Small Block V8 Performance Guide - Square Body Trucks

Overview

The small block Chevrolet V8 engine is the heart of most GM Square Body trucks from 1973-1987, offering tremendous potential for performance enhancement. Whether you're working with a 305, 350, or 400 cubic inch engine, these powerplants respond exceptionally well to modifications ranging from simple bolt-on upgrades to complete engine rebuilds.

This guide covers performance modifications for all small block V8 engines found in C10, C20, K10, K20, K5 Blazer, and Suburban models. The modular design of these engines allows for extensive customization while maintaining reliability and driveability suitable for both daily driving and weekend projects.

Performance modifications can transform your Square Body from a basic work truck into a capable street performer, improving acceleration, throttle response, and overall driving experience while maintaining the classic character that makes these trucks so desirable.

Common Issues

Pre-1980 Engines

• Smog equipment restrictions: Early emissions equipment significantly reduces performance
• Low compression ratios: 1975-1980 engines typically run 8.0:1 to 8.5:1 compression
• Restrictive exhaust manifolds: Cast iron manifolds create significant backpressure
• Inadequate fuel delivery: Stock mechanical fuel pumps often insufficient for performance modifications

1981-1987 Engines

• Computer-controlled carburetion: Early CCC systems limit modification potential
• Restrictive air cleaner assemblies: Large plastic housings reduce airflow
• Lean factory tuning: Emissions-focused tuning sacrifices performance
• Weak ignition timing: Conservative timing curves limit power output

Universal Issues

• Heat management: Square Body engine bays retain heat, affecting performance
• Transmission limitations: Stock TH350 and TH400 may require upgrades for increased power
• Rear axle capacity: 10-bolt rear ends have power limitations with significant modifications

Parts & Materials Needed

Basic Performance Package ($800-1,500)

• Intake manifold: Edelbrock Performer #2101 (non-EGR) or #2116 (EGR)
• Carburetor: Holley 600 CFM #0-1850S or Edelbrock 600 CFM #1405
• Headers: Hooker Super Comp #5204HKR (2WD) or #5208HKR (4WD)
• Exhaust system: 2.5" dual exhaust with H-pipe or X-pipe
• Air filter: K&N drop-in replacement or open element with heat shield
• Ignition: HEI distributor upgrade with performance module

Intermediate Build ($2,500-4,000)

• Camshaft: Comp Cams XE268H #CL12-224-4 or similar hydraulic flat tappet
• Valve springs: Comp Cams #26918-16 (check installed height)
• Rocker arms: Comp Cams Magnum 1.5:1 ratio #1418-16
• Timing chain: Cloyes True Roller #9-3100
• Oil pump: Melling high-volume #M155HV
• Gasket set: Fel-Pro performance gasket kit

Advanced Build ($5,000+)

• Rotating assembly: Forged pistons, I-beam rods, forged crankshaft
• Cylinder heads: Vortec heads #12558062 or aftermarket aluminum
• Roller camshaft: Comp Cams XR270R or similar
• Fuel system: Electric fuel pump, larger fuel lines
• Cooling system: Aluminum radiator, high-flow water pump

Essential Tools

• Torque wrench (10-150 ft-lbs)
• Engine hoist and stand
• Compression tester
• Timing light
• Vacuum gauge
• Basic hand tools and sockets

Step-by-Step Guide

Phase 1: Bolt-On Modifications

Intake and Carburetor Upgrade

1. Remove stock air cleaner and carburetor
   - Disconnect battery negative terminal
   - Remove air cleaner housing and element
   - Disconnect throttle linkage, fuel line, and vacuum lines
   - Label all connections for reassembly

2. Install performance intake manifold
   - Remove old intake manifold (torque sequence: outside bolts first)
   - Clean all gasket surfaces thoroughly
   - Apply thin coat of RTV to china walls
   - Install new intake with Fel-Pro gaskets #1205
   - Torque to 25-30 ft-lbs in proper sequence

