Don't be fooled into believing...
Don't be fooled into believing that the "double hump" stampings on the ends of these two heads make them alike. There actually are minor differences that can influence how much power an engine equipped with these heads will make. Jeff Huneycutt
When it comes to old-school stock car racers and stock cylinder heads, Chevrolet's long-running "double hump" cylinder heads have always been the gold standard. They are among the best stock heads ever allowed in racing, they are still relatively plentiful in junkyards and machine shops and they don't require a lot of machine work to make ready for racing service.
These heads got their nickname because of the raised identification shape on the each end. It's two half-ovals connected by a straight bar-most easily described as "two humps." Some of these are also nicknamed "Fuelie" heads because a version appeared on fuel-injected 327-powered Chevy Corvettes. These heads appeared on 327 and 350 cubic inch engines across most of the 1960s (beginning in 1962). Although there are some variations, they generally have small chambers for the high-octane unleaded fuel readily available at the time, smaller intake ports and valves that measured either 1.94 for the intake and 1.50 for the exhaust or 2.02 and 1.60 respectively.
UNCC engineering student Dallas...
UNCC engineering student Dallas Dallman sets up the flow bench for extensive testing on the two double-hump cylinder heads. Jeff Huneycutt
Because of these traits, these heads are very popular in Street Stock and Pure Stock classes that require many stock components-including cylinder heads-with minimal modifications. The ports may be a bit small for big street motors, but they are well suited for a class that usually requires a 500-cfm two-barrel. Plus, the combustion chambers are a closed-chamber design and already optimized for a smaller chamber size. You can cut any head down to the minimum 62cc size that most rulebooks require, but an open-chamber design just isn't going to respond as well. Finally, even if you have the heads with the smaller valves, they are easily modified to the larger 2.020/1.600 valve sizes by cutting larger seats and opening up the bowl area slightly on a seat-and-guide machine (and it's all legal).
Because of this, many racers will assume that just because they've found or purchased a set of Chevy heads with the infamous raised double hump on the ends, they've got the best thing going. In general terms, yes. But did you know that not all double-hump heads perform equally? Chevy actually made several running changes to the heads over the years, changing the part numbers as it went. So even though two pairs of heads may both have the double-hump branding, checking part numbers is the only way to know exactly what you've got.
The difference may look slight,...
The difference may look slight, but the small reduction in the diameter of a portion of the valve stem in an undercut valve (left) can make a difference in both flow through the port as well as reduced mass in the valvetrain. Jeff Huneycutt
Finding out how much different part numbers can vary within the double-hump family was actually a project undertaken by the Motorsports and Automotive Research Center at the University of North Carolina at Charlotte (UNCC). UNCC's mechanical engineering program offers a specialty allowing students to concentrate their studies on motorsports-related topics. The University also offers a very well-stocked motorsports laboratory that supports student-led racing teams (including a Legends car, a drag racer, a Formula SAE open-wheel racer, and others) with the intention of helping them get real-world racing experience. UNCC Engineering graduates can be found working on several NASCAR Sprint Cup teams as well as other racing organizations and even working for the OEs.
The good news here is that the university has recently opened its doors to Circle Track to work together on upcoming projects. But while discussing the possibilities with Steve Johnson and Luke Woroniecki, who lead the student's activities in the Motorsports Lab, the two showed us the work being done by engineering student Dallas Dallman to compare different cylinder heads within the double-hump family.