How much lift can GT40 heads handle?
In general, GT40 heads can accommodate valve lifts in roughly the 0.500" to 0.600" range when paired with appropriate springs, retainers, rockers, and matching pistons. The exact maximum depends on the head variant (GT40, GT40P, aluminum versions), the valvetrain you choose, and how carefully you clear everything for high RPM operation.
That question touches on several interrelated factors. GT40 heads were designed to be high-flow performance pieces for Ford Windsor-based engines and have seen wide use from mild street cams to aggressive race setups. The practical lift limit is not one fixed value but a product of the head’s geometry, the springs and retainers installed, the rocker ratio, and the clearance between the valve, piston, and valve train components at full lift. Below is a breakdown to help you assess and plan accordingly.
GT40 heads at a glance
GT40 family heads came in several variants used on Windsor-based engines, including cast-iron GT40 and the later, lighter GT40P designs. Each variant has its own port sizes, valve sizes, and flow characteristics, which in turn influence how much lift you can safely run. The key takeaway is that while these heads are capable of supporting relatively aggressive profiles, the lift ceiling rises with improved supporting hardware and careful assembly.
Variants explained
Different GT40 derivatives have different valve seats, springs, and geometry. Matching the cam lift to the head’s flow path and the valvetrain’s capacities is essential for reliable operation at high RPM.
Before moving on to the specifics, it’s important to note that the numbers below describe common ranges observed in practice. Always verify with your head and spring manufacturers, as installed heights and seat pressures can shift these values.
What lifts are practical with GT40 heads?
To give you a practical sense of the available range, here are general guidelines across typical street to moderate race applications. These ranges assume a well-built valvetrain with appropriate springs and correct clearance checks.
- Street/torque-oriented builds: 0.500" to 0.525" valve lift
- Moderate performance builds: 0.525" to 0.550" valve lift
- Higher-rpm or more aggressive street/strip builds: 0.550" to 0.600" valve lift
- Extreme or specialty setups (requires upgraded springs and careful clearance work): above 0.600" valve lift
These figures assume properly matched intake valve springs, seat pressure, and sufficient piston-to-valve clearance, plus secure retainer/guide compatibility and a compatible rocker ratio. As lift increases, the risk of coil bind, retainer-to-guide interference, and piston contact grows if any one part is out of spec.
How to determine your maximum safe lift
To move from general ranges to a precise, safe limit for your engine, follow these steps. The goal is to ensure the valvetrain can reach the target lift without coil bind or interference and with adequate piston clearance at full lift.
- Identify your GT40 head variant and valve sizes for accurate geometry and port characteristics.
- Determine your chosen rocker ratio and cam lobe lift. Compute target valve lift as lift_at_cam = lobe_lift × rocker_ratio.
- Check installed spring height and seat pressure. Obtain max recommended lift data from the spring manufacturer for your installed height.
- Calculate piston-to-valve clearance at TDC with the intended lift. A conservative minimum clearance is typically around 0.080" or more depending on build specifics; consult your machinist for a precise target.
- Verify coil bind and retainer clearance by simulating the full lift with the chosen springs and retainer/guide combinations. Use valve spring testers and, if available, a camshaft with a similar lobe profile for testing.
- Assemble the valvetrain and physically check for any contact between pistons, valves, springs, and retainers at full lift, ideally with the engine at top-dead-center and the timing set for the intended ramp rate.
Following these steps helps ensure you don’t overdrive the GT40 heads and risk damage or unwanted wear. If in doubt, work with a qualified engine builder who has experience with GT40 variants and Windsor-based setups.
Summary
GT40 heads are capable of supporting a range of lift profiles, typically from about 0.500" up to 0.600" with proper hardware and careful clearance checks. The exact limit varies by head variant, spring choice, rocker configuration, and piston-to-valve clearance. Always verify with spring manufacturers and a knowledgeable machinist, and perform thorough physical and dimensional checks before committing to a high-l lift cam. With careful matching, GT40 heads can deliver strong performance across street and semi-race applications.
What is better, GT40 or GT40P heads?
GT40P heads are a budget-friendly alternative to GT40 heads, offering similar performance with a smaller combustion chamber and slightly smaller exhaust valve, but they have a different spark plug angle that can cause header clearance issues. GT40 heads have a larger combustion chamber and exhaust valve and are generally considered easier to fit with headers, making them the preferred choice for most builds despite their similar performance potential.
| Feature | GT40 Heads | GT40P Heads |
|---|---|---|
| Combustion Chamber | 63−6663 minus 6663−66 cc | 58−6158 minus 6158−61 cc |
| Exhaust Valve | 1.541.541.54 inches | 1.461.461.46 inches |
| Spark Plug Angle | Standard angle | Angled outward for better burn, can cause header clearance issues |
| Header Fitment | Easier, most headers fit without modification | Can require 90-degree spark plug boots or specific headers due to clearance issues |
| Thermactor/EGR | Has provision for thermactor/EGR tubes | Does not have thermactor/EGR provision, uses external connections |
| Performance | Similar power gains to GT40P heads, around 20−4020 minus 4020−40 horsepower | Similar power gains to GT40 heads, around 20−4020 minus 4020−40 horsepower |
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What is the max lift on GT40 head?
SPRING KIT, FORD GT40 / GT40P CYLINDER HEAD, . 570" MAX LIFT COMP Cams GT40 Valve Spring Kit is designed to maximize the potential of GT40 and GT40P Cylinder Heads when using a flat tappet camshaft. This kit allows up to . 570" lift and provides spring rates that can handle significantly more aggressive lobe profiles.
What is the max lift on TBI heads?
You will max them out with 480 lift. I would stay around 430 lift you'll benefit more from less lift and a lil more duration with stock tbi heads and intake. TBI heads have very restricted intake runners. Not meant for performance.
What are the benefits of GT40 heads?
These heads were designed for improved emissions and fuel efficiency, making them a more environmentally friendly option. While they also offer good performance gains, GT40P heads have smaller intake and exhaust ports compared to the original GT40 heads.
