The head or heads of an engine may seem, at a quick glance, to be simply a shaped block of aluminum or cast iron, but there is a bit more to their structure than that. Understanding the job a head does may not be complex, but similarly, there is more to it than it may at first seem. The intricacy that goes into designing a head that makes power efficiently is very complex. Simply put, the head in a gas or diesel engine generally provides a home for the valves and allows them to open and close as the camshaft(s) “tell” them what do to. The valves do this to let fuel and air in and exhaust out of the combustion chamber at certain times, while at other times, the head (and valves) hold explosive forces (compression) in to push the piston down. Of course, intake and exhaust port shape and size, combustion chamber size and shape, valvetrain oiling, engine cooling, rocker arms, valvesprings and seats, and a precision fit are all important aspects of a head’s function, as are the construction of the fasteners and gaskets that keep everything in their place. As off-roaders who want reliable and efficient torque and power, what can we learn about heads that help us understand what they need to do to and how they need to do it?
To get an idea of how heads work, what parts are good, what’s bad, and what can be improved, we visited our friend and machinist Joe Ali for a little lesson in heads. Ali is mostly a motorcycle guy that has been working on heads for many years (more than he will admit) and works on all kinds of heads from 50cc Honda mini bikes to diesel truck engines, tractors, common pushrod V-8s, modern import engines, and even a few AMA racing superbike engines. Ali is the expert (he warns us to use that term loosely), and we’re here to try to simplify his knowledge into what you may find interesting without screwing up too much of what he told us. We also lean on our pals at Tilden Motorsports, specialists in LS engines, parts, and accessories, for a few images and advice. Ali also correctly points out that almost all mass-produced heads can be improved upon as vehicle manufacturers are limited by production line efficiency and costs of more expensive processes and parts. For example, a manufacturer can save lots of time (and thus money) by not grinding down a metal flashing line. That said, modern mass produced heads do work very well.
Old Heads vs. New HeadsHead technology has evolved over time as cars and trucks have become more and more a part of just about everyone’s lives. The first engine’s heads were hardly more than a plate that capped off the top of the engine and held a spark plug. Modern heads can hold multiple valves, intricate ports, specially designed combustion chambers, spark plugs, cams, variable valve timing components, and direct injection technology. With hundreds of years of technological advancements, modern heads are more reliable, clean, and efficient than ever. Clearly not all heads are created equally and a modern engine with a simple flathead design is all but unheard of. Modern heads almost all contain overhead valves and are clearly more efficient at making torque and power. Despite this, purists still run flathead engines in plenty of 4x4s around the world, and while not at the cutting edge of technology, flathead engines still do what is expected well. If your 4x4 still has old-school technology like a flathead running and driving in the tail end of 2016, we are happy to support your firm grasp on the past, but in general, we recommend a bit more modern approach to head technology for off-roading.
Gas Heads vs. Diesel HeadsChances are you know some of the differences between gasoline and diesel trucks. Mainly, one runs on diesel fuel and the other gasoline. We don’t want to oversimplify the difference, but with all the stories we’ve heard of diesel fuel tanks full of gasoline and gasoline tanks full of diesel, maybe we should? The basic mechanical differences between gas and diesel engines is in their compression ratio and how ignition occurs in the engine. Compression ratio is the ratio of the volume of a given cylinder when the piston is at the bottom of its stroke (max volume) over the volume of a given cylinder when the piston is at the top of its stroke (minimum volume). Compression ratios for gas engines range from 6:1 up to about 13.5 for common road-driven vehicles (with flathead engines being at the bottom). Diesel engines have compression ratios ranging from 14:1 up to about 23:1. Ignition in the cylinder of a gasoline engine is initiated by the spark plug, but in a diesel, ignition is the result of the compression of the fuel and air mixture to the point when combustion occurs. The largest difference between diesel heads and gasoline heads is where the compression ratio comes from. Most gasoline heads have the combustion chamber as part of the head. With the piston at the top of the stroke and nearly at the top of the block, the vast majority of the combustion chamber is recessed into the head. In common diesel engines, the combustion chamber is within the dished piston. The reason for this difference has to do with how ignition occurs in the two different types of engines and how the air/fuel charge burns.
Head Gasket vs. Head GasketThe old myth is that hot rodders, way back in the good ol’ days, would fashion cereal boxes for head gaskets. The idea is that the cereal boxes were thinner than other head gaskets and using them was a cheap way to bump up compression. That might be true, but that slight bump in compression came at what cost to sealing? The head gasket has to keep fluids (coolant and oil) and gases (the intake charge and then exhaust gases) where they are supposed to be without leaks (internal or external). The enemies of all head gaskets are few: improper installation (bolts overtightened, unevenly torqued bolts, undertightened bolts, gasket poorly aligned), surface issues (dirt, surface damage, or non-flat surfaces), and engine overheating (because of pre-ignition (pinging) or other mechanical cooling issues). Arguably the best, most-modern head gasket design in use today is known as MLS, or multi-layer steel gaskets. MLS gaskets employ three to five layers of carbon and/or spring steel formed with beads that help seal larger fluid or gas particles and elastomer coatings that seal smaller fluid or gas particles. These gaskets have all but rendered other head gasket designs obsolete. Its design allows for higher sealing potential and higher ignition pressures, all with lower clamping force. These head gaskets rely on a clean, flat surface. Oils, other sealants, and small particles can damage them (just like other types of head gaskets). MLS gaskets are found on many of the most modern and efficient engines you are likely to drive or drive past. They are highly engineered to very exacting tolerances.
