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.

>>> Big-Block Chevrolet Production Cylinder Heads
Casting Part No. Application Chamber Int Exh
336781 336780 74 454/270 113 2.06 1.72
353049 353047 73 454/275 113 2.06 1.72
3999241 3999240 72 454/270 105 2.06 1.72
3993820   71 454/365 113 2.06 1.72
3964291 3964287 69 427/435 118 2.19 1.88
3964290   69 427/390 & 400 & 70 454/390 106 2.06 1.72
3946074   69 427/430 L88 & ZL1 & 71 454/LS6 118 2.19 1.88
3931063   69 427/390 & 400 100 2.06 1.72
3919842   68 427/430 & 435 L88-L89 & 69 L89 106 2.19 1.72
3919840   67-69 427/435 L71 (very late 1967) 106 2.19 1.72
3917215   68 427/390 & 400 100 2.06 1.72
3909802   67 427/390 & 400 (late) 100 2.06 1.72
3904392   67 427/430 L88 & L89 106 2.19 1.72
3904391   67 427 435 L71 106 2.19 1.72
3094390   67 427/390 & 400 (early) 98 2.06 1.72
3873858   66 427/425 108 2.19 1.72
3872702   66 427/390 98 2.06 1.72
3856208   65 396/425 108 2.19 1.72

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.