For decades, the Ford 302 engine has been the heartbeat of countless hot rods, Mustangs, and custom builds. Among the many components that enthusiasts hunt for to unlock hidden horsepower, Gt40 heads stand out as a legendary upgrade. Originally sourced from high-performance Ford production vehicles like the Mustang Cobra and the Ford Explorer, these cylinder heads became the gold standard for budget-minded builders looking to bridge the gap between restrictive factory heads and expensive, aftermarket aluminum units.
Understanding the Legacy of Gt40 Heads
The Gt40 heads are cast-iron cylinder heads that significantly improved airflow compared to the standard “E7” heads found on factory 5.0L engines. While modern CNC-machined aluminum heads offer higher peak numbers, the Gt40 series provides a robust, cost-effective, and period-correct solution for street-driven small-block Fords. There are two primary versions that enthusiasts frequently discuss: the original Gt40 and the later Gt40P heads.
The distinction is vital for any builder. The original Gt40 heads feature a more traditional spark plug angle, making them compatible with almost all standard headers. Conversely, the Gt40P heads are highly sought after for their improved combustion chamber design, which promotes better swirl and flame propagation, though they require specific headers due to the unique angle of the spark plugs.
Performance Gains and Flow Characteristics
When you swap to Gt40 heads, you are essentially increasing the volumetric efficiency of your engine. The intake ports are designed with better geometry, allowing the engine to breathe deeper at higher RPMs. If you are currently running stock heads, you can expect noticeable improvements in torque and power throughout the mid-range.
Here is a comparison of typical flow characteristics for the most common 302-based heads:
| Head Type | Intake Flow (CFM @ .500 Lift) | Exhaust Flow (CFM @ .500 Lift) |
|---|---|---|
| Standard E7TE (Stock) | ~165 CFM | ~115 CFM |
| Gt40 (Iron) | ~205 CFM | ~160 CFM |
| Gt40P (Iron) | ~210 CFM | ~170 CFM |
Key Considerations for Installation
Installing these heads is a straightforward process for anyone familiar with a torque wrench, but there are specific technical hurdles to navigate. Because these heads were produced over several years, ensure you are starting with a clean set that has been pressure-tested for cracks, as cast-iron heads can fatigue over time.
- Check Valve Springs: Factory springs are often weak. If you are running a performance camshaft with higher lift, upgrade the springs to prevent valve float.
- Bolt Compatibility: Ensure your head bolts match the year of your block. Some 302 blocks use 7/16" bolts, while others use 1/2". You may need bushing kits if there is a discrepancy.
- Header Fitment: As mentioned, Gt40P heads have a different spark plug angle. Attempting to use standard headers will often result in burnt plug wires or physical interference.
- Pushrod Length: Always check your pushrod length with an adjustable checker whenever you change heads or cams to ensure the rocker arm geometry is centered on the valve stem tip.
💡 Note: Always use a high-quality head gasket set when performing this swap. Because the Gt40 heads have different coolant passage configurations than earlier small-block heads, ensure you are using the correct gasket designed specifically for the Gt40 casting to avoid overheating issues.
Optimizing the Rest of the Induction System
Putting Gt40 heads on an engine with a restrictive intake manifold or a tiny throttle body is akin to running a marathon while breathing through a straw. To fully capitalize on the increased airflow provided by these heads, you should pair them with an appropriate intake manifold.
The Gt40 tubular intake is the classic companion for these heads. Its long runners are designed to maximize low-to-mid-range torque, which is perfect for street performance. Alternatively, many enthusiasts opt for aftermarket intakes that allow the engine to pull harder in the upper RPM bands. Pairing the heads with a slightly more aggressive camshaft—usually in the .480" to .520" lift range—will complete the "top-end" package and turn a mundane stock 5.0L into a true performer.
Common Challenges and Solutions
One of the most frequent frustrations when working with Gt40 heads is the difficulty of changing spark plugs on the “P” version. Because the plug wire boots sit very close to the header tubes, they are prone to heat soak. Use high-quality, heat-shielded spark plug wires to prevent misfires and ensure that you have the correct “P” specific spark plugs, which have a different reach and seat profile compared to standard plugs.
Additionally, pay attention to the EGR and thermactor passages. Some Gt40 heads have air injection holes that need to be plugged if you are running a simplified vacuum and emissions system. Utilizing hardened steel plugs or threaded fittings ensures a leak-free setup.
Final Thoughts on the Upgrade
Investing in Gt40 heads remains one of the most reliable ways to increase the performance of a classic Ford small block without breaking the bank. By choosing the right version, verifying your supporting components like pushrods and headers, and ensuring the valve train is properly set up, you can unlock a level of power that transforms the driving dynamics of your vehicle. While newer technology exists, the proven track record of these heads ensures they will continue to be a staple in the performance world for many years to come. Whether you are aiming for a period-correct build or simply need a reliable power bump, these iron powerhouses offer an excellent balance of cost, durability, and performance that is hard to ignore.
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