With cast-iron heads stacked up in the background, we gathered enough proof that aluminum
It seems as though we're surrounded by myths in our everyday lives. "An apple a day keeps the doctor away," and "The check is in the mail," are just two examples. Building high-performance engines has its fair share of fallacies also. Most everyone has an opinion about something. Unfortunately, not everyone's opinion is as right as it should be.
We recently sat down with Bill Mitchell, of Bill Mitchell Products and World Castings fame, and discussed several myths that always seem to be floating around in our industry. Bill brought up some very simple and basic truths that most of us never seem to think about, because we've been "brainwashed" to just accept them. But, when you sit down and really think about these, they have a lot of truth.
A dilemma Mitchell lives with everyday is, "Do you make a part the right way, or do you just sell a customer what he wants." While Mitchell will sell you what you want, he commented, "I want to make sure a customer gets what he or she really needs, and sometimes that takes educating the consumer a little. Yes, that sometimes takes a little more effort than just giving somebody what he wants, but it's important that we at least lay down the facts and then let them make the choice."
What follows here are the facts. Some of these ideas might seem to contradict today's style of thinking. However, if you think about them using everyday common sense, you'll realize that they all have merit.
Myth #1 - Aluminum Heads Make More PowerBuilding power requires one important ingredient: heat. You've all probably heard the term, "Heat equals horsepower," and it's true. When the spark plug fires in each cylinder, it creates an explosion of the fuel and air mixture. The containment of that explosion creates pressure on the piston and forces it back down the cylinder. The faster it is forced back down, the more power it ultimately transfers to the crankshaft and, eventually, out to the tires. So far, that's relatively simple to understand, but where does heat come into play?
The explosion obviously creates heat. The amount of heat is then divided up, and most goes towards the creation of pressure on the piston, but some is transferred into the block and head casting and, eventually, finds its way into the water jackets (and is cooled). There's no way you could contain all the heat without transferring some of it out, but theoretically, if we could, well, that's just dreaming.
Let's get back to heat. Aluminum is great at dissipating heat. Heat up two equal-size blocks of aluminum and iron, and you'll quickly understand that theory. The heat is transferred very quickly out of the aluminum, while the iron will take longer to cool. That's why you're seeing more aluminum radiators around lately. Now the question remains, "If we want to keep heat in a combustion chamber, why do we use an aluminum head?" A cast-iron head will undoubtedly hold the heat in more and will make more power than an equally modified aluminum piece. But, unfortunately, there are drawbacks.
To start with, a cast-iron, big-block Chevy cylinder head just about requires the use of a small crane to lift on and off a motor. There's another term that's floating around, and it's, "Weight equals horsepower." Using the general statement (which is close to being true), "100 pounds equals a 10th of a second," it then stands to reason that the 75-85-pound savings from using aluminum heads on a big-block is worth close to a 10th in e.t.
Acceleration is also an important factor. In the 1600s, the Greek astronomer, Galileo, reasoned that both light and heavy objects fall at the same rate of speed. However, we're not sure if his testing involved big-block Chevrolet dragsters, with and without aluminum heads. The plain fact is that, while they both might fall at the same speed if dropped from a tree, the lighter car will accelerate faster. Can losing a 10th of second due to using cast-iron heads be picked up in the extra power made by them? Probably not.
All cast-iron cylinder heads from World Castings utilize integral valve guides for strengt
Another argument can be made for the use of aluminum heads and it's called repair-ability. Cast iron does not weld very well. Break a piston and it'll quickly tear through a cylinder head. There are methods to repair a cast-iron head, but they're expensive and not easily done. And most times you'll end up with a crack down the line. Mitchell says, "So what; a cast head is typically very inexpensive, so throw it out and get a new one." This is fine unless you've got a ton invested in porting or valve work. Starting with a new head, you'll have to invest that money all over again. But Mitchell continues, "We sell a number of cast-iron heads that will make good power without any extra port work, so replacing one down the road because it's broken is not an expensive affair." Any heli-arc welder worth his weight, though, can usually bring a torched aluminum head back to life. A little bit of clean up and it'll be as good as new.
So what's better, aluminum or cast iron? That depends, and it'll have to be a decision for you to make. But don't be fooled into believing the myth, "Aluminum heads make more power."
Myth #2 -Bronze Valve GuidesWho ever thought of the idea that bronze valve guides are better than any others? Years ago, when we used to rummage through scrap yards for a good set of cylinder heads, typically, we'd come across a good casting, but the valve guides would be worn out. What we then did was bore out the guide and press in another one. While bronze guides were easily available to most cylinder head rebuilders, that's what was used. Basically, they were just a cheap rebuilder's trick. But are they better?
Let's first talk about aluminum heads. It's very important to use another material for valve guides in aluminum heads because a steel valve will wear out the aluminum, but in reality, cast iron has better wear characteristics than anything. Mitchell says, "The fact is, Chevrolet uses cast-iron guides in their aluminum Corvette heads, even in the exotic Z06 models. If it's okay for them, why do we want to re-engineer it?"
