Engine Swaps & Builds - Square Body Trucks

Overview

Engine swaps and builds represent one of the most popular modifications for 1973-1987 GM Square Body trucks, offering dramatic improvements in power, reliability, and fuel economy. The Square Body's spacious engine bay and robust frame make it an ideal platform for everything from mild small block upgrades to radical big block conversions and modern LS swaps.

This comprehensive guide covers the most common engine swap options, from period-correct GM powerplants to modern fuel-injected engines. Whether you're replacing a worn-out original engine or seeking serious performance gains, understanding the mechanical, electrical, and regulatory considerations is crucial for a successful build.

The Square Body's original engine lineup included various small blocks (305, 350) and big blocks (366, 400, 454), plus diesel options (350 and 379 Detroit Diesel). Most swaps involve upgrading to more powerful or efficient variants of these engines, though modern LS and LT-series swaps have become increasingly popular.

Common Issues

Original Engine Problems

• Smog-Era Performance: 1973-1986 engines were heavily restricted by emissions equipment, resulting in poor power output
• Carburetor Issues: Rochester Quadrajet and 2-barrel carburetors require frequent rebuilding and tuning
• Computer Control Problems: 1981-1987 CCC (Computer Command Control) systems are prone to failure and difficult to diagnose
• TBI Fuel Delivery: 1987 TBI systems often suffer from failed fuel pumps, injectors, and pressure regulators

Swap-Specific Challenges

• Motor Mount Compatibility: Different engines require specific mount configurations
• Transmission Compatibility: Not all engines mate to original transmissions without adapters
• Cooling System Capacity: Higher output engines may overwhelm original cooling systems
• Electrical Integration: Modern engines require extensive wiring modifications
• Exhaust Clearance: Headers and exhaust routing can be challenging with certain combinations

Parts & Materials Needed

Universal Swap Components

• Engine Mounts:
• Small Block Chevy: GM 14044666 (driver), GM 14044667 (passenger)
• Big Block Chevy: GM 3991906 (driver), GM 3991907 (passenger)
• LS Series: Hooker 12635HKR or ICT Billet 551557
• Transmission Crossmember: May require modification or replacement
• Radiator: Consider aluminum upgrade (Champion CC138 for most applications)
• Electric Fans: Flex-a-lite 180 or SPAL 30102120 for increased cooling capacity

LS Swap Specific Parts

• Wiring Harness: PSI LSXH-4L60E or Howell EFI LS1K10
• Oil Pan: Holley 302-2 (rear sump) or ICT Billet 551138X (front sump)
• Headers: Hooker 70101303HKR or Hedman 69590
• Fuel System: Aeromotive 17109 in-tank pump or external 11169 setup
• ECU: HP Tuners or EFI Live for custom tuning

Tools Required

• Engine hoist (minimum 2-ton capacity)
• Transmission jack
• Complete socket set (metric and standard)
• Torque wrench (10-150 ft-lbs)
• Wiring crimpers and electrical supplies
• Cutting/welding equipment (for custom mounts)
• Fuel line tools and fittings

Step-by-Step Guide

Phase 1: Planning and Preparation

1. Determine Swap Goals
   - Target horsepower and torque figures
   - Budget constraints ($3,000-$15,000+ depending on scope)
   - Emissions compliance requirements
   - Daily driver vs. weekend warrior usage

2. Engine Selection
   - Budget Option: Rebuilt 350 small block with mild cam and carburetor
   - Performance Option: 383 stroker or 454 big block
   - Modern Option: LS1, LS3, or LT1 with fuel injection
   - Diesel Option: 6.2L or 6.5L Detroit Diesel for fuel economy

3. Transmission Compatibility
   - TH350/TH400 bolt to most Chevy engines
   - 700R4/4L60E provide overdrive for highway driving
   - Manual options include SM465, NV4500, or T56

Phase 2: Removal of Original Engine

1. Disconnect Battery and Drain Fluids
   - Remove negative battery cable
   - Drain coolant, oil, and transmission fluid
   - Recover A/C refrigerant if equipped

2. Remove Accessories and Connections
   - Unbolt radiator and cooling fans
   - Disconnect fuel lines (use proper fuel line tools)
   - Remove air cleaner and carburetor/TBI unit
   - Disconnect all electrical connections
   - Remove exhaust manifolds or headers

3. Engine Extraction
   - Support transmission with jack
   - Remove transmission-to-engine bolts
   - Attach engine hoist to lifting points
   - Remove motor mount bolts
   - Carefully lift engine from bay

Phase 3: New Engine Installation

1. Prepare Engine Bay
   - Clean and inspect motor mounts
   - Check frame for cracks or damage
   - Install new motor mounts if required
   - Route new wiring harness (LS swaps)

2. Engine Installation
   - Lower engine into position using hoist
   - Align with transmission (use alignment dowels)
   - Install motor mount bolts to specification
   - Torque transmission bolts: 35 ft-lbs (small block), 45 ft-lbs (big block)

3. Connect Systems
   - Install headers/exhaust manifolds
   - Connect fuel system (upgrade pump if necessary)
   - Install radiator and cooling system
   - Connect electrical harness and ECU
   - Fill fluids and check levels

