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.

Can you guess what we learned? Zilcho. As in zero difference anywhere in the power or detonation characteristics of the iron versus aluminum heads. Even the optimum total ignition timing was the same at 36 degrees. Regardless of coolant temp, rate of acceleration, steady state, or through a sweep, the dyno curves for the two styles of heads were identical. If anything, we could squint and guess and mumble that maybe aluminum heads were better by 2-3 hp. But the one thing we could never say is that the iron heads retained more heat and made more power than the aluminum. Maybe it's different on some engines with a drastically different water-jacket design, but we'll stand up and say that the old bench-racing line just ain't true.

We're not going to be quite so cocky about debunking the claim that you can run higher compression on pump gas with aluminum than with iron. Our test does not definitively prove that. It may not have even tested it. We can say with complete confidence that we did not encounter detonation at any point during our testing, even at 10.88:1 compression. That could be because the cam was pretty big, with an intake-closing point 79 degrees ABDC. With cranking compression in the 185s, it was not taxing the detonation point even with 91 octane. Also, the Engine Masters thing has pretty well demonstrated that a dyno seems to have far more detonation tolerance than do actual driving conditions in a car. So, ultimately, we don't think we pushed that limit enough to make a positive finding.