By Aaron Bonk
The valves are airflow’s last obstacle on its trip into the combustion chamber and of course serve as yet another obstruction on its way out. The air enters the intake system through a filter, travels through the throttle body, into the intake manifold, into the cylinder head’s ports and then, ultimately past the valves. Of course, said airflow only travels past the valves when the valves open and this is all controlled by the camshaft. But the camshaft does not interact directly with the valves; it’s the job of the rocker arms, valvesprings, retainers, and keepers (and in some cases the pushrods) to act as intermediaries and make everything happen. We refer to this series of parts as the valvetrain.
Valves: What They Do
A valve is nothing more than a flow-control device and is as diverse as the number of engines, machines, and appliances they may be found in. In terms of the internal combustion automobile engine, the poppet valve is exclusively called upon for use. Poppet valves pop up and down against a corresponding shaped seat, thus opening and closing access to the combustion chamber in accordance with the camshaft. You can look at them as a means of opening and closing the intake and exhaust ports.
Other designs have been introduced over the years including rotary valves, sleeve and piston valves, and slide valves, but the poppet valve has proven to be the best compromise between cost and effectiveness as far as automobile engines are concerned. Its shape and design allows for one of the most ideal flow coefficients with little friction and the need for little lubrication. Perhaps the only disadvantage of the poppet valve besides its non-instantaneous response, which we talked about when discussing camshafts, is its poor cooling characteristics – a problem especially relative to exhaust valves since their temperatures far exceed that of the rest of the engine.
A typical poppet valve, when viewed from the bottom, features a circular-shaped disc, or seat, which mates against that of the cylinder head. Its side profile however, reveals a mushroom shape that tapers into a long, thin stem to allow air to flow past itself. Valves may be comprised of multiple pieces and of multiple materials; more on that in a bit though. In closed position, the valve’s face is forced against the cylinder head’s seat with the help of the valvespring, a retainer, and a series of keepers, which help hold the assembly together under tension.
Besides how well air is allowed to flow around it, the biggest challenge facing a valve is heat. Remember, it’s extremely hot inside the combustion chamber and there’s little there to cool things off. It’s common for temperatures to well exceed 1,500 degrees F. The intake valves see a reduction in temperature thanks to the incoming air and fuel mixture’s cooling effect while the exhaust valves, which get significantly hotter, are only cooled when they come into contact with the valve seat. Here, heat is transferred back to the head for that brief moment. Heat is also transferred through to the valve guide as well. As mentioned, it’s the exhaust valves that get the hottest, and it isn’t just their face. As hot air is released from the combustion chamber and past the valve, everything all the way up to the mid-portion of the valve stem is heated up. With temperatures like these, it would be a fair assumption to say valves just aren’t as simple as they look.
Valve Diameters and Their Effects
Among other things discussed in previous chapters, it’s also the diameter of the head of the valve that determines the amount of air that may enter the combustion chamber. Generally speaking, increasing the valve’s diameter will result in better breathing and the availability of more high-RPM power. Of course, there’s a point of diminishment and this is all relative to intake manifold size as well as cylinder head port size and shape and the number of valves present. By simply fitting a larger valve in place, don’t get your hopes too high for much of a power gain. Additionally, valve size is also relevant to piston size; therefore, larger-diameter valves are best suited to large-displacement engines and vice versa. The down side: increasing valve diameter significantly makes for a heavier valvetrain, which may negate any of the newfound horsepower. Besides, sometimes there’s simply not enough room to fit any larger of a valve into place due to a particular combustion chamber shape or size.
Often times, manufacturers make up for the lack in flow simply by upping the valve diameter slightly. But running slightly larger valves has other advantages besides promoting additional airflow. As an engine begins to show its age, its valves tend to protrude lower and lower into their respective seats, forming a sort of dish. This is generally caused by excessive heat and poor gasoline and is most prominent on the exhaust side. This is a problem for two reasons: first, the farther down the valve moves into its seat, the more shrouding will occur – a bad thing. Second, since the valve is now offset from its seat, its sealing surfaces, or angles, will no longer match up – also a bad thing. Bigger valves and bigger seats tend to prolong this process to some degree. In some cases, larger valves may be fitted to their existing seats.
While increasing valve diameters is certainly one method of improving airflow OEMs have opted for that of multiple valves. By fitting the cylinder head with three, four, or more valves per combustion chamber, flow values far greater than that of just two valves – one intake and one exhaust – have successfully been realized. Unintended benefits of using multiple valves include reduced temperatures which translate into less thermal stress placed on the valves. In fact, even at equal temperatures, it’s been proven that thermal stresses decrease in multiple-valve scenarios. Most engines today worth anything feature four valves per cylinder.
To be sure, when concerning airflow, the most important part of the valve is the seat – the portion that actually permits and restricts air from entering the combustion chamber by sealing itself up against the cylinder head. But the rest of the valve, specifically the shaft, has its own almost equally important job. The shaft is the portion of the valve that guides itself in its up-and-down motion. This, however, cannot take place without the assistance of the valve guide.
