Big Block Cylinder Heads - Big-Block Cylinder Head Test

The Setup
There are a boatload of variables involved in testing five different Rat motor heads. We quickly discovered that each of them demanded wildly different pushrod lengths based on changes in valve length. For example, both the TFS and Dart heads use 0.250-inch-longer intake valves that require a taller pushrod. Brodix adds 0.100 inch to both the intake and exhaust valves, while the Edelbrock valves are stock length. And don't forget, we skewed this whole deal with a hydraulic roller cam that uses a much taller lifter, which effectively shortens the pushrods. Mix in the fact that all Rat motors use different-length intake and exhaust pushrods and you've got a major science project just in pushrod lengths. After much gnashing of teeth, we ended up with six different pushrod lengths for our five subject heads. Just for fun, many of the heads also required lash caps on some of the valves to create sufficient clearance between the Comp steel Magnum 1.7:1 roller rockers and the valvespring retainers. Whew! The next time you hear a buddy complain about how much money custom engine builders ask for assembling an engine, smack them upside the head and tell them we put a solid 20 hours into setting up these heads so that they would perform (and survive) as intended.

But wait-there's more. We also realized that in order to generate a truly fair comparison, all the heads would require the same valvespring pressure. This would eliminate valve float as a variable, which was very important. One item we had no control over was that larger-diameter valves automatically weigh more than smaller valves. To accommodate all the different valve weights, we decided on a Comp Cams PN 933 spring with a common installed height of 1.950 inches. The 933 measured 175 pounds at 1.950-inch seat height with 440 pounds of open pressure at 0.550 inches of valve lift.

Finally, all but one of these heads required its own specific head bolt set as well. It took a big stack of ARP fasteners to get the job done, and we appreciate all the help that ARP gave us to make this test successful.

Pushrod Chart
Because these heads were fitted with several different valve lengths, this demanded multiple custom pushrod lengths. Comp's adjustable checking pushrods and broad Hi-Tech 31/48-inch pushrod selection made it easy to set up the proper rocker geometry with custom pushrod lengths in 0.050-inch steps. The following chart lists the pushrod lengths we established for our hydraulic roller-cammed 496, but you should still take the time to measure your own required lengths to ensure accuracy. The chart lists each Comp Hi-Tech pushrod length (in inches) with each part number in parentheses underneath. These Comp Hi-Tech pushrods can be ordered either individually or in sets of 16.

We used a Height Mic from Powerhouse Tools to establish a common installed height of 1.950 inches for all five sets of heads. That meant measuring a total of 80 valves and shimming each one within 0.010 inch to ensure each valve and spring combination produced similar loads.

All aluminum heads require a steel spring insert to protect the soft aluminum from hard steel springs. Setting the installed height often requires shims (arrow), which are placed between the spring insert and the cylinder head.