The Ultimate Duramax Frankenstein Build

Engine Block: L5P (2017 to Present)

• Why it wins:
  • Strongest Duramax block ever cast
  • Reinforced main webbing and thicker cylinder walls
  • Capable of handling 800 HP before needing reinforcement
  • Updated oiling and cooling for modern power levels
• Honorable Mention:
  • LBZ block (2006 to 2007) for builds under 650 HP — proven and affordable

Crankshaft and Rotating Assembly: LBZ (2006 to 2007)

• Why it wins:
  • Forged steel crank
  • Proven under heavy towing and moderate performance
  • Cheaper and easier to work with than LML or L5P rotating assemblies
• L5P internals are slightly heavier-duty, but harder to modify

Cylinder Heads: L5P

• Why it wins:
  • Highest flowing factory Duramax heads
  • Improved valve and injector geometry
  • Less prone to cracking than earlier castings
• Avoid:
  • LB7 heads — known for cracking and injector cup issues
  • LLY heads — suffered from overheating if not upgraded

Injectors: L5P (Denso Solenoid)

• Why it wins:
  • Durable, efficient, and capable of high flow rates
  • Not piezoelectric like the LML, which were prone to failure
  • Better atomization and control than earlier Bosch designs
• Honorable Mention:
  • LBZ/LMM Bosch solenoid injectors — solid mid-power option, widely available

Injection Pump: Denso HP4 (L5P)

• Why it wins:
  • Replaces the failure-prone CP4
  • Supports up to 36,000 PSI fuel rail pressure
  • Reliable under both stock and tuned conditions
• Avoid:
  • CP4 (LML) — known for catastrophic failure
  • CP3 (LB7–LMM) — reliable, but outdated for 700 HP without dual setups

Turbocharger: Variable Geometry Turbo (2011 to 2016 LML)

• Why it wins:
  • Best factory balance of spool and towing performance
  • Integrated VGT brake
  • Larger than earlier VGTs on LBZ/LMM
• Runner-up:
  • L5P turbo — stronger, better design, but harder to retrofit

Transmission: Allison 1000 6-Speed (2006 to 2010 LBZ/LMM)

• Why it wins:
  • Best balance of strength, simplicity, and tuning support
  • Proven under heavy towing and performance use
  • Plenty of parts, converters, and valve body upgrades available
• Avoid:
  • Early 5-speed Allison (2001 to 2005)
  • 10-speed (L5P) — promising but less proven in high HP builds

ECM: LBZ (2006 to 2007)

• Why it wins:
  • Most tunable Duramax ECM
  • Fully supported by EFI Live
  • Simpler than newer ECMs with fewer emission system complexities
• Runner-up:
  • L5P E41 ECM — more powerful, but much harder to tune and currently requires unlocking and special tools

Cooling System: L5P

• Why it wins:
  • Best factory cooling ever installed in a Duramax
  • Massive radiator and fan
  • Upgraded charge air cooler and improved coolant routing
  • Designed for towing under extreme load and temperature

Best OEM Duramax Build Summary

Component	Best Year(s)	Why It Wins
Engine Block	2017 L5P	Strongest, best factory casting
Rotating Assembly	2006 to 2007 LBZ	Proven crank and rods
Cylinder Heads	2017 L5P	Highest flow, most durable
Injectors	2017 L5P	High pressure, solenoid-style reliability
Injection Pump	2017 Denso HP4	Replaces CP4, excellent rail pressure
Turbo	2011 to 2016 LML	Best OEM towing performance with VGT
Transmission	2006 to 2010 Allison 1000 6-speed	Tunable and proven
ECM	2006 to 2007 LBZ	Easy to tune, EFI Live compatible
Cooling System	2017 L5P	Best airflow and radiator design

Final Thoughts

If you could build the ultimate Duramax using GM-only parts, this Frankenstein would combine the strength of the L5P, the tunability of the LBZ, and the proven Allison 6-speed. Whether you're building a tow rig, a street sleeper, or just want something that won’t leave you stranded, this combo delivers unmatched balance between power, reliability, and drivability.

By: Adam