This here's what we call last-minute filler. At least we're good at it, and have had plenty of practice lately. Most recently, the engine for our long-lost '77 Nova project finally came back from the machine shop, but with egg-shaped big ends on the rods and ring gaps way too tight for the nitrous we plan to throw at it. It happens to you, it happens to us. So, while this story was supposed to show you how we built that Disco 350, we instead used the time we could have spent fixing it to bring you this story: a shootout of iron and aluminum heads that are identical except for the material they're made from.
Coming next, a Freiburger versus Brule cholesterol shootout.
Why? Because we question stuff. You know-bench racing stuff like, "you can get away with more compression ratio with aluminum heads because they dissipate more heat," and "iron heads make more power because they keep the heat in the chambers." We've heard those things a million times, and we've written it a bunch too. But has anyone really tested these claims, or is it just theory? To find out, we put some Dart heads through the ringer. One set was the Iron Eagle Platinum Series (PN 10511122P) with 215cc intake runners, and the other was the aluminum Pro 1s (PN 11511122). Both had 64cc chambers, angled plugs, 2.05/1.60 valves of the same design, and 1.437-inch-diameter valvesprings. We tested these as complete assemblies supplied by Dart. Aside from the material they are made of, the heads seem to differ only in the spark-plug type: the iron heads use short-reach, tapered-seat plugs, and the aluminums take long-reach, gasket-seat style. Externally they appear to be the same casting, and to verify that they are functionally identical, we cc'd the chambers (around 63.8 cc each) and intake runners (almost 217 cc on both). We also threw both heads on Westech Performance Group's Superflow flowbench. There were some variances of a few cfm here and there, but nothing we'd call relevant overall.
By now you've figured out that we're talking about a small-block Chevy again. The engine was a 388-incher (4.06x3.75 bore and stroke) with a China crank and 5.7-inch rods and Keith Black forged-flattop pistons with two valve reliefs that we've hand-enlarged to a dish volume of 6.5 cc. The pistons are set 0.005 inch below the deck. Add that up with the 63.8cc head chambers and the 0.041x4.166-inch Fel-Pro head gaskets, and the static compression ratio is 10.88:1. You'd presume that was too much compression for iron heads on pump gas. The cam is a Comp Cams 306S (PN 12-445-4), a mechanical-flat-tappet grind with 260/260 duration at 0.050 tappet lift, 0.555/0.55-inch lift with our Comp Pro Magnum roller rockers, and a lobe displacement angle of 110 degrees.
Our test procedure involved changing no hard parts except the two styles of Dart heads. With both the iron and the aluminum, we ran sweeps to find the best total ignition timing, thinking perhaps the differing heat-retention characteristics would require different timing settings. We ran each set of heads with 91-octane pump gas then backed up our findings with a load of Rockett Brand 118-octane fuel just to ensure that there was no possibility of detonation that we did not discern audibly or through checking spark plugs. Also, we tested each set of heads with a beginning coolant temp first at 110 degrees, and second at 185 degrees, thinking that perhaps the differences in the heat characteristics of each head material would be revealed by wildly disparate coolant temps; would one material lose more power at higher temps than the other? Finally, we executed a steady-state test at 190 degrees of coolant temp with the dyno holding the engine at peak torque for 10 seconds and capturing data 10 times per second to see if heat buildup in the head would cause power to fall at a different rate with the varied metal of the heads. We beat the engine pretty hard.
Here's our handiwork on the piston domes. Don't blame us for the grooves that lead outward from the valve reliefs, placing the heat at the thinnest part of the ring land. | 
Iron head: 55 pounds. Aluminum head: 24.35 pounds. Two assembled iron heads from Summit Racing: $1,075.90. Two assembled aluminum heads: $1,331.90. So you pay $256 to save 61 pounds from the nose of your car. It's worth it. | 
Lucas oil was kind enough to load up Westech with free oil for us to use, so we were kind enough to give them this plug. Our 383 used the 10/30 synthetic. |