What is the volume of a GT40P cylinder head?
The combustion chamber volume of a GT40P cylinder head is about 60 cubic centimeters (cc) per chamber, totaling roughly 120 cc for both chambers combined. Exact figures vary by casting and how the head has been machined or decked.
What GT40P heads are and why their volume matters
GT40P is Ford’s designation for a family of aluminum cylinder heads used on certain 4.6-liter modular V8 engines. The “P” variant refers to a specific casting within that family. The combustion chamber size directly affects compression ratio and how the engine responds to changes in gasket thickness, deck height, and cam choice. Builders rely on precise chamber measurements to predict performance and fuel requirements.
Typical chamber size for GT40P heads
Before listing the commonly cited figures, note that stock measurements can vary by casting batch and subsequent machining. The figures below reflect typical, widely cited ranges for this head family.
- Chamber volume per cylinder (per chamber): approximately 60 cc, with a common range of about 58–62 cc.
- Total chamber volume for a complete pair: about 120 cc (roughly 7.3 cubic inches) for both cylinders combined.
- Variation factors: deck milling, gasket thickness, and light machining changes can alter each chamber by a few cc, which in turn affects compression ratio and combustion characteristics.
In practice, if you’re tuning a engine that uses GT40P heads, you should measure the actual chamber volume after any decking or gasket changes to ensure your compression targets are accurate.
How to verify the exact volume on your GT40P heads
Before diving into measurements, understand that precise results require careful procedure and consistent conditions. The following outline describes a common approach used by machinists when quantifying combustion chamber volume.
- Prepare the head: remove valves and seats to expose the true chamber volume.
- Seal the chamber: gasket surfaces must be clean and free of leaks; seal any ports as needed to prevent water from escaping during filling.
- Level the head: place the head so the combustion chamber sits flat against a stable surface to avoid air pockets.
- Fill with water: using a graduated cylinder or burette, fill the chamber with distilled water until the deck surface (gasket line) is reached.
- Record volume: note the exact volume of water needed to fill the chamber to the deck line; repeat for each chamber for accuracy.
- Account for gasket and deck: if you’re calculating compression ratio for a complete assembly, include the gasket thickness and any deck milling in your final volume calculation.
Concluding: measuring each chamber directly gives you the most reliable data for compression predictions and for comparing different builds or decking options.
Practical implications for engine builders
Understanding the chamber volume helps in planning compression ratio, piston selection, and gasket choices. Small changes in chamber size can shift compression enough to influence knock resistance, fuel requirements, and overall performance. When selecting heads, gaskets, and any milling, it’s wise to re-calculate compression and re-verify chamber volumes to avoid unintended results.
Summary
For GT40P cylinder heads, expect about 60 cc per combustion chamber, roughly 120 cc for the pair, with real-world figures varying by casting and machining. Accurate measurements should be taken whenever you deck or gasket a head, and those numbers should feed into your compression ratio and tuning calculations to ensure the engine performs as intended.
What is the difference between GT40 and GT 40p heads?
The GT40P cylinder heads had a small combustion chamber ranging from 58-61cc while the GT40 heads had a 63-66cc combustion chamber. The GT40 heads have thermactor holes built in to connect to the EGR/smog equipment whereas the GT40P heads use an external EGR connective system.
How to tell GT40P head?
You can identify GT40P heads by looking for four vertical casting marks on the front of the head, a "GT40P" casting stamp (often on the intake side below the valve cover), and a spark plug angle that protrudes at nearly an 80-degree angle. The presence of a four-bar mark on the head's end differentiates them from standard GT40 heads, which have three bars.
This video shows the location of the four bars that identify GT40P heads: 57sFlorida Cracker KustomsYouTube · Mar 26, 2024
Key identifiers
- Casting marks: Look for four vertical hash marks or bars on the front end of the cylinder head. Standard GT40 heads have three bars.
- Stamping: Check for "GT40P" cast into the head, most often found on the intake side below the valve cover. Other identifying stamps like "P" or "GTP" may also be present.
- Spark plug angle: The spark plugs on GT40P heads are set at a much steeper angle, protruding almost straight out from the side of the head. This is a common source of header fitment issues, requiring specific headers designed for this angle.
- Intake/Exhaust ports: GT40P heads typically have a 1.84-inch intake valve and a smaller 1.46-inch exhaust valve, with a smaller combustion chamber (58−6158 minus 6158−61 cc) compared to standard GT40 heads (63−6663 minus 6663−66 cc).
- Thermactor tube provision: Unlike standard GT40 heads, the GT40P heads from an Explorer do not have a hole for the thermactor (air injection) crossover tube.
You can watch this video to learn about the differences in spark plug angle between GT40 and GT40P heads: 50sKASL CustomsYouTube · Mar 12, 2023
What is the volume of the GT40P head?
GT40 combustion chambers are slightly larger, with volumes ranging from 63-66 cc. The Explorer/Mountaineer heads (GT40P) are somewhat smaller, measured between 58-61cc chamber volume. Thus, GT40P cylinder heads may give a small bump in compression.
What is the volume of the combustion chamber on a GT40?
The P heads also come with 59cc combustion chambers, which will increase your stock 5.0HO engine compression to about 9.5:1 while the GT40's 65.5cc chamber hurts your compression dropping it to about 8.8:1.
