Big-Block Chevrolet Heads - CHP How It Works

Conventional 24-Degree Big-Block Chevy Heads are Giving Spread-Port Castings a Run for their Money

Stephen Kim Apr 1, 2011 0 Comment(s)
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Now this is our kind of war. Over the last decade, hot rodders have watched with glee as the war between cubic inches and cylinder heads rages on. Just as the enormous displacement made possible by today’s stroker cranks and aftermarket blocks push the limits of airflow, cylinder head manufacturers have come roaring back with so much cubic feet per minute that now any short-block seems too small.

1105chp 01  Big Block Chevy Heads Port 2/2

The new crop of big-block Chevy cylinder heads can top 500 cfm, more than enough to feed the appetite of a 632ci short-block turning 8,000-plus rpm. Making things even more interesting is that there’s a mutiny brewing within the walk of Rat motor heads. In the not-so-distant past, such astonishing airflow was the exclusive territory of spread-port, Pro Stockstyle cylinder heads. These days, however, big-blocks routinely produce 1,000-plus horsepower with conventional 24-degree castings.

Curious as to how this is even possible, we contacted Jason Neugent of Brodix, David Canfield of Trick Flow, Tony McAfee of Dart, Rick Roberts of Edelbrock, and Tony Mamo of Air Flow Research. In addition to dissecting the state of modern big-block cylinder head technology, we wanted to know the pros and cons of conventional 24-degree heads versus spread-port heads, price differences between the two, the types of specialized components that each require, and which style of heads reign supreme in the horsepower department.

Before getting to the good stuff, we’re obligated to clarify that Big Chief is a term that’s often used generically to describe spread-port heads. In truth, Big Chief is Dart’s brand name for its spread-port heads. Likewise, Brodix calls its spread-port heads Big Dukes. With that factoid out of the way, let’s get to the good stuff.

Conventional Evolution

Jason Neugent: From the factory, stock 26-degree big-block Chevy heads don’t offer the best architecture from an airflow standpoint, but the design has been highly refined over the years. Their strength is in their popularity, and therefore all the off-the-shelf parts that are built around it. On the other head, a 26-degree valve angle isn’t conducive to airflow and not much can be done about it. A common practice is flattening the valve angle to 24 degrees, which improves airflow, torque, and horsepower gains. The combination of refining port shape, moving valve centerlines, testing out different valve jobs, improving combustion chamber efficiency, and utilizing wet flow bench technology has allowed us to create a great cylinder head. The oval ports used on some of our high-end conventional heads have deeper bowls, a taller shortside radius, and better transitions in the corners. Also, bigger valves and flatter valve angles move more air at a faster pace. These refinements, along with advances in CNC-porting, have resulted in conventional-style heads with flow numbers that are comparable to spread-port, Pro Stockstyle heads. Some of the specialized parts needed to run a high-end 24-degree cylinder head are a shaft-mount rocker system, special pistons, and an oval port intake manifold. Fortunately, all these parts area available off the shelf. Shop wisely, and you could save approximately 20-40 percent on the heads and valvetrain by going with a high-end 24-degree versus a Big Duke cylinder head.

David Canfield: The factory 26-degree big-block Chevy head’s biggest flaw is its large combustion chambers. The factory chamber is comparatively large and creates several issuesdifficulty in raising compression being one of the primary drawbacks. Also, the fact that the ports are not mirrored creates unique issues. By changing the valve angle from 26 to 24 degrees, along with increasing the side cant of the valves helps unshroud the valves and increase airflow. Advances in valvetrain technology have proved instrumental, as well as computer modeling technology that has contributed to port design and research. Most castings are designed with extra material to allow for porting techniques that were impossible just 10-15 years ago. Unlike spread-port heads that require an intake manifold, valvetrain, and pistons that are specific to that head design, conventional 24-degree heads maintain a stock-type setup. While this may not offer the same horsepower potential of a spread-port casting, there are serious financial advantages to using a conventional 24-degree head.

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