How much horsepower do GT40 heads add?
On a typical Ford Windsor small-block, upgrading to GT40 heads adds roughly 20–60 horsepower, with the exact number depending on accompanying components and tuning.
Context: What GT40 heads are and why they matter
GT40 heads are Ford's high-performance cylinder heads designed for the Windsor family of engines, most notably the 289/302. They are prized for their improved flow over stock heads, which can translate into meaningful power gains when paired with a balanced performance package. The actual horsepower boost depends on multiple factors, including the camshaft, compression ratio, intake, exhaust, and any porting or valve work performed alongside the head swap.
Key variables that influence the power gain
The numbers below reflect common dyno results reported by enthusiasts and engine builders for Windsor-based builds. Results will vary based on the condition of the bottom end, fuel system, ignition timing, and tuning.
Before diving into the figures, note that the following scenarios illustrate typical outcomes for a 302 (or similar displacement) using GT40 heads with varying levels of supporting upgrades.
Typical horsepower gains by upgrade path for Windsor small-blocks with GT40 heads
- Bolt-on upgrade (stock cam, stock intake/exhaust): roughly 20–35 horsepower gain.
- With a cam upgrade and headers plus a better intake: roughly 40–70 horsepower gain.
- With higher compression (around 9.5–10.5:1) and minor port work plus tuned EFI or improved induction: roughly 60–90 horsepower gain in ideal conditions.
Concluding paragraph: In practice, GT40 heads unlock more airflow and mid- to high-rpm performance, but the actual horsepower bump hinges on the rest of the build and tuning. The most dramatic gains come when the heads are part of a coordinated package including cam timing, intake design, exhaust flow, and appropriate compression.
Takeaways and practical guidance
If you’re planning a GT40 head swap, approach it as a top-end upgrade that rewards complementary improvements. You’ll typically see a tangible power increase, but to maximize the benefit, pair the heads with a suitable cam, a matching intake, quality headers, and careful tuning. Have a reputable engine builder map the tune to your specific goals and exhaust setup.
Summary
GT40 heads are a proven upgrade for Ford Windsor engines, delivering measurable horsepower gains when used as part of a balanced performance package. Expect about 20–60 horsepower on typical builds, with larger gains possible when combined with cam, compression, and induction upgrades. Real-world results depend on the entire system and precise tuning, so plan accordingly and consult a qualified engine builder to tailor the setup to your goals.
