Tony McAfee: What’s really pushed the development of high-end 24-degree big-block Chevy heads over the last few years are several amateur racing classes to prohibit the use of spread-port heads and limit displacement to 565 ci. Even so, these motors are producing more than 1,100 hp. Ironically, the idea behind banning spread-port heads was to keep costs down, but as usual, racers with the deepest pockets are winning once again because they’re putting so much money into the heads. Consequently, the cost of building a max-effort motor with conventional heads versus building an engine with Big Chiefs is very close. With conventional heads on a typical 565ci big-block that use standard rockers, stud girdles, and standard lifter locations and pistons, making 1,050 hp isn’t that uncommon using out-of-the-box parts. To get into the 1,100-1,200hp range with conventional heads, however, you usually have to move the valveguides. This can take hundreds of hours of work, and at the end of the day you still can’t match a Big Chief head airflow-wise because you can’t raise the intake ports. All this head work adds up to lots of labor, which drives up costs. These days, it’s just as easy to build an engine with Big Chief heads as it is to build an engine with conventional heads for not that much more money.
Rick Roberts: The reason why conventional 24-degree cylinder head technology has advanced so much in recent years is because spread-port heads are prohibited in many racing classes. One class that stands out in particular is Texas Pro Stock, which caps displacement at 565 ci, and requires conventional heads and a single Dominator carb. These rules were intended to reduce costs, but the exact opposite has happened. While spread-port heads are a little more expensive, by the time you factor in the expense of shaft-mount rockers and titanium valves, the top end package costs $6,000-$8,000 for a conventional-headed engine combo, and roughly $1,000 more for a spread-port top end. You might be able to get away with stainless valves to further cut down on costs with conventional heads, but that’s not an option with spread-port heads because their valves are so big and heavy. Ultimately, you’re not really saving that much money with a set of conventional heads. Using conventional heads to beat someone else who has a big, bad spread-port motor, which should be more capable, is part of the appeal.
Which Makes More Power?
Jason Neugent: Although a high-end 24-degree big-block Chevy head can come close to a spread-port head airflow-wise, spread-port heads still have more potential and will make more power. From top to bottom, a spread-port head is just a better design. They have shallower combustion chambers, better ports, and a flatter valve angle, all of which are crucial in making horsepower. With valve angles ranging from 12-18 degrees, air enters the intake ports at a straighter line-of-sight trajectory than with a 24-degree head. Consequently, spread-port heads will make more horsepower in the long run.
Tony McAfee: Even though a high-end, 24-degree head can come close to a Big Chief head airflow-wise, the Big Chiefs will still make more power. That’s because what a cylinder head flows on a flow bench and what it flows once fuel is added into the mix are completely different. With a conventional 24-degree head, air and fuel enter the port below the short-turn radius, are raised up, then turned back down into the bore. With this type of flow path, the fuel tends to separate from the air, puddles down into the motor, and goes back out the exhaust in unburned form. Thanks to its raised ports, a Big Chief head keeps the fuel in suspension much better. So if you have a conventional 24-degree head and a Big Chief head that both flow 500 cfm, the Big Chief head will still make more power.