Chevy Big Block Cylinder Heads PCV valves - Proper Breathing Techniques

The Good, The Bad, And The Ugly Of Big-Block Chevy Heads

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Identifying marks cast into original cast iron heads tell the real story. "PASS" (shown here) identifies this casting as one having oval intake ports.

A third type of big-block head is the one used on truck engines. These heads have almost round intake ports and very small valves. They're about as applicable as a one-barrel carburetor for a Corvette engine, but some people mistake them for passenger car oval port castings.

In addition to open and closed chambers with either rectangular or oval intake ports, GM big-block Chevy heads are also available in either cast iron or aluminum. As you might expect, all factory-installed aluminum heads incorporated rectangular ports. It wasn't until the Performance Parts division made a serious entry into the aftermarket cylinder head business that oval port "GM" aluminum heads became available.

In 1992, the world of big-block Chevy heads got even more diverse with the introduction of the Generation V engine. Although this was long after the last big-block Corvette was produced, Gen V heads are similar enough to original-style big-block heads that it isn't all that unusual for someone to attempt to install them on a Mark IV engine-something that can't be done without a few modifications.

The term "open chamber" refers to heads with combustion chambers having a nominal volume of 118 cc, while "closed chambers" are nominally rated at 108 cc. In spite of these "official" specifications, real world combustion-chamber volumes vary somewhat according to casting.

Regardless of intake port configuration, all big-block heads produced from 1965 to 1969 had closed chambers. The first open chamber appeared in '69 ZL-1 aluminum heads. Cast iron heads with open chambers first appeared in late 1970 for the '71 model year.

Without question, a big-block will produce more power with open-chamber cylinder heads. In fact, the improved air flow and combustion efficiency offered by the open-chamber design is frequently sufficient to more than offset the loss in compression ratio (which can be over a full point) produced by the increased combustion-chamber volume.