Dallman took the two most popular part numbers in the double-hump lineup-3890462 and 3917291-measured the critical dimensions of each and then subjected them both to several tests to determine their respective strengths and weaknesses. There are other part numbers within the double-hump family, but these two (called 462 and 291 for the last three digits in their part numbers from here on out) are generally considered the best of the bunch.
All the tests were performed back-to-back, and every precaution was taken to eliminate as many variables as possible. For example, Dallman put both heads on the flow bench and tested them with standard as well as undercut valves. But instead of using two sets of valves, he used the same physical valves in each head to ensure that any variances between two different valves could not cause a false analysis. Tests were also only performed to a maximum of 0.500-inch of valve lift because in the real world these heads are only raced in Street-Stock level classes, and the rules almost always limit maximum valve lift to half an inch or less. The result of testing at higher valve lift levels could only skew the results away from a racer's real-world needs.
So, without any further ado, here are the results.
Dallman uses a pitot probe to map out how flow varies in different portions of the ports.
Although they may look alike to the naked eye, careful measurements show that there are small differences between the two heads. We're only talking about differences in a couple hundredths of an inch when it comes to port width and height, but these are both areas-unlike the seat angle or depth-that you usually aren't allowed to touch, and when you are looking for the absolute maximum in performance it may make a real difference.
Ignore measurements here that are usually allowed within the rulebook to be changed during the course of a valve job-things like seat angle, valve diameter, and even valve stem diameter-instead, look at the height of the ports, the average port width, and even the port length. Port height is important not only because it creates a larger cross-sectional area, but also because the straighter a line you can draw from the entrance of the port to the valve seat, the better air should flow through it. So a higher average port height should be better.
On the Flow Bench
Dallman ran several different tests with both heads on the flow bench. We won't bore you with all the details, but we will pass along his more interesting findings. Dallman tested both heads alone, and with a dual-plane intake, a two-barrel carburetor, and an air cleaner in place. He also tested two different types of valves (an intake valve with a standard, straight stem, and an intake with an undercut stem) with both setups. Keeping with the real-world aspects of these tests, Dallman used a Holley 500 cfm two-barrel carburetor and an Edelbrock 2101 dual-plane intake with a two-barrel adapter plate. It's a typical Street Stock setup.
Many racers assume that the higher-priced valves with undercut stems are for improving flow. After all, they reduce the volume of the valve stem in the port, which should mean more room for the air/fuel charge to get by. In fact, they usually do the opposite. As you will see in the flow bench results (grey chart, page 70), undercut valve stems actually are a small detriment to airflow. Their true worth, according to Woroniecki, is to cut the overall weight of the valve. So even though the flow numbers don't show an advantage, you may be able to gain better valve control at higher rpm levels by using the lighter undercut valves.