671 Detroit Diesel Long Block Assembly Part 1- DIESELPRO.COM Video

March 25, 2025

Detroit Diesel 6-71 Engine Variants – Specifications & Applications

Overview

The Detroit Diesel 6-71 is one of the most recognizable diesel engines in the history of heavy equipment, transportation, and marine propulsion. Originally introduced in 1938 and refined over decades, this inline six-cylinder, two-stroke dieselengine remains relevant today, especially in the aftermarket for fleet restoration, repowers, and rebuilds. It has appeared in multiple configurations: the non-turbo 2-valve, the non-turbo 4-valve, and the turbocharged (6-71T) variant.

All three versions share the same displacement and general architecture but differ significantly in power output, airflow characteristics, and application focus. These differences affect everything from injector selection to cylinder head design and air handling.

At Diesel Pro Power, we support every version of the 6-71 engine with a full catalog of premium aftermarket parts, including overhaul kits, cylinder heads, turbochargers, blower components, and water pumps. This guide details each variant’s specifications and highlights the industries and equipment types that depend on them.

Engine Block for Detroit Diesel 671 Engine
Engine Block for Detroit Diesel 671 Engine

Detroit Diesel 6-71 Non-Turbo – 2-Valve Head Type

Technical Specifications

  • Engine Type: Inline six-cylinder, two-stroke diesel
  • Head Design: Two valves per cylinder (one intake, one exhaust)
  • Displacement: 426 cubic inches (6.99 liters)
  • Bore x Stroke: 4.25 in x 5.00 in
  • Compression Ratio: Approximately 17:1
  • Aspiration: Naturally aspirated via Roots blower only
  • Fuel System: Mechanical unit injectors
  • Governed Speed: 2,100 RPM
  • Horsepower Range: 165 to 210 HP @ 2,100 RPM
  • Peak Torque: 400 to 500 lb-ft @ 1,200 to 1,600 RPM
  • Oil Capacity: ~5 to 6 gallons
  • Coolant Capacity: ~4 to 5 gallons

The 2-valve 6-71 was the original configuration and remained in production for decades. Its airflow is limited by the valve count, making it best suited for low-load and steady-RPM applications.

Applications

Marine Industry

  • River workboats
  • Harbor service barges
  • Small ferries with minimal propulsion demands
  • Classic pleasure boats and converted tugboats

Industrial Applications

  • Stationary generator sets (prime or standby power)
  • Fire pumps and irrigation units
  • Light-duty compressors and small drilling equipment
  • Small airfield power units

Ground-Based Equipment

  • Graders and pavers (pre-1970s)
  • Utility trucks
  • Old crane carriers and material handlers
  • Military cargo trucks (WWII era)

Key Traits

  • Easy to service in remote environments
  • Lower cost due to simpler head design
  • Good fit for repower projects where maximum HP is not required
  • Long-lasting when used below rated capacity

Detroit Diesel 6-71 Non-Turbo – 4-Valve Head Type

Technical Specifications

  • Valve Configuration: Four valves per cylinder (two intake, two exhaust)
  • Aspiration: Roots blower (no turbo)
  • Horsepower Range: 210 to 238 HP @ 2,100 RPM
  • Torque: 475 to 525 lb-ft
  • Fuel System: Mechanical unit injectors
  • Compression Ratio: ~17:1
  • Other Specs: Displacement, oil capacity, and cooling system remain identical to 2-valve version

The 4-valve head enhances airflow and fuel burn efficiency, offering higher output without increasing displacement or requiring a turbocharger. This makes it a logical upgrade path or factory option for operations needing more power without added complexity.

Applications

Marine Use

  • Mid-size fishing vessels
  • Harbor pilot and inspection boats
  • Auxiliary drives on larger ships
  • Converted classic vessels requiring moderate HP increase

Commercial Equipment

  • Transit buses (especially GM or RTS platforms)
  • Cement mixers and dump trucks
  • Rail maintenance vehicles
  • High-torque portable pumps and compressor trailers

Key Traits

  • Enhanced airflow improves throttle response
  • Easier starting in cold conditions
  • Lower emissions compared to 2-valve
  • Factory-preferred for many post-1965 6-71 installations

Detroit Diesel 6-71 Turbocharged (6-71T)

Technical Specifications

  • Aspiration: Turbocharged with Roots blower assist
  • Head Configuration: Four valves per cylinder (required for turbo airflow)
  • Displacement: 426 cubic inches
  • Horsepower Range: 238 to 310 HP @ 2,100 RPM
  • Peak Torque: 550 to 675 lb-ft @ 1,200 to 1,600 RPM
  • Turbocharger Boost: 14 to 22 psi
  • Compression Ratio: 16.5:1 (slightly lower to accommodate boost pressure)
  • Fuel Injection: Matched to turbo output; typically larger injector sizes (e.g., N70 or N75)
  • Oil and Coolant Capacities: Same as naturally aspirated versions

The turbocharged 6-71 dramatically improves power output and efficiency, making it suitable for demanding environments, high-altitude performance, and dynamic throttle conditions. It is commonly referred to as the 6-71T and is instantly recognizable by the presence of the turbocharger mounted in conjunction with the Roots blower.

