The big-block Chevy first saw the light of day back in the '60s when it first appeared as a NASCAR powerplant known as the "Mystery Engine." Here we are some 45 years down the road, and getting real horsepower from this engine is still somewhat of a mystery to many, so here's the fix: we're going to ask--and answer--the 10 most important questions you should be considering when buying cylinder heads for your big-block. But first some background.

When Chevrolet introduced the mystery engine, it looked (with its then less-than-orthodox canted-valve heads) like a pretty exciting deal. Shortly after its intro, it became known as Chevrolet's "porcupine-headed" engine, and subsequently, because a porcupine is sort of a member of the rat family, it was known as the Rat Motor. With its canted valves, Chevrolet had obviously made an effort to improve the breathing capability of its then-new offering, and that brings us squarely to the first commonly asked question for new-millennia cylinder heads in an age of new-millennia power:

1. Why can't I use my stock heads as they are?
The whole issue with the canted-valve heads was to produce an engine that could take on Chrysler's deep-breathing Hemi. While Chevy's big-block was a step forward for the day, those seemingly high-tech canted-valve heads did not in fact, breathe quite as well as it appeared they might.

If we fast forward to 2009, even the best of the production factory heads are way short of satisfactory in a world of conventional, but highly refined designs. To make the point, let's look at a comparison of the valve size per cube of a typical performance small-block 350 versus a 454 big-block with stock valves. The most important factor toward making horsepower is the circumference of the intake valve in relation to the cylinder's cubic inches. With a typical 350 small-block, each inch of intake valve circumference has to feed 6.89ci of displacement. Even without the usual 0.060-plus overbore or even a stroker crank, a lowly 454 with the stock 2.19-inch intake valve has 8.25 cubes to feed for every inch of valve circumference. This means that even a 454 is potentially air starved, and your common quarter-inch stroker with a 0.060-inch over bore is, at 496 inches, really air starved.

2. Can't I just port the stock heads?
Any one of the stock big-block heads can be ported to show big gains in airflow. That said, most of the so-called high-performance castings (and especially the big rectangular port castings) were produced at a time when casting techniques were far less capable of producing a clean and precise port shape that would be conducive to airflow. Sure, bigger valves could be installed, but at the end of the day, porting a set of factory castings involves removing so much metal it becomes a career instead of a weekend hobby. But the time to port is far from the only issue. To get these stock heads (shown) in shape for a dyno test, I had to install new guides and valves, as well as cut a high-performance valve seat job. By the time the cost of springs, retainers, and more, was factored in, the bill was about $700, and I still only had a set of 30-year-old heads that were still in serious need of 50 hours of port work. That money could have been put toward new aftermarket high-flow castings, which would've gotten the job done right out of the box.