Iron vs. Aluminum Heads

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.

Still, that's one bench-race axiom that's still up for grabs and another one chucked out the door. Not bad for page filler.

Dart's modern heart-shaped...        read full caption Dart's modern heart-shaped combustion chambers and 0.046-inch quench distance help quell detonation even with iron heads and are far superior to old factory castings in this regard. The Dart "solid flat-tappet" heads come with 1.437-inch-diameter valvesprings installed at 1.800 inch and had 130 pounds on the seat and 325 open (0.555-inch valve lift). Valve float with the Comp 306S cam was not a problem even at 6,500 rpm.

All testing was done with...        read full caption All testing was done with an 850-cfm Speed Demon carb, a Wilson 1-inch spacer, an Edelbrock Victor Jr. intake, MSD ignition, Hooker 1 3/4-inch headers, 18-inch collector extensions, and Hooker Aero Chamber mufflers.