Applications

Marine Use

  • Fast fishing vessels and offshore patrol boats
  • Twin-engine sportfishing yachts
  • Turbo-upgraded shrimpers and scallopers
  • Ferries with time-sensitive docking routes
  • Crew boats for offshore oil platforms

Industrial Use

  • Large compressors for mining operations
  • Frac fluid power units
  • Fire engines and emergency pump platforms
  • Drilling rigs in oilfields and water wells
  • Heavy-load conveyors in quarries

On-Highway and Defense Use

  • Fire trucks and emergency response vehicles
  • Tank retrievers and tracked military equipment
  • Armored buses and homeland security vehicles
  • Vintage racing truck conversions

Key Traits

  • High horsepower-to-displacement ratio
  • Excellent for high-RPM loads and uphill demand
  • Better combustion efficiency and reduced black smoke
  • Requires more precise maintenance (especially on turbo and cooling systems)

Side-by-Side Comparison Table

Feature 6-71 Non-Turbo (2-Valve) 6-71 Non-Turbo (4-Valve) 6-71 Turbocharged (6-71T)
Valves per Cylinder 2 4 4
Aspiration Method Blower only Blower only Turbo + Blower
Horsepower Range 165–210 HP 210–238 HP 238–310 HP
Torque Output 400–500 lb-ft 475–525 lb-ft 550–675 lb-ft
Fuel Efficiency Basic Improved Superior under load
Maintenance Complexity Low Moderate Higher (turbo maintenance)
Best For Simplicity & Low RPM Mid-power Steady Ops Dynamic Load High Torque

Choosing the Right Variant

When to Choose 2-Valve Non-Turbo

  • You need mechanical simplicity and low maintenance
  • The application runs at constant speeds and light loads
  • Parts interchangeability and field serviceability are priorities
  • Your equipment predates 1965 or requires historical accuracy

When to Choose 4-Valve Non-Turbo

  • You want more horsepower without adding a turbo
  • Airflow and combustion efficiency are a concern
  • Your application involves variable but not extreme load changes
  • Emissions and temperature stability matter

When to Choose Turbocharged 6-71 (6-71T)

  • You require more than 250 HP per engine
  • The application involves frequent acceleration, climbing, or load shifts
  • You are operating at high altitudes or humid coastal zones
  • You’re replacing an existing 6-71T or repowering a vessel for commercial use

Additional Resources For The Detroit Diesel 6-71

Practical Guide To Servicing Your Detroit Diesel 71 Series Inline 6-71 Part 1 

Practical Guide To Servicing Your Detroit Diesel 671 – Part 2

Engine Components and Their Roles (Covering Detroit Diesel 671 Engines)

Key Maintenance Procedures (Covering Detroit Diesel 671 Engines)

Assembly and Disassembly Guidelines (Covering Detroit Diesel 671 Engines)

Safety Protocols for Technicians (Working On Detroit Diesel 671 Engines)

Torque Specifications and Wear Limits (Covering Detroit Diesel 671 Engines)

Exact Torque Specifications for Detroit Diesel Series 71 Inline Engine Components (Covering Detroit Diesel 671 Engines)

Wear Limits for Critical Engine Parts and How to Measure These During Inspections (Covering Detroit Diesel 671 Engines)

Lubrication System For Detroit Diesel 71 Series Inline Engines (6-71)

Cooling System and Heat Exchanger For Detroit Diesel 71 Series Inline Engines ( 671)

Introduction to the Cooling System and Heat Exchanger For Detroit Diesel 71 Series Inline Engines ( 671)

Routine Cooling System Checks and Maintenance For Detroit Diesel 71 Series Inline Engines ( 671)

Water Pump For Detroit Diesel 71 Series Inline Engines (671)

Thermostat Operation For Detroit Diesel 71 Series Inline Engines (671)

Ventilation and Crankcase Vapor Collection For Detroit Diesel 71 Series Inline Engines (671)

Troubleshooting and Specifications For Cooling & Lubrication In Detroit Diesel 71 Series Inline Engines (671)

Overview of Troubleshooting in Lubrication and Cooling Systems For Detroit Diesel 71 Series Inline Engines (671)

Systematic Approach to Troubleshooting For Detroit Diesel 71 Series Inline Engines (671)

Cooling System Troubleshooting For Detroit Diesel 71 Series Inline Engines (671)

Torque Specifications for Key Components For Detroit Diesel 71 Series Inline Engines (671)

Parts Grading and Condition Assessment For Detroit Diesel 71 Series Inline Engines (671)

Troubleshooting Tips for Improved Efficiency In Detroit Diesel 71 Series Inline Engines (671)

Summary of Troubleshooting and Specifications In Detroit Diesel 71 Series Inline Engines ( 671)

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