Best description froA2Speed web site

What is a “HEMI Big Gas Head”?
Think of it as a Hellcat head with a 6.4 intake manifold bolt pattern.

This is the head used in the RAM Truck equipped with the 6.4L HEMI, first made available in 2014. The HEMI Big Gas head was extensively redesigned by Dodge SRT engineers with keen focus on durability in a heavy duty environment.

Some key features:
Castings and Material: The casting is derived from the SRT8 Hellcat head, giving the Big Gas Heads substantially improved internal reinforcement and improved water jackets. They are also made of the same 356 T-6 aluminum alloy found in the Hellcat Head. (vs. the T319 modified alloy used in the Apache 6.4L head).

Valves: By our measurements the Big Gas intake valves are 54.3 mm (2.138”); exhaust valves are 42 mm (1.654”); the same size as the Apache’s. The Big Gas valve guides are stainless steel and seats are a modified steel alloy. Both the Big Gas and Apache valves have hollow stems to reduce inertial mass allowing for higher RPMs. Exhaust valves are sodium filled; when the exhaust valve gets hot, the sodium liquefies which in turn transfers heat from the valve head to the valve guides via a “cocktail shaker” effect induced by valve’s motion. This style of exhaust valve helps prevent hot spots in the combustion chamber & valve head which reduces the possibility of pre-ignition. Visually it’s apparent that the Big Gas valves are a different material or have a different coating then the Apache’s; but as of this writing I haven’t been able to get a spec for this.

Intake Ports: On both the 6.4L Apache and Big Gas HEMI heads no clean-up machining is necessary on the intake ports. When the heads are cast, a fine sand (AFS 90) is used around the intake port cores. This provides the intake ports with a smooth surface finish and exceptional flow characteristics. Intake valve angle on both heads is 18°.

Exhaust Ports: Experience has shown that the greatest gains in Gen III HEMI heads are found in the exhaust ports. The Big Gas exhaust ports have improved long turns, guide support and ceiling. Un-like the Apache, the Big Gas HEMI head has fine sand cast exhaust ports, the resulting smooth surface finish improves flow characteristics dramatically. Exhaust valve angle on both heads is 16.5°.

Combustion Chamber: The Apache and Big Gas heads have similar quench-free combustion chambers. The Apache chamber is 73.4 cc’s, we will be pouring both chambers next week, will report results in a new thread.

Valve Springs: The Big Gas and Apache heads come with the same valve springs; meaning that replacement is required for aftermarket cams.

In summary: In summary, the Big Gas head is an outstanding yet affordable option for boosted applications where cylinder pressure and thermal stresses are anticipated. It’s a great option for a naturally aspirated motor as well, particularly if it's bottle-fed.