3. Mount new carburetor
   - Install carburetor with new gasket
   - Connect throttle linkage (may require bracket modification)
   - Connect vacuum lines per diagram
   - Install fuel line with new filter

Header Installation

1. Prepare vehicle
   - Raise front of truck securely on jack stands
   - Disconnect battery and remove spark plug wires
   - Apply penetrating oil to exhaust manifold bolts

2. Remove stock manifolds
   - Remove heat shields and air injection components
   - Remove exhaust pipes at manifold connection
   - Remove manifold bolts (expect some breakage on older trucks)
   - Clean head surface thoroughly

3. Install headers
   - Test-fit headers before final installation
   - Use anti-seize on header bolts
   - Install with new gaskets (Remflex #3003 recommended)
   - Torque to 25 ft-lbs in sequence
   - Connect exhaust system with new hardware

Phase 2: Internal Engine Modifications

Camshaft Replacement

1. Engine preparation
   - Remove radiator, fan, and water pump pulley
   - Remove intake manifold and valve covers
   - Remove timing cover and chain
   - Remove rocker arms and pushrods (keep in order)

2. Camshaft removal
   - Remove lifters with magnetic tool
   - Remove camshaft retainer plate
   - Carefully slide camshaft out (protect bearings)

3. Installation
   - Apply cam assembly lube to all lobes and journals
   - Install new camshaft carefully
   - Install new timing chain and gears
   - Degree camshaft for optimal performance
   - Reassemble in reverse order

Cylinder Head Work

1. Head removal
   - Drain coolant completely
   - Remove exhaust manifolds/headers
   - Remove intake manifold
   - Remove rocker arms and pushrods
   - Remove head bolts in reverse torque sequence

2. Machine work (Professional shop recommended)
   - Pressure test for cracks
   - Mill heads for increased compression (0.030" maximum)
   - Three-angle valve job
   - Install hardened valve seats if needed

3. Reassembly
   - Install new head gaskets (Fel-Pro #1003 or similar)
   - Torque heads to specification in proper sequence
   - 1973-1980: 65 ft-lbs
   - 1981-1987: 70 ft-lbs

Year/Model Differences

1973-1974 Models

• High-compression engines available: Some 350s still had 9.0:1+ compression
• Minimal emissions equipment: Easier to modify
• Points ignition: HEI upgrade highly recommended
• Quadrajet carburetor: Good performance potential with proper tuning

1975-1980 Models

• Catalytic converter required: Headers must accommodate cats
• HEI ignition standard: Good performance potential
• Lower compression: 8.5:1 typical, head milling beneficial
• Extensive emissions equipment: Some can be removed legally depending on local laws

1981-1987 Models

• Computer Controlled Carburetion: CCC system limits modifications
• Feedback carburetor: Difficult to tune, aftermarket replacement recommended
• Knock sensor equipped: Allows more aggressive timing
• Improved cylinder heads: Better flow than earlier castings
• 4WD differences: K-series trucks require specific headers due to front axle clearance

Model-Specific Considerations

C10/C20 (2WD)

• Header clearance: More options available
• Weight distribution: Better for performance applications
• Transmission cooling: Less demanding than 4WD models

K10/K20 (4WD)

• Header fitment: Limited options due to front axle
• Transfer case load: Additional stress on drivetrain
• Ground clearance: Oil pan selection critical

K5 Blazer/Suburban

• Cooling requirements: Larger radiator recommended for performance builds
• Weight considerations: Heavier vehicles require more aggressive modifications
• Fuel economy: Balance performance with practicality for family use

Tips & Best Practices

Planning Your Build

• Set realistic goals: Match modifications to intended use
• Budget for supporting modifications: Don't forget drivetrain upgrades
• Plan in phases: Allows for testing and refinement between stages
• Document everything: Keep records of parts and settings

