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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteThe Gen II 426 HEMI and Gen III HEMI share a name and a hemispherical-chamber heritage, but they are different engine families—not old and new versions built from interchangeable parts. Here, “Gen II” means the classic 426 (7.0L); “Gen III” means the modern family that began with the 5.7L for the 2003 model year and later grew to include 6.1L, 6.2L and 6.4L versions. The biggest changes are the modern family’s range of displacements, newer design and application-specific equipment such as cylinder deactivation and variable valve timing.
What Gen 2 and Gen 3 mean in this comparison
Generation labels can be used loosely, so this comparison uses them narrowly: Gen II is the classic Chrysler 426 HEMI, while Gen III is the modern HEMI family launched with the 5.7L engine. The 5.7L arrived for the 2003 model year as an all-new truck V8, marking what the SAE paper abstract calls “the return of the hemispherical combustion chamber architecture.” SAE International’s abstract for technical paper 2002-01-2815 describes that debut.
That shared chamber concept does not make the engines the same design. COMP Cams says modern 5.7L and 6.1L HEMIs do not share parts with classic HEMI engines. Treat the generations as distinct when identifying, rebuilding or shopping for components; confirm fitment for the specific engine and vehicle rather than relying on the HEMI name.
How the modern HEMI family differs in size
The Gen II example is a single displacement in this comparison: the 426-cubic-inch (7.0L) engine. Gen III is a family of several displacements. The figures below are reported by SlashGear’s 2024 comparison article; they illustrate how bore and stroke vary, but are not a parts-interchange guide.
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| Engine in this comparison | Displacement | Bore | Stroke |
|---|---|---|---|
| Gen II 426 | 426 cu in (7.0L) | 4.25 in | 3.75 in |
| Gen III 5.7L | 5.7L | 3.917 in | 3.578 in |
| Gen III 6.1L | 6.1L | 4.055 in | 3.578 in |
| Gen III 6.2L | 6.2L | 4.090 in | 3.578 in |
| Gen III 6.4L (392) | 6.4L | 4.090 in | 3.72 in |
SlashGear’s comparison reports these dimensions. The 6.4L is also called the 392. The cited comparison attributes its increased displacement relative to the 6.2L to a longer stroke; a 2012 service description characterizes that application’s 6.4L as a bored-and-stroked 5.7L-derived engine. That description should not be generalized to every year or application.
What changed in the design—and what did not
The 2003 5.7L was described as an all-new engine, not a refresh of the 426. Both generations use the HEMI identity, but the modern engine family has its own design and parts. COMP Cams’ technical information specifically distinguishes the modern 5.7L and 6.1L from classic HEMIs on parts sharing. A component that looks or sounds familiar is not proof of compatibility.
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The 2012 Dodge Charger SRT-8 6.4L service description identifies a hemispherical-style chamber, splayed valves, dual spark plugs and MDS for that particular engine. These are details of that named application, not a universal specification for every Gen III HEMI. Operation CHARM’s 2012 Charger SRT-8 6.4L engine description provides the application-specific details.
Why MDS and VVT depend on the engine version
Some Gen III engines use Multi-Displacement System (MDS), which deactivates cylinders under conditions suited to the system. Later development also brought variable valve timing (VVT). Neither feature should be assumed on every Gen III engine: equipment depends on the engine version, year and application. Check the vehicle’s service information or exact engine specifications before buying parts or planning modifications.
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That variation matters for performance work. COMP Cams notes that installing an aftermarket performance cam on an MDS-equipped engine may require deactivating MDS and adjusting the engine computer tune. It is not a universal requirement for every cam or every Gen III; verify the cam’s instructions and the engine’s configuration.
Are Gen 2 and Gen 3 HEMI parts interchangeable?
Do not assume they are. COMP Cams explicitly says its modern 5.7L and 6.1L engines do not share parts with classic HEMIs. The bore-and-stroke differences also describe displacement, not component compatibility. For any part, establish the exact engine variant, model year and vehicle application, then verify the manufacturer’s fitment listing or service documentation.
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Even within the Gen III family, one listing should not be treated as universal fitment. For example, Mopar’s performance catalog lists pushrod set P5153629 for specified 5.7L, 6.1L and 392 HEMI engines and gives two pushrod lengths. That listing is a useful reminder to verify the application and dimensions—not evidence that every Gen III engine takes the same pushrods. Mopar’s performance parts catalog is the place to check the listing and its fitment details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare two specific HEMI engines
“Gen 3” alone does not identify an exact specification. Before comparing output, planning a swap or ordering parts, pin down the engine and application:
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- Identify the engine. Record displacement, model year and the vehicle it came from. A 5.7L and a 6.4L are both Gen III, but that label does not make their specifications identical.
- Check the exact factory specification. Compare output and equipment only for the particular engine-year-vehicle combination. The figures in the table above are bore and stroke dimensions, not factory power ratings.
- Confirm installed systems. Determine whether the engine has MDS, VVT or other application-specific equipment before selecting a cam, control parts or tuning approach.
- Verify each part by fitment. Use the manufacturer’s application listing or service information. Do not use generation labels or displacement alone as a compatibility check.
Factory output is not included here because the available specifications do not establish a complete, authoritative year-by-year comparison. Ratings can vary by application, so a fair power comparison needs matched, engine-specific sources.
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