Reiterating the engine as an air pump analogy, the cylinder head ports represent the most significant restriction in that pump. Porting and polishing cylinder heads is one of the most effective ways to increase naturally aspirated engine power by improving airflow into and out of the combustion chamber. In this article, we’ll explore the science behind this critical performance modification.
Goals of Porting
Porting aims to optimize the airflow characteristics of the cylinder head through several approaches:
Increase Flow Volume
The primary goal is to increase the total Cubic Feet per Minute (CFM) the head can flow, measured on a flow bench. This is accomplished by:
- Removing material to increase cross-sectional area
- Smoothing rough surfaces that create turbulence
- Optimizing the port shape for better flow characteristics
Flow bench testing provides objective data on improvements, showing CFM gains at various pressure differentials (typically 10″, 15″, 20″, and 25″ of water).
Increase Flow Velocity
In some cases, reducing port volume can actually increase power by increasing air speed:
- Higher velocity improves cylinder filling at lower RPMs
- Increased velocity helps maintain momentum through port turns
- Velocity improvements can enhance low-end torque
This approach requires careful balance – too much velocity restriction can limit peak power.
Remove Obstructions
Factory heads often contain features that restrict flow:
- Short Side Radius: The tight radius where the port turns into the valve pocket is often a major restriction point that benefits from smoothing
- Bowl Blending: The transition between the port and valve seat area often has sharp edges that create turbulence
- Gasket Matching: Matching the port shape to the intake/exhaust manifold gasket opening eliminates steps and restrictions
The Process
Professional porting is a precise process requiring specialized tools and experience:
Tools
Different stages of porting require specific tools:
- Carbide Burrs: For rough shaping and removing large amounts of material
- Die Grinder Cutters: For medium material removal and shaping
- Cartridge Rolls: For fine smoothing and finishing
- Flow Bench: For measuring and validating improvements
Intake vs. Exhaust
The approach differs between intake and exhaust ports:
- Intake Ports: Often left with a slightly rough texture (around 60-80 grit) to promote fuel atomization in carbureted or port-injected applications
- Exhaust Ports: Polished to a mirror finish (400+ grit) to help prevent carbon buildup and reduce thermal stress
In direct-injection applications, intake port finish is less critical for fuel atomization.
Beyond Porting
Porting is most effective when combined with complementary modifications:
Valve Job
A multi-angle valve job complements porting by improving flow at low valve lifts:
- Standard valve jobs use a single angle (typically 45°)
- Multi-angle jobs (3 or 5 angles) create a smoother transition for airflow
- Improvements are most noticeable at low lift when the valve is only slightly open
Oversized Valves
Installing larger valves can further improve flow:
- Increases the flow potential at high lift
- Requires sufficient space in the combustion chamber
- May require custom pistons to maintain proper compression
Trade-offs include increased valve train stress and potential clearance issues.
Conclusion
Porting and polishing is a true art form that requires both technical knowledge and experience. A well-ported head can provide the single most effective power modification for a naturally aspirated engine, often yielding 15-30% power increases. Unlike many modifications that focus on one aspect of engine performance, porting improves the fundamental breathing capability of the engine, benefiting the entire RPM range.
For serious performance enthusiasts, investing in quality head porting pays dividends in both power and efficiency. Whether you’re building a street cruiser or a race engine, optimizing airflow through the cylinder heads should be at the top of your modification list.