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.
Among the factory heads, the "049" casting shown here are among the best for all-around re
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.