Another type of commonly used head gasket is the composite-type head gasket, usually made of graphite or asbestos, with the latter being less and less popular for obvious health reasons. These head gaskets are made of a tanged metal sheet with composite material rolled on both sides. The combustion chamber is sealed with a fire ring and some of the fluid passages may be sealed with an elastomer ring and the composite material impregnated with compounds that seal them from fluids. As the head is torqued in place, a composite gasket deforms slightly, sealing the various openings.
Occasionally, a copper sheet with a wire ring around each cylinder is used as a head gasket. Some of these gaskets require special machining for these rings, while some do not. Copper head gasket proponents tout the metal’s ability to better transfer heat than composite materials, helping with cooling. Also, copper’s elasticity is said to be a benefit, allowing the head gasket to resist extremes in pressure, like when you’re running lots of boost or nitrous. In these scenarios, a composite material would yield or tear and a leak would develop while the copper would “spring back” and hold the seal. Copper head gaskets still require the use of some type of sealant to ensure the metal-to-metal surfaces are fluid and gas-proof.
Arguments for and against all types of head gaskets can be made, and what you end up using in an engine is generally determined by what the end goal is. Copper is aimed at forced-induction race-like engine conditions, while MLS is more appealing to the everyday user and or vehicle manufacturer. A good engine builder who is knowledgeable about building power in the type of engine you want (gas, diesel, naturally aspirated, efficient, low rpm torque, or high rpm and high power) will be able to help you determine what head gasket is best for you and your 4x4’s engine.
Aftermarket vs. Factory Head BoltsThis one’s fairly straightforward and there’s not a lot to learn or debate. All head bolts need to be installed correctly. This includes using a bolt lubricant when specified and being sure the threads in the engine block are clean, undamaged, and not rusty. Torque-to-yield (TTY) bolts cannot be reused. They are designed to stretch and become “plastic,” applying a specific clamping force on the head. Since they stretch, reusing TTY bolts could cause one to fail, bottom in the threads, or result in uneven clamping forces. That’s bad. With new TTY bolts, the specific tightening sequence must be followed to ensure the bolts are not too tight or too loose.
Aftermarket head bolts like those from ARP use one of several different materials that consistently exceed the strength of factory head bolts. This is because a vehicle manufacturer will use bolts that are the most cost effective for that engine rather than those that exhibit the greatest mechanical properties. Because of this, high-quality aftermarket head bolts or studs can be reused without concern, but that does not mean that there are not TTY aftermarket fasteners, and no TTY fasteners can be re-used—unless you’re mending a fence with them. Aftermarket TTY head bolts do exist, so you need to know if the head bolts for your specific application can be reused. When in doubt, don’t cheap out. If you’re into pinching pennies, we can relate, but this is probably not the area to cheap out. We are happy to pay for value, and try to avoid the glam and glitter when it’s not needed. Head bolts are an area—literally deep within your engine—where it’s better to be safe than sorry.
Bolts vs. StudsMaking a comparison between head studs and head bolts makes sense fairly quickly when you begin to look into it. Both do their job, and neither are necessarily going to make more power or torque although in some ways head studs do their job better than head bolts, but the difference to most 4x4 enthusiasts is nominal. What head studs do is important to assembly, maintenance, and the exacting tolerances of clamping things together in high-performance scenarios. For one, head studs make installing and removing heads easier. Studs help easily locate the head on the engine block and some are designed to help locate the head gasket precisely between the head and block. This is important in a race engine where things are changed or inspected and parts need to be swapped out quickly with exacting tolerances. On your daily driver or trail rig, that is less important, making head bolts more than adequate. Another area where studs have a slight advantage is when it comes to torqueing things together. Head bolts by their nature and when torqued to spec, see two different forces. One is parallel to their long axis (clamping force, holding the head to the block) while the other is a side effect: rotational (because they add this clamping force as you turn the head of the bolt). Studs, when properly installed, should only see the clamping force parallel to their long axis. Torque is applied to the nut on the stud and when done properly, should not impart a rotational force to the stud when at a specific torque setting. This means the torque specification is more precise, and this generally makes the job of the head gasket a little less tenuous. Despite the benefits of head studs, the vast majority of those of us who play in the dirt won’t reap the benefits of using studs. Now, if you’re going fast in the dirt as a livelihood this may be different.
Aluminum vs. IronThe debates can rage from election year politics to what part works best, and with the Internet at your disposal, you can be sure you too can find someone who will tell you what you want to hear. Whether that is the truth or not, you (or we) may never know. One debate engine builders and those who’ve imagined building engines have had for years is whether are aluminum heads better than cast iron. Why one may be better the other seems to be based in opinions on which has the best thermal properties and more. We won’t entertain those opinions here. Rather, we’ll talk about some nearly scientific investigations we’ve bumped into in the past. In the late ’90s, Car Craft magazine did a heads-up test (see what we did there) between identical iron and aluminum heads on the same engine on the same dyno. The results were nominal, meaning there is little if any benefit to either type of head in terms of performance. One thing Car Craft addressed that may be the only real difference is in weight. Since iron is denser than aluminum, using aluminum heads may help save about 40 pounds total, but that’s kind of splitting hairs at this point. Also, Engine Masters magazine, in an apples-to-oranges–type comparison, tested prepped factory iron Vortec production heads versus aluminum CNC-ported Blueprint Engines heads and found the aluminum units (CNC ported, larger intake runners, and larger valves) made more power and torque but only picked up 12.7 lb-ft of torque and 36 hp on a built GM 383ci V-8. What does that mean? It means more expensive heads that flow better make more power. No surprise there.
We’ll recommend running what you’ve got unless it’s stock and has restricted flow. In that case either aftermarket aluminum or at least certain prepped production iron heads would help torque and power.