Valve guides are cylindrical in shape, hollow, and are of a stationary design. Once they’re pressed into the head, they don’t move. As mentioned, their main job is to keep the valves on track, but they take a lot of abuse in doing so and are frankly called upon to do much more than simply lining the valves up. For one, the valve guide must restrict oil from within the cylinder head – needed to lubricate the valve shaft – from passing through the guide and into the combustion chamber. Tight clearances, proper material selection, and good valve stem seals all help fight against this. Oil in the combustion chamber means poor combustion due to octane compromising, an increased chance of detonation, potentially less power in the short run and carbon-coated valves in the long run, all bad situations. Keep in mind though, this is less of an issue on the exhaust side due to the higher pressure present here. Secondly, the valve guides take on a significant amount of heat transferred from the valves, specifically the exhaust side. For this reason, material selection is crucial, as poorly chosen valve guides can warp and/or loosen their clearance in respect to the valve shaft. Lastly, valve guides may often pose a restriction to airflow both entering and exiting the combustion chamber. Many times valve guides must protrude into the port in order to maintain a sufficient length in relation to the accompanying valve shaft. Often times, there’s a compromise to be made between an acceptable and safe length and the slight restriction this may pose to airflow. Many choose to grind off the portion of the valve guide that protrudes into the port. While this may perhaps improve airflow in some instances by eliminating this obstacle, often times its consequences may be far greater. Rocker arm-equipped engines tend to inhibit side load forces on the valve guide through the valve shaft. The shorter the guide is, the less area there will be for this load to be spread across. The results are prematurely worn guides so be careful here.
While older cast-iron cylinder heads often make due with cast iron valve guides, modern-day aluminum heads are best suited by using bronze alloy guides. What bronze guides do best, in comparison to other materials, is transfer heat away from the valve stem, thus reducing combustion chamber temperatures. They are also much more durable in comparison to other materials meaning their tight clearances will remain tight for longer periods of time, and tight clearances mean there’s less of a chance that oil will be able to pass through itself into the combustion chamber.
Having new valve guides pressed into place is not always the way to go though. Sometimes the originals may be difficult to remove or a less costly alternative is desired. Valve guide inserts are a less expensive route to take and can offer similar results to replacing the guides themselves. The process is relatively simple: the guides are drilled out and the tube-like inserts are pressed within them, providing a new orifice for the valve shaft to slide inside of. Once in place, the inserts are expanded and reamed a number of times to ensure a tight fit. This is by far the easiest way to remedy oversized, damaged or worn guides.
Valve Seats and More on Valve Guides
Previosuly [link] we discussed the problems associated with airflow around the valve seat and how improving the cut of the seat and the area around it may significantly help performance. Well, the seat of the valve itself is equally as important as the portion it mates up to on its respective cylinder head. Remember, even when the valve is moved as far out of the way as possible, air must still travel past it. The easier it can do this, the better off you’ll be performance-wise. In contrast, the cut of the seat is also directly related to its ability to seal, keeping unwanted gases out of the combustion chamber at the wrong times.
You may recall in our discussion of valve diameters and their effects the scenario in which, over time, the valves recess farther into their respective seats. While larger valves may prolong this from happening, some new high-tech valve seat materials, and guide materials, almost prohibit it completely. The problem lies in the temperature of the valves; the hotter they get, the hotter the seats and guides get since much of their heat is transferable. Bronze guides will take on much of the heat and then transfer it back into the cooling system. But the more worn out the guide becomes, its chances at doing this efficiently decrease. Without a tight, interference fit for the valve to slide through, heat is unable to transfer through the guide evenly, and the results are hot spots along the guide and premature wear. You may ask why we’re talking about valve guides again here. The point is that if the proper guides are chosen, heat from the valves may be dissipated through the guides, negating the need for replacing the valve seats with anything other than what you currently have.
What Rocker Arms Do
Whether you’re talking OHV or OHC engines, the basic premise of the rocker arm is the same: it transmits the motion of the camshaft onto that of the valves. Without it, the camshaft would spin and the valves would just sit there doing nothing. Rocker arms have been compared to a sort of teeter totter; this is due to their fulcrum-like design in which the two ends teeter back and forth from a fixed central position, reacting to the camshaft lobe on one end and reacting against the valve stem on the other. Most rocker arms are fixed to a roller shaft or a pivot ball which allow them to freely teeter back and forth yet remain in the same location. As the camshaft’s eccentrically shaped lobe rotates, this imparts an up-and-down motion onto the rocker arm which then imparts an opposite motion to its other end. This, in turn, transmits into the valve’s upward and downward motion. When the camshaft moves the rocker arm up, the valve goes down and vice versa.