This cutaway of a cylinder head demonstrates that if the guide is bored out to accept a br
Cast-iron heads are another story. Whenever you bore out a valve guide, you weaken the casting in that area. Add in the fact that the interference fit between the guide and casting might not be right, coupled with the fact that the valve job must now be completely redone (because it's no longer concentric with the valve bowl), and you've created more problems than it's worth. Actually, bronze guides aren't worth much to begin with.
Fortunately, today we don't have to go scrounging junkyards to find a good set of castings. Mitchell's company (World Products) casts and sells a number of top-quality cast-iron (as well as aluminum) heads that make good power right out of the box. They also incorporate integral cast-iron guides on all their cast heads. Why would you want to take a brand-new casting and trash it by boring out the guides and installing bronze replacements?
Here's one other point to bring up on the subject of guides. Titanium valves, for those of you who use them, actually like the softer bronze material. It all really comes down to the interaction between two dissimilar metals. Don't be fooled by the myth, "Bronze valve guides are better.
Myth #3 - Splayed Four-Bolt Main Caps And Other MetalsAnother myth that goes back to our "junkyard days" is that four-bolt main caps that have the outer two bolts splayed (or angled outward) are better. When all we had to work with years ago were two-bolt blocks, a common practice was to install four-bolt main caps. This was a must for anyone making power, because the standard cast-iron two-bolt caps were not only weak (metal-wise) but caused web-splitting of the blocks. An easy fix was to install a four-bolt cap.
Both Merlin and Motown blocks from World utilize four-bolt, nodular iron main caps that ar
However, a problem occured simply because of the placement of the outer two bolts. To just add two bolts right next to the stock ones, and drill them in the same plane, meant that the block had very little meat (or metal) for those bolts to tie into. The only solution was to drill them at an angle, splaying them outward, because there was enough metal there for them to bite into. Henceforth, from that time forward, "Splayed caps are essential." Another myth exploded.
Mitchell says, "We cast all our Merlin big-block and Motown small-block cast-iron blocks with enough reinforcement in the main bearing web area to alleviate that problem. All our blocks have four-bolt caps that have all the bolts drilled in the same plane."
Another problem with splayed caps is that they tend to distort the bottom of the cylinder bore. It's the same reason that we hone blocks with a torque plate in place of the cylinder head. Because of the close proximity of the head bolts to the cylinder bore, they distort the top of the block. Splayed bolts within close proximity to the bottom of the bore will do the same thing.
While we're on the subject of main caps, let's talk about steel caps. Again, another myth that harkens back to the "dark ages," where we didn't know any better. When somebody got the idea to install four-bolt caps, they didn't just run out to the hardware store and buy them. They had to be made. What easier and available material was there besides steel? So steel caps were made, and along with them came another myth.
The nodular iron caps used on the World Products blocks have the same rate of expansion as the cast-iron block, something steel caps don't have. This translates into the top and bottom bearing halves' keeping the same clearances. Steel caps will elongate more, allowing the bearing half in the cap to loosen up more than that of the block half.
Big intake ports might look good on the flow bench, but they won't work all that well when
"If you look at a Caterpillar diesel engine," says Mitchell, "it's got 22:1 compression, and it runs simply by detonation. There's no spark plug to fire it. Every one of those motors has nodular iron caps that are located by the use of dowel pins. Like I said earlier about Chevrolet's using cast-iron guides, if nodular iron caps live in a Cat engine, there's no reason to reinvent the wheel and think we can do it any better."
Myth #4 - Big Manifolds And Big Ports Make PowerBigger is not always better. Making power requires not only making heat, like we alluded to earlier, but it's also a combination of the camshaft, cylinder head flow, bore, and stroke. After all, you can't make a fire if you get fuel in the cylinder.
Big ports, though, will flow air but lack air velocity that gives a good, clean signal to the carburetor. The lack of air velocity means that not enough air/fuel mixture will reach the cylinder before the spark is ignited. And we know that the more air/fuel mixture you can pack into the cylinder, the greater the explosion and the greater the power.
An example of this is a water hose. Turn the spigot on and watch how much water flows out of a typical 5/8-inch water hose. Now install a 1-inch hose, and while you'll see more water flow out, you'll notice that it flows with less pressure. Liken that pressure drop to the signal seen by the carburetor. With less signal, the carburetor will feed less fuel to the cylinders. Less fuel, less explosion. Simple.
There are some engines that will work with big ports, but it's generally a trade-off. Big ports also reduce the driveability of a combination. High winding, stick-shift cars that maintain rpm in a short band could benefit by big ports. But, a 468ci engine (that tops out at 7,000 rpm), put in front of a tight converter and an automatic transmission, will not like 350cc intake ports. A combination like that would go faster with a set of 250-280cc ports. It's all a matter of velocity.
Bigger isn't always better, and it's best to have a matched combination with smaller ports than to brag to your buddies that you've got cylinder heads that flow like sewer pipes.
Bill Mitchell Products
51 Trade Zone Ct.