The Grumpy Grease Monkey mechanical engineer.
Hot Topic: Iron vs. Aluminum
On the subject of drag race cylinder heads, there’s considerable debate over which material—aluminum or cast iron—provides the ultimate power advantage. Our experts discuss the merits of each.
By John F. Katz
The weight-saving advantages of aluminum heads versus cast iron are obvious and widely accepted—especially in drag racing, where weight high up in the front of the car is especially undesirable.
“Cast iron has been used for making cylinder heads since the early days of the internal combustion engine,” noted Jack McInnis of World Products, Louisville, Kentucky. “Its primary advantages relative to aluminum are lower cost and durability. Aluminum’s significant advantages are light weight and relative ease of repair by welding.”
But could the thermal properties of iron offer a power advantage over aluminum? That question gets complicated.
“Thermal conductivity has been debated for many years,” said Kevin Feeney of RHS, Memphis, Tennessee. “Historically, an iron head was considered more durable, and able to make more power due to the fact that it would not dissipate the combustion heat as quickly. With everything else remaining equal, there exists some merit to this argument.”
Mike Downs of Trick Flow Specialties, Tallmadge, Ohio, pointed out that the thermal conductivity of aluminum is “four or five times that of iron. This means an iron head will usually operate hotter. On the positive side, this means the fuel is pre-heated in the intake runner and easier to ignite in the cylinder.
On the negative side, it means the preheated air/fuel mixture will expand, reducing the effective flow into the engine and increasing the risk of pre-ignition. A properly designed aluminum head will transfer heat more quickly to the coolant, leaving the intake runner cooler and therefore able to flow more air-fuel mixture into the cylinder. High-energy ignition systems easily compensate for the cooler intake charge and help achieve maximum fuel burn.”
“The greater thermal conductivity of aluminum is a great advantage,” agreed Chris Frank of Frankenstein Racing Heads, Joshua, Texas, “especially in power-adder applications. That ability to dissipate heat quickly allows for more aggressive tune-ups.”
“Aluminum heads dissipate heat quicker than cast iron,” echoed Torrance, California-based Edelbrock’s Smitty Smith. “This can be an advantage in elimination-style drag racing, keeping the head temperature consistent round after round.”
With aluminum, Tony Mamo of AFR (Air Flow Research), Valencia, California, concurred, “detonation is less likely in an engine on the ragged edge, as it won’t hold as much heat. But that also firms up the argument that a cast iron head on an engine without detonation issues would make more power for the very same reason!”
“You need to build more heat with aluminum to make the same power,” agreed Bill Mitchell Jr. of Bill Mitchell Products, Ronkonkoma, New York, “or compensate by coating the chambers to keep more heat in the cylinders.”
“The machinability of cast iron is actually pretty good,” said Sonny Leonard of Sonny’s Racing Engines and Components, Lynchburg, Virginia. “The main hurdle would be during the CNC porting process. You tend to be machining in slender cavities, which dictates using less rigid tooling. This may require longer cycle times, which would reduce profitability.” That said, “cast iron does offer some advantages over aluminum. Cast iron has greater stiffness and damping characteristics, which may aid valvetrain stability. But the weight penalty is too great to really explore this option. The weight issue will always be the deciding factor.”
Added Carroll Carter of C & C Motorsports, Manassas, Virginia: “Aluminum is preferred over iron due to its light weight, intrinsic metal characteristics, and the ease of working with it. Aluminum is easier to machine and easier to repair—and it creates a better-looking product. Today’s advanced CNC machining centers and other computerized shop equipment are turning race engines into works of art.”
There are three main advantages (that I can think of) for a cast iron block over an aluminum one:
- Dimensional Stability: Aluminum grows more during the heating process than does iron. Extra precautions must therefore be built into aluminum blocks so as counteract this condition and prevent issues.
- No Cylinder Liners: If you ever want to rebuild an aluminum block where the cylinder liners are toast, they must be replaced. This is a large machining expense in comparison to having to just bore the cylinders in a cast iron block.
- Cost: Cast iron has been used in industry for many years and is fairly easy to produce. Aluminum, on the other hand, costs a lot more to refine from bauxite ore. Just in materials alone, the cost is greater. Then look at the cost for the cylinder liners and special casting processes which must be employed to get the aluminum block correct and the cost goes even higher. There is some trade-off, however, in the heating process (doesn't take as much energy to melt aluminum v. cast iron), and aluminum is easier to machine (less wear on the machine tooling/fixtures v. cast iron).