Conventional vs. Roller Rockers
Excluding pushrod engines, there are essentially two types of rocker arms widely used today. The first is the conventional type. It relies on a steel pad in which the camshaft lobe rests upon. The pad is lubricated with oil in order to avoid damage from its almost constant contact with the camshaft lobe. Like anything else, rocker arm pads can wear and show signs of damage. As the pads wear, the effectiveness of the camshaft decreases. Maximum lift and duration may no longer be realized due to a thinning or worn rocker arm pad.
The second type of rocker arm is similar, but makes us of a roller bearing instead of a steel pad. As you’d expect, the roller bearing type potentially reduces friction and has the ability of freeing up some additional horsepower. They’re also usually more expensive. Specially designed roller cams must be used with these roller rockers since regular camshafts that use conventional rockers aren’t compatible.
Rocker Arm Ratios
Rocker arms, like any teeter totter, involve leverage. Altering the rocker arm in relation to its fixed central point will transmit more or less force onto the stem of the valve. This is dictated by its rocker ratio, which is simply a comparison between how far the valve actually opens and where the rocker pad is located in comparison to the rocker arm’s center. Increasing an engine’s rocker arm ratio is effectively the same is increasing its valve lift but is certainly a less than common way to go about doing it.
What Valvesprings Do
It’s been established that the rocker arms force the valves open via its reaction to the camshaft, but how exactly do the valves close? This is the job of the valvespring; not only do valvesprings assist the valves in closing, thus creating a positive seal against the head, they also help keep the rocker arm in contact with the camshaft lobe in order to properly control high-speed valvetrain motion. Actually, the valvespring is asked to do a lot; it must support not only the valve but also its keepers, retainer, pushrod (if used), as well as its own weight. But valvesprings are not as utilitarian as they may sound at first; they affect an engine’s longevity and even its horsepower potential and are certainly not something to be underestimated.
The valvespring stays pretty busy, working throughout most of the engine cycle, but not all of it. When a given valve is opening, pressure is exerted upon it through the rocker arm by means of the camshaft lobe. It’s the initial opening point of acceleration that the valvespring gets a brief break. Its work starts up again shortly though as the valvespring is called upon to stop the valve from going any farther.
Valvesprings must be strong enough to support all of their components so as not to lose contact with the camshaft lobe at any useable engine speed. Of course, as engine speed rises, this job becomes much more difficult. And if the assembly of parts is not brought to rest at full lift of the camshaft then valve float will occur – a bad situation.
As engine speeds rise and the camshaft spins faster, greater amounts of force are applied to the valvetrain through the camshaft; this, in turn, makes it much more difficult for the valve and its components to come to a stop. When the valvespring is no longer able to withstand these forces and valve float occurs, the rocker arm or follower briefly loses contact with the camshaft lobe. All the while, the camshaft keeps spinning while the valve has temporarily missed a beat. The results can seriously impair intake and exhaust timing events resulting in major power losses, or worse, damaged camshaft lobes, rocker arms and pushrods if applicable. Over-revving the engine is a common cause of valve float, but perhaps a more common cause is using springs simply not up to the task, something that seems to be more and more common with that which is supplied with OEM cylinder heads.
Valve float is not the only consequence of insufficient valvespring stiffness, valve bounce – when the valve hits its seat so hard as to bounce off of it – is yet another problem. This often occurs, like valve float, when the acceleration of the camshaft lobe far exceeds that which the valvespring is capable of handling.
While stiffer springs are the solution to both valve float and valve bounce, there’s more than one way to go about devising a stiffer valvespring. Common methods include using thicker coils, different and stronger materials, and larger-diameter coils. Multiple springs fit inside one another are another solution.
Single vs. Dual Valvesprings
In many cases, the methods mentioned above are perfectly acceptable means of stiffening up valvesprings; however, in some cases, these solutions may create their own set of problems. As the coils become thicker, often times the spring is not able to compress far enough to open the valve fully. In other words, there is no space left in between the individual coils. This situation known as coil bind can be avoided by taking careful measurements prior to installing any aftermarket springs though. Often times, the solution lays in dual or triple valvesprings. With smaller springs placed inside larger ones, stiffness may be increased while making use of thinner and smaller coils. Generally, the inner spring is inversely wound in comparison to the outer spring so as to avoid them tangling up with one another.
It’s important to note here that stiffer is not always better. Too stiff of a valvespring will make the camshaft work significantly harder to open the valve, resulting in a potential loss of power due to the added resistance. Camshaft lobes and rocker arm pads may also see additional wear due to the excessive loads placed upon them due to the stiffer spring.
Valve keepers, or collets, are a small yet intricate part of the valvetrain. Without them, things would fly all over the place. Their job is simple: to hold the valve, valvespring, and retainer together as one unit. They do so by fitting themselves into a machined groove near the tip of the valve. The pressure of the valvespring ensures they stay in position. In most cases, the factory-supplied keepers will do just fine. Be aware though that some aftermarket valve manufacturers require the use of their specialized keepers for a proper fit.