K-Series Truck Performance Modifications - Square Body Trucks

Overview

The K-series Square Body trucks (1973-1987) represent some of the most capable and modifiable 4WD platforms ever produced by General Motors. These trucks came equipped with robust NP203, NP205, and NP208 transfer cases, Dana and GM 10-bolt front axles, and a variety of engine options that respond well to performance modifications. Whether you're building a weekend trail rig, improving towing capacity, or creating a balanced street/off-road machine, the K-series platform offers excellent potential for enhancement.

This guide covers engine, drivetrain, suspension, and auxiliary modifications specific to K10, K20, K30, K5 Blazer, and K-series Suburban models. The Square Body's body-on-frame construction and generous engine bay accommodate significant upgrades while maintaining reliability and functionality.

Common Issues

Transfer Case Problems

• NP203 full-time cases (1973-1979): Chain stretch, planetary gear wear, and vacuum actuator failures
• NP208 cases (1981-1987): Aluminum case cracking around mounting points under high torque
• Thermal management: Stock cases run hot under sustained load

Front Axle Limitations

• GM 10-bolt front axles (most K10s): Ring gear weakness above 35" tires or high torque applications
• Dana 44 units: U-joint failures at the axle shafts, particularly with larger tires
• Steering geometry: Bump steer and poor handling with lift modifications

Engine Bay Heat Management

• Inadequate cooling for modified engines, especially with A/C and larger cams
• Carburetor heat soak issues affecting performance and reliability

Parts & Materials Needed

Engine Performance Modifications

Carburetor Upgrades:
- Edelbrock 1406 (600 CFM) for 350/305 engines
- Holley 0-80457S (600 CFM vacuum secondary) for street use
- Holley 0-4160C (750 CFM) for 454 big blocks

Intake Manifolds:
- Edelbrock 2101 Performer (small block) - Part #2101
- Weiand 8120 Stealth (big block) - Part #8120
- GM Performance 12496822 (LS swap applications)

Exhaust Systems:
- Hooker 5204HKR headers (1973-1987 small block)
- Flowmaster 17204 cat-back system
- Borla 140189 ATAK system (premium option)

Drivetrain Upgrades

Transfer Case Modifications:
- NP205 swap kit from Advance Adapters - Part #50-9400
- Atlas II transfer case (premium option) - Part #AT-40
- Heavy-duty chains: NP208 chain upgrade Part #15570A

Axle Upgrades:
- Dana 60 front axle assemblies (junkyard or Dynatrac ProRock 60)
- Yukon Gear YA D44-30SPL chromoly axle shafts
- ARB RD117 air locker (Dana 44 applications)

Suspension Components

Lift Kits:
- Rough Country 1309 (4" suspension lift)
- BDS 144H (6" lift with replacement springs)
- ORD dual shock hoops - Part #DSH-7387

Steering Upgrades:
- AGR Super Box steering gear
- Borgeson 999016 steering shaft upgrade
- PSC Motorsports hydraulic assist steering kit

Step-by-Step Guide

Engine Performance Build (Small Block 350)

Phase 1: Bolt-On Modifications

1. Install Performance Intake and Carburetor
   - Remove stock Rochester carburetor and intake manifold
   - Clean intake mounting surface with brake cleaner
   - Install Edelbrock 2101 Performer manifold with supplied gaskets
   - Torque intake bolts to 25 ft-lbs in proper sequence
   - Mount Edelbrock 1406 carburetor with 12 ft-lbs on mounting bolts

2. Headers and Exhaust System
   - Remove stock exhaust manifolds (save bolts for reuse)
   - Install Hooker 5204HKR headers with high-temp gaskets
   - Use anti-seize compound on header bolts, torque to 25 ft-lbs
   - Connect to 2.5" dual exhaust system with H-pipe crossover

3. Ignition Upgrade
   - Install HEI distributor if equipped with points system (pre-1975)
   - Upgrade to Accel 140043 Super Coil
   - Gap spark plugs to 0.045" for performance applications

Phase 2: Internal Engine Modifications

1. Camshaft Selection
   - Comp Cams XE268H for street/towing applications
   - Edelbrock 2102 for mild performance increase
   - Requires valve spring upgrade and pushrod length verification

2. Head Work
   - Port match intake manifold to cylinder heads
   - Consider Vortec head swap (1996+ 5.7L heads) for significant improvement
   - Requires specific intake manifold (Edelbrock 2116)

Transfer Case Upgrade (NP208 to NP205)

Required Components

• NP205 transfer case (1969-1991 GM truck)
• Advance Adapters 50-9400 adapter kit
• Custom driveshafts (front and rear)
• Crossmember modification brackets

Installation Process

1. Preparation
   - Drain transmission and transfer case fluids
   - Support transmission with jack and remove crossmember
   - Mark driveshaft positions before removal

2. Transfer Case Removal
   - Disconnect speedometer cable and shift linkage
   - Remove transfer case-to-transmission bolts
   - Lower case carefully (NP208 weighs approximately 85 lbs)

3. NP205 Installation
   - Install Advance Adapters kit per instructions
   - Modify crossmember or install replacement (ORD part #NP205-CM)
   - Set transfer case with proper alignment to transmission
   - Torque mounting bolts to 35 ft-lbs

4. Final Assembly
   - Install custom driveshafts (measure and order from Tom Wood's Custom Drive Shafts)
   - Connect shift linkage and speedometer cable
   - Fill with 80W-90 gear oil (4 quarts capacity)

