Big-Block Chevrolet Heads - CHP How It Works

Conventional 24-Degree Big-Block Chevy Heads are Giving Spread-Port Castings a Run for their Money

Stephen Kim Apr 1, 2011 0 Comment(s)
View Full Gallery

Tony Mamo: Obviously, with the same fortitude and dedication to product design as with a high-end 24-degree head, a spread-port head’s flatter valve angle and raised-runner configuration blows it away on paper and when it comes to achieving large airflow figures. It’s all relative, though, and a lot of that potential will depend on the finished volume and cross-sectional area of the head in question as well as the valve job angle. These factors play a very key role in the higher-flowing heads, both spread-port and conventional. Steeper valve angles will always flow more peak airflow if they are designed properly, but there will always be a trade off in the lower and middle lift portion of the flow curve, so beware that steep valve angles are very camshaft and application sensitive. Also, keep in mind that steeper valve angles affect reliability and tend to need a more frequent freshening up of the seat and valve faces because they wedge more under operating conditions. One of the greatest strengths of our 385cc castings is the fact we are getting big peak numbers of 465 cfm with exceptional low- and mid-lift flow as well. The no-compromise, have-your-cake-and-eat-it-too scenario made the execution of that head even more challenging. What makes these heads even more appealing is that they’re still truly a bolt-on piece, no different than installing a much smaller 315 or 335cc head. The only difference with the 377/385 series is the intake valve is about 0.040-inch longer to facilitate the use of an optional triple spring we can supply that’s meant to install at 2.100 (the height of the spring at rest) if so it can’t handle 0.900 lift. Both styles of heads accept stud-mount conventional rocker arms, and the exhaust ports are in the exact same location of the rest of our lineup.

Tony McAfee: Many design factors distinguish a Big Chief cylinder head from a conventional 24-degree big-block Chevy head. To improve upon a conventional head’s limitations, a Big Chief’s casting is much larger physically. This frees up space to move the valveguides substantially closer toward the center of the cylinder bores. Furthermore, Big Chiefs have flatter 18-degree valve angles with different cant angles as well. The intake ports are raised an inch over stock and spread 0.400-inch away from each other. This helps equalize port volume and airflow from one port to the next. When Big Chiefs were first used in Pro Stock, the problem was trying to stabilize the valvetrain with such radically repositioned valves. The solution was Z-rocker arms, which had a 0.900-inch offset on intake and a 0.600-inch offset on the exhaust. If not for Z-rockers, the Big Chiefs wouldn’t have come into existence. In today’s world, however, stability is no longer an issue. Now we use multi-angle rockers with thrust bearings along with offset lifters, so offset rockers are no longer required.

Short-Turn Negotiation

Rick Roberts: One of the biggest downsides of a conventional 24-degree head is trying to work around the stock intake port location. This results in a very sharp turn at the shortside radius. The short-turn radius is a very finicky part of the port, and it requires a delicate touch, especially in applications where the ports can flow up to 500 cfm. The idea is to make the short-turn broad at its highest point, then widening it out and laying it down before making a quick transition into a circular seat. If you saw the short-turn radius on some of the heads these high-end builders putting together, it would scare you. It looks so radical, but it works. It’s possible to hit water in the short-turn area, so we’ve revised the water jacket location on our Victor heads to prevent this. That said, in order to get 500 cfm out of a conventional head, you’ll give something up at 0.300-inch lift, but if you need a head that flows to 0.850-inch lift, you probably don’t care too much about low-lift flow.


Connect With Us

Get Latest News and Articles. Newsletter Sign Up

subscribe to the magazine

get digital get print