Iron vs. Alloy

There was a “what-if” question addressed a few months ago in our Performance Q&A column that piqued our interest. The question was simple: If you had a set of iron and aluminum cylinder heads that were as identical as iron and alloy can be, which would make more power? Even the most ardent aluminum head supporter will admit that iron heads have the power advantage because aluminum transfers too much heat out of the chamber during combustion.

So the real question is “How much horsepower?” Those are the kinds of questions we live for, so CHP decided to find out. The plan, because we’re such modest gearheads, was simple. Bolt a motor together, stuff a big cam in it, and then use the short-block to test iron versus aluminum to really nail down the difference. Then there’d be one less question in the world to ponder. Or so we thought.

The Heads

Before we began, we decided to look around for a pair of small-block heads that were identical in port size, shoe size, airflow, and valve diameters. Our search took us to World Products where the folks who crank out dozens of iron castings an hour revealed that they offer a pair of Motown 220 heads in both iron and aluminum that work for our comparison.

The original Motown heads initially appeared in iron with 220cc intake ports, 2.055/1.60-inch stainless steel valves, and 64cc combustion chambers. The heads are available with either 1.437- or 1.550-inch valvespring diameters, with the larger springs intended for mega-lift roller cams. There’s also a choice of 67cc or 72cc chambers. Both Motown 220 heads are machined for both perimeter and centerbolt valve covers.

The Motown 220 Lite aluminum version is almost a clone of its cast-iron cousin with the addition of CNC-ported combustion chambers. The aluminum heads also differ in that they use a 14mm gasket-style spark plug while the iron versions employ a 14mm tapered seat plug.

These large-port heads are best matched with larger-displacement small-blocks like 383ci, 406ci, or 415ci engines that can really move some air. Given this, we decided to construct a 383ci small-block for our test. The 383 is a stroker version of a 350, using a 400ci small-block, 3.75-inch stroke crank in a 0.030-over 350 block. With the 64cc chambers, a 0.005-inch negative deck height, and a 0.039-inch Fel-Pro 1014 head gasket, the compression computed to 9.9:1.

The Engine

To ensure this effort came together quickly, we called upon our trusty engine-builder Ed Taylor of Ventura Motorsports to assemble the small-block. Starting with a properly relieved, bored, and align-honed block, ably machined by Pete Christensen of Jim Grubbs Motorsports, Taylor chose to drop in an interesting rotating assembly from Scat Industries. Scat has recently produced a kit offering an internally balanced cast-iron crank matched with a set of 4130 steel, 6-inch I-beam connecting rods and a set of SRP forged aluminum pistons.

Chevy originally built the 400 crank as an externally balanced unit to reduce the size of the crank counterweights. This requires a more expensive externally balanced harmonic balancer and flywheel when building a 383ci small-block. Scat moves the weight around to create a neutral balance package that allows the engine-builder to use a standard 350 balancer and flywheel/flexplate and also to avoid using expensive Mallory metal to balance the rotating assembly. Taylor combined these parts with a Milodon oil pump and pan assembly to finish off the bottom end, sealed with a slick one-piece Fel-Pro oil pan gasket. Even though this is a comparison test, we wanted to see how much power we could squeeze out of this 383, so we added a healthy Crane mechanical roller camshaft.

With 232 degrees of duration at 0.050-inch tappet lift and 0.525-inch intake lift, we estimated this would help the peak horsepower while still building a respectable torque curve for street use. The remainder of the valvetrain is also all Crane: Gold 1.5:1 roller rockers, Crane-spec’d roller style dual valvesprings, steel retainers, hardened pushrods, and a Crane dual roller timing set. All of this fits under a set of World Sportsman II cast-aluminum valve covers.

For the intake side, World sent one of its dual plane manifolds that we felt would work best as a street combination when teamed with a Holley 750-cfm mechanical secondary carburetor. A Moroso DuraFire HEI distributor and a set of Moroso wires light the spark. Finishing the engine off is a neutral-balanced 6.6-inch diameter harmonic balancer from Summit Racing and a set of Hooker 1¾-inch headers feeding into a pair of 2-½-inch Borla stainless mufflers.

The Test

Taylor assembled the engine, pressure-lubed it, and set it on Ken Duttweiler’s dyno. After a suitable break-in and warm-up period, it didn’t take long before we were making steam. The first test was with the iron Motown 220 heads to ensure that we didn’t have a problem with detonation on 91-octane pump gas with the iron castings. Taylor idealized the timing at 37 degrees of total timing and then started yanking the dyno handle.

He made a few tweaks to jetting and timing, but the power didn’t really change very much, cranking out 449 lb-ft of torque at 4,200 rpm with a horsepower peak of 407 at 5,600 rpm. While these numbers are powerful, we were only halfway through the test. Now it was time to do the head swap to see where the differences would come in.

While Taylor was swapping heads, the bench-racing discussion centered on where the power difference would occur. Some estimated that the entire power curve would be down ever so slightly while others postulated that there would be less of a difference at higher engine speeds because the heads would have less time to transfer the heat. Some engine-builders plug as much as another full point of compression in their aluminum-head engines compared to iron head engines.

Not everything goes the way you expect it, however. Soon after we began wailing on the aluminum head combo, it was clear the aluminum version made more power! Comparing the best power curve from the iron heads against the aluminum heads produced a significant difference, so we decided to take an average of three runs from each combination to create the two curves produced here. These runs were the result of several tests to optimize the timing and jetting.

The aluminum heads made an average of 8 lb-ft of torque more than the iron heads throughout the entire curve, which contradicts our theory that iron heads should produce more power. For this to work, the theory assumes the heads are identical. There was a slight difference in flow between the iron and aluminum castings when Todd McKenzie flow-tested both sets of heads. This demanded some minor tweaking on the aluminum heads to bring them within a few cfm of the iron. We attributed this slight flow discrepancy to a minor lip created when the hardened steel seats are driven into the aluminum heads. But after McKenzie’s tune-up, both sets of heads flowed very similar numbers.

The other significant difference between the iron and aluminum heads was the CNC chamber machining on the aluminum Motown heads. The CNC’d chambers are the same size as the iron heads, but CNC work did reshape the chamber slightly. Chamber shape is a difficult item to quantify, so it’s tough to state with any certainty that the chamber mods contributed to the difference in power, but this appears to be the most likely explanation for the aluminum head power increase.

Conclusion

We still believe that iron heads have the ability to make more horsepower than aluminum heads, but we also think that the difference is probably less than an average of 5 lb-ft of torque. Given this, the power difference is slight enough that other variables can contribute as well. In this case, the two sets of heads were not identical enough to reveal the basic inherent advantage of the iron heads.