When looking at aftermarket cylinder heads, almost all of the options these days are aluminum. But what about the cast-iron aftermarket cylinder head options? There has been much debate on which material is best used for a cylinder head for the past decade or so. Nowadays, many customers automatically approach their engine build as though they will be using aluminum cylinder heads. But, the question remains, is aluminum the best choice?
There have been many tests performed over the years trying to prove which material — iron or aluminum — will make more power, but something that needs to be addressed here is that we shouldn’t look at the differences in the alloy for a power gain. We need to focus on the engine’s application to determine the material to be used for the engine build.
Withstanding the Test of Time
This topic of cylinder-head alloy came about from a question that a customer asked me. The question was: “Why are iron heads still being manufactured?” At first, I was a little taken back as to why this was being asked, but then it hit me, I was now talking to someone from the “new generation” of gearheads.
These days it’s commonplace to see different manufacturers offer aluminum cylinder heads ranging from as-cast to full CNC-ported at reasonable prices. Even billet-aluminum heads are becoming common. Cast-iron heads really don’t get the advertisement they used to.
When the question was asked, it took my mind back to the late-‘80s at the local drag strip when modified engines were built from a lot of stock components. We didn’t use aftermarket blocks, cranks, rods, heads, and valvetrain components all that often.
A common practice was to offset-grind a good crank for a little more stroke, recondition the rods with new bolts, bore the OEM block, and use a set of TRW pistons. For the heads, we ported our cast-iron pieces without the use of a flow-bench. Then, we put in an aftermarket flat-tappet camshaft — hopefully, chosen correctly — and hope for the best.
The significance of the story is this — I remember the time when a regular competitor, who had a fast bracket car, purchased a new set of aluminum cylinder heads. They were a set of Phase 6 GM Bowtie small-block aluminum cylinder heads. During the shakedown runs on a Saturday night, the car actually lost e.t.
We were puzzled at the time as to why the engine lost power. The aluminum cylinder heads were supposed to be the Holy Grail of making power because they literally cost a small fortune. This was supposed to propel the car into warp speed at the cost of a space shuttle ride, but it didn’t.
Later, the competitor removed the aluminum heads and sought someone’s help with a flow-bench to help properly port them. After some porting and a camshaft change, the car came back to the drag strip and ran the best e.t. it ever had.
Being intrigued by the internal combustion engine and this incident, flow and camshaft design became something of interest to me. Power was hidden somewhere in the aluminum cylinder head that the professional porter found, and it wasn’t the alloy of the cylinder head. If you think about it, the iron and aluminum cylinder head both performed very well.
The goal of the conversation is to find the attributes of both types of cylinder heads and where their application is best suited. The goal here is not to make a blanket statement and say one is better than the other, but rather understand critical differences of each that will help decide which heads you should use for your engine build. We spoke with the cast-iron experts at World Products to get some technical info on the cast-iron heads it has produced for what seems like forever.
The Inherent Differences
The most obvious difference that consumers notice is the weight. Yes, the cast-iron cylinder head does weigh quite a bit more than its aluminum counterpart. This is one of the major factors consumers look at when choosing cylinder heads. Yet, for most street applications, weight is not going to be a factor.
Yes, cast-iron heads will put a few more pounds on the front end of the vehicle, but will it really be noticed? When you are racing, the additional weight on the front of the car can be an issue. Several racing classes require the use of cast-iron cylinder heads to help even the playing field, but, if the racers in these classes were given a choice, they would choose the aluminum head. The weight penalty will always be the deciding factor in that area.
While the cast-iron head may weigh more, it does offer the benefits of additional strength and durability compared to aluminum. This is the most prominent attribute of a cast-iron head. By being durable, the cast-iron cylinder head can take abuse that would damage — or even destroy — an aluminum head.
When thinking in terms of cylinder heads, there are forms of abuse that are often overlooked. Think about expansion as the cylinder head and engine start to reach operating temperature. Aluminum does tend to grow and expand with heat, more so than cast-iron. As the team at World Products pointed out, that movement between the cast-iron block and aluminum head due to the dissimilar expansion rates can lead to head gasket failures.