Phase 4: Initial Startup and Tuning

1. Pre-Start Checks
   - Verify all connections are secure
   - Check fluid levels
   - Prime oil system by removing distributor and spinning oil pump
   - Set initial timing (if carbureted)

2. First Start
   - Use break-in oil for new/rebuilt engines
   - Monitor oil pressure, temperature, and charging system
   - Allow engine to warm up slowly
   - Check for leaks and unusual noises

3. Tuning and Break-In
   - Adjust carburetor or program ECU
   - Follow cam manufacturer's break-in procedure
   - Change oil after first 500 miles
   - Professional dyno tuning recommended for performance builds

Year/Model Differences

1973-1980 Models

• Simpler Electrical: No computer controls make swaps more straightforward
• Carburetor Compatible: Easy to retain carburetor setup with period engines
• Frame Differences: Some early models have different motor mount locations
• Emissions: Less restrictive in many states for pre-1976 models

1981-1987 Models

• Computer Controls: CCC system may need to be retained or completely replaced
• Wiring Complexity: More extensive electrical modifications required
• TBI Systems: 1987 models have Throttle Body Injection that can be upgraded
• Transmission Options: 700R4 overdrive transmission available

Model-Specific Considerations

C10/C20 (2WD Models)

• Oil Pan Clearance: More options for oil pan selection
• Exhaust Routing: Easier header installation and exhaust routing
• Weight Distribution: Better balance with heavier engines

K10/K20 (4WD Models)

• Transfer Case Clearance: Oil pan selection limited by transfer case
• Front Axle: May require different headers due to differential clearance
• Lift Requirements: Lifted trucks may need custom exhaust work

K5 Blazer/Suburban

• Wheelbase Considerations: Longer vehicles handle big blocks better
• Cooling Requirements: Larger vehicles may need upgraded cooling systems
• Fuel Tank Access: In-tank fuel pump upgrades more challenging

Tips & Best Practices

Planning Phase

• Budget 20% Extra: Unexpected issues always arise during swaps
• Document Everything: Take photos before disassembly
• Source Quality Parts: Cheap engine mounts and gaskets cause problems
• Consider Professional Help: Complex swaps benefit from experienced shops

Installation Best Practices

• Use Proper Torque Specs:
• Motor mount bolts: 50 ft-lbs
• Oil pan bolts: 10 ft-lbs (aluminum), 15 ft-lbs (steel)
• Header bolts: 25 ft-lbs
• Quality Gaskets: Use Fel-Pro or equivalent gasket sets
• Break-In Procedure: Follow cam manufacturer specifications exactly
• Cooling System: Use 50/50 coolant mix and burp system thoroughly

Performance Optimization

• Exhaust System: Don't restrict flow with undersized pipes
• Air Intake: Cold air intake systems provide measurable gains
• Fuel System: Upgrade pump and lines for high-output engines
• Ignition System: HEI or aftermarket ignition improves reliability

Common Mistakes to Avoid

• Inadequate Fuel System: Starved engines run lean and fail
• Poor Wiring: Electrical issues cause endless problems
• Cheap Motor Mounts: Polyurethane mounts reduce vibration
• Ignoring Cooling: Overheating destroys engines quickly

Troubleshooting

Engine Won't Start

1. Check Fuel Delivery
   - Verify fuel pump operation (listen for buzzing)
   - Check fuel pressure: 6-8 PSI (carburetor), 58-64 PSI (LS engines)
   - Inspect fuel lines for leaks or restrictions

2. Verify Ignition System
   - Test for spark at plugs
   - Check timing: 8-12 degrees BTDC initial (varies by engine)
   - Ensure proper firing order: 1-8-4-3-6-5-7-2 (SBC), 1-8-7-2-6-5-4-3 (BBC)

3. Compression Issues
   - Perform compression test: 140-180 PSI normal range
   - Check valve adjustment if equipped with solid lifters
   - Verify cam timing if recently assembled

Overheating Problems

1. Cooling System Capacity
   - Upgrade to aluminum radiator for high-output engines
   - Install electric fans with proper CFM rating
   - Check water pump flow direction (reverse rotation pumps exist)

2. Airflow Issues
   - Remove debris from radiator fins
   - Ensure proper fan shroud installation
   - Check for air pockets in cooling system

Oil Pressure Issues

1. Low Oil Pressure
   - Check oil level and condition
   - Verify proper oil pump pickup installation
   - Normal pressure: 10 PSI per 1000 RPM minimum

2. High Oil Pressure
   - Check for restriction in oil filter or lines
   - Verify proper oil viscosity for temperature range
   - Inspect pressure relief valve operation

Electrical Problems (LS Swaps)

1. ECU Communication
   - Verify ground connections are clean and tight
   - Check power and ground circuits to ECU
   - Use scan tool to check for diagnostic codes

2. Sensor Issues
   - MAP sensor: 1V at idle, 4-5V at WOT
   - TPS sensor: 0.5V closed throttle, 4.5V WOT
   - Coolant temp sensor: 2.5V at 70°F, 0.5V at 210°F

Fuel System Diagnosis

1. Pressure Testing
   - Install fuel pressure gauge at rail
   - Key on, engine off: Should hold pressure for 5 minutes
   - Running pressure should remain steady under load

2. Injector Problems
   - Check injector pulse with noid light
   - Test injector resistance: 12-16 ohms typical
   - Clean or replace clogged injectors as needed