the operating range. For direct-fit upgrades on platform-specific applications, the GT3794VA and GT4094VA kits remain the right starting point. For a build that has outgrown those kits and is targeting the 800-1,585 horsepower range, the G42 G-Series II delivers the Garrett engineering that the application requires.