Performance Tuning

• Start conservative: Begin with mild modifications and increase gradually
• Monitor vital signs: Watch oil pressure, coolant temperature, and fuel pressure
• Tune for your fuel: Optimize timing and jetting for available gasoline
• Break-in properly: Follow cam manufacturer's break-in procedures exactly

Common Mistakes to Avoid

• Mismatched components: Ensure all parts work together as a system
• Ignoring fuel delivery: Upgrade fuel pump and lines for performance modifications
• Overlooking cooling: Performance engines generate more heat
• Cheap gaskets: Quality gaskets prevent comebacks and leaks
• Rushing assembly: Take time to do it right the first time

Money-Saving Tips

• Buy complete packages: Often cheaper than individual components
• Watch for sales: Stock up during vendor promotions
• Consider used parts: Inspect carefully but can save significantly
• Do your own work: Labor costs can exceed parts costs
• Plan ahead: Avoid rush shipping charges

Troubleshooting

Poor Performance After Modifications

Symptoms: Lacks power, poor throttle response

Possible Causes:
- Incorrect timing: Verify initial timing and advance curve
- Vacuum leaks: Check all intake connections
- Fuel delivery issues: Test fuel pressure and volume
- Carburetor tuning: May need different jets or metering rods

Solutions:
1. Set initial timing to 10-12° BTDC
2. Check vacuum at idle (should be 18-22" Hg)
3. Test fuel pressure (5-7 PSI for carbureted engines)
4. Perform carburetor tune-up or replacement

Symptoms: Engine knock or ping

Possible Causes:
- Too much timing advance: Reduce initial or total timing
- Carbon buildup: Perform decarbon service
- Low octane fuel: Switch to premium gasoline
- Lean fuel mixture: Richen carburetor settings

Overheating Issues

After Performance Modifications

Common Causes:
- Insufficient cooling capacity: Stock radiator inadequate
- Poor airflow: Check fan operation and shroud
- Timing too advanced: Retard timing slightly
- Lean fuel mixture: Causes excessive combustion temperatures

Solutions:
1. Upgrade to aluminum radiator with higher capacity
2. Install electric fan with proper CFM rating
3. Verify thermostat operation (160°F recommended for performance)
4. Check water pump flow and impeller condition

Carburetor Problems

Flooding or Rich Running

• Float level too high: Adjust per manufacturer specifications
• Power valve blown: Common with backfire through intake
• Jets too large: Step down primary jets
• Accelerator pump diaphragm: Replace if torn or hardened

Lean Conditions

• Vacuum leaks: Spray carb cleaner around gasket surfaces
• Clogged jets: Remove and clean all metering circuits
• Fuel pressure low: Test and replace fuel pump if needed
• Air leaks: Check carburetor base gasket and intake manifold

Ignition System Issues

Hard Starting or Rough Idle

• Worn distributor: Check shaft play and advance mechanisms
• Incorrect plug gap: Set to 0.045" for HEI systems
• Coil problems: Test primary and secondary resistance
• Module failure: Common HEI problem, carry spare

Testing Procedures:
1. Check spark at plugs with timing light
2. Test coil output with spark tester
3. Verify pickup coil resistance (500-1500 ohms)
4. Check module with known good unit

Mechanical Problems

Oil Pressure Issues

• High-volume pump cavitation: May need to restrict pickup
• Bearing clearances: Check with plastigauge
• Relief valve problems: Clean or replace spring and valve
• Oil viscosity: Use appropriate weight for clearances

Compression Loss

• Ring seal: Perform wet compression test
• Valve problems: Check valve adjustment and condition
• Head gasket: Look for coolant in oil or combustion gases in coolant
• Timing chain stretch: Check chain deflection and gear wear

By following this comprehensive guide, Square Body truck owners can successfully modify their small block V8 engines for improved performance while maintaining reliability and driveability. Remember that performance modifications should be approached systematically, with proper planning and attention to supporting systems to achieve the best results.