Additionally, the expansion of the aluminum carries over to the steel components inside the cylinder head as well — valve seats, spring seats, guides, and rocker bosses. Aluminum heads tend to fatigue in these areas. That is why special attention has to be given to components such as the valve seats when installing them. Also, think about high spring-seat pressures and the effects on the surface of the spring seat bosses.
The valve guides have to be installed with the proper interference-fit, so they don’t fall down or drop. Consider the rocker arm stud bosses and the cyclic pressures endured as the rocker arm opens and closes the valves. When it comes to cast-iron, its durability makes them far less prone to issues in these notorious trouble spots.
The counterargument (ad infinitum) is if cast-iron cylinder heads get too hot, they can crack. While this is true to a point, it is an issue seen mostly on OEM castings. Aftermarket cast-iron heads are thicker on the deck surfaces, making cracking from temperature differences a thing of the past.
Plus, if the engine were hot enough to crack a cast-iron head, then think about what it would be like for an aluminum cylinder head. Any number of issues could come up, not the least of which would be expanding so much the valve guides and seats could fall right out of it.
Another theory regarding the inherent differences in material properties is thermal conductivity. Since there is heat energy in the combustion process, is there any lost with an aluminum cylinder head?
Because an aluminum cylinder head dissipates heat more quickly, there could be some merit to this argument in terms of efficiency. Many engine builders will raise the compression one point for an aluminum cylinder head, or even coat the chambers, to help keep the heat in the engine and not dissipate as quickly.
World’s engineers explain that in certain applications, such as when using methanol or ethanol blends, cast iron heads can be more efficient. This is due to their ability to retain combustion heat, and in alcohol applications, temperature equals cylinder pressure. Aluminum cylinder heads dissipate that combustion heat quicker, so you have reduced combustion temperature.
On the flip side, some engine builders feel that since the cast-iron head retains most of the heat, the retained heat then radiates to the intake runners. Heated intake runners would pre-heat the incoming air/fuel mixture, causing it to expand, which in turn reduces flow and increases the risk of pre-ignition. The validity and significance of these theories are still debated amongst engine builders today.
Differences You Might Not Expect
An interesting test was performed at Hendricks Motorsports quite a few years ago in comparing aluminum to cast-iron. A problem they were experiencing was out-of-round cylinder bores. How in the world could you get out-of-round cylinder bores with state-of-the-art machining equipment? Once the cylinder head was bolted down and the engine ran on the dyno, they were finding the bores were not remaining concentric.
After some research, it turns out the problem was with the torque plate used in the honing process. The torque plate was aluminum and was not securing the deck of the block during the honing process. After that, a steel torque plate was used during the honing process and found to keep the bores more concentric.
This “unequal” pressure on the deck surface was found by using a very costly pressure film. Pressure film was placed between the torque plate and engine block and torqued down. The pressure film has small capillaries of red dye that will burst and form the shape of whatever you are torquing against it to create an impression. Engineers noticed the aluminum torque plate was not as rigid and therefore was not applying the same amount of pressure around the bores.
Cost Doesn’t Equal Performance
The object here is not to say bad things about either alloy of the cylinder head, but rather explain there are factors you must consider when building an engine based on the application for which it is designed.
Besides the cost differences in raw materials, aluminum cylinder heads are made with far more machining steps in the manufacturing process in order to increase their durability. Those factors usually result in a higher cost to the end-user over cast-iron cylinder heads.
Because cast-iron costs less, it is usually recommended for budget builds. Even though it will perform well and be durable, the cost difference and use in “budget” builds have led to a stigma of cast-iron being inferior to the more expensive options.
So back to that original question: “Why are cast-iron heads still being made today?” The answer is simple. Cast-iron cylinder heads have a lot of merits when it comes to cost and durability. They have come a long way from the hand-ported OEM head of the mid-20th Century. Manufacturers like World Products are offering excellent castings with thick decks and various runner sizes for big-power, big-cubic-inch builds.
While the balance of popularity may have shifted from one material to the other over the years, that hasn’t diminished the cast-iron cylinder heads’ abilities to make substantial power without spending a ton of money. Add in the inherent strength advantages of iron heads over aluminum, and they start to look more and more advantageous in a street-driven build, whether it’s making 400 or 1,400 horsepower.