Suspension Lift Installation (4" System)

Front Suspension

1. Coil Spring Replacement
   - Remove wheels and support lower control arms
   - Compress coil springs with appropriate tool
   - Install 4" lift springs (Rough Country part #1309)
   - Replace shock absorbers with extended travel units

2. Steering Corrections
   - Install dropped pitman arm to correct steering geometry
   - Consider steering stabilizer for larger tire applications
   - Align front end after completion

Rear Suspension

1. Leaf Spring Modifications
   - Add leaf spring blocks or replacement springs
   - Install longer U-bolts and hardware
   - Maintain proper pinion angle (2-4 degrees down)

2. Shock Installation
   - Mount extended travel shocks
   - Verify adequate clearance at full compression and extension

Year/Model Differences

1973-1980 Models

• Transfer Cases: NP203 full-time (chain drive), NP205 part-time (gear drive)
• Front Axles: Dana 44 standard on K20/K30, GM 10-bolt on most K10s
• Engines: 250 I6, 292 I6, 350 V8, 400 V8, 454 V8
• Fuel System: Mechanical fuel pumps, carburetor fuel systems
• Emissions: Minimal emissions equipment, easier modification

1981-1987 Models

• Transfer Cases: NP208 aluminum case (chain drive), NP205 available through 1986
• Front Axles: Increased use of GM 10-bolt, Dana 44 on heavy-duty applications
• Engines: 305 V8 becomes common, 350 TBI introduced in 1987
• Fuel System: Electric fuel pumps, some TBI applications
• Emissions: Computer controls (1981+), more restrictive emissions equipment

Model-Specific Considerations

K10 (1/2-ton)
- Lighter duty components limit extreme modifications
- GM 10-bolt front axle requires upgrade for serious off-road use
- NP208 transfer case adequate for moderate builds

K20 (3/4-ton)
- Dana 44 front axle handles more abuse
- 14-bolt rear axle (full-floating) very robust
- Better platform for heavy-duty modifications

K5 Blazer
- Shorter wheelbase affects handling with lifts
- Removable top creates structural considerations
- Same drivetrain options as comparable pickup trucks

Suburban
- Longer wheelbase improves stability with modifications
- Additional weight requires consideration for performance mods
- Excellent platform for family-friendly builds

Tips & Best Practices

Engine Modifications

• Maintain cooling system capacity: Upgrade radiator and fan when increasing power
• Fuel system scaling: Ensure fuel pump and lines can support modifications
• Tune for altitude: Adjust carburetor jetting for local elevation
• Break-in procedures: Follow cam manufacturer specifications for new camshafts

Drivetrain Upgrades

• Driveline angles: Maintain proper U-joint angles to prevent vibration and wear
• Gear ratio matching: Consider axle gear changes with tire size increases
• Fluid specifications: Use appropriate lubricants for modified applications
• Heat management: Add transmission coolers for heavy-duty use

Suspension Modifications

• Progressive approach: Start with moderate lifts and build experience
• Component quality: Invest in quality shocks and steering components
• Alignment critical: Professional alignment essential after modifications
• Test incrementally: Verify handling characteristics before extreme off-road use

Safety Considerations

• Brake system scaling: Upgrade brakes proportionally with performance increases
• Rollover awareness: Higher center of gravity requires adjusted driving techniques
• Tire selection: Choose tires appropriate for intended use and vehicle capabilities

Troubleshooting

Engine Performance Issues

Symptom: Loss of power after carburetor upgrade
- Cause: Incorrect jetting or vacuum leak
- Solution: Verify all vacuum connections, adjust mixture screws, check float level

Symptom: Overheating with performance modifications
- Cause: Inadequate cooling system capacity
- Solution: Upgrade radiator, install electric fans, verify thermostat operation

Symptom: Rough idle with performance cam
- Cause: Increased overlap, vacuum leak, or timing issues
- Solution: Adjust idle mixture, verify timing, consider idle air control modifications

Drivetrain Problems

Symptom: Transfer case noise after NP205 swap
- Cause: Incorrect fluid level or type, misalignment
- Solution: Verify 80W-90 gear oil level, check case alignment with transmission

Symptom: Driveline vibration after lift
- Cause: Improper driveline angles
- Solution: Install transfer case drop kit or adjust pinion angle

Symptom: Front axle noise with larger tires
- Cause: Increased stress on CV joints or differential
- Solution: Upgrade to chromoly axle shafts, verify differential setup

Suspension Issues

Symptom: Harsh ride quality after lift installation
- Cause: Incorrect shock valving or spring rate
- Solution: Install shocks designed for lifted applications, verify spring specifications

Symptom: Steering wander or bump steer
- Cause: Altered steering geometry from lift
- Solution: Install dropped pitman arm, steering stabilizer, verify alignment

Symptom: Premature tire wear patterns
- Cause: Incorrect alignment, worn steering components
- Solution: Professional alignment, inspect and replace worn steering parts

Electrical System Problems

Symptom: Charging system issues after engine modifications
- Cause: Increased electrical load, alternator inadequacy
- Solution: Upgrade to higher output alternator, verify belt tension and wiring

Symptom: Fuel delivery problems with electric pump conversion
- Cause: Incorrect pump pressure or volume
- Solution: Verify pump specifications match engine requirements, check fuel pressure