Correctly viewed, a pushrod engine's valvetrain assembly stands at the gateway of improved power. It is not a collection of components only intended to time and provide the correct valve motion. Rather, its proper operation should be considered satisfactory when it is characterized by minimal deflection and an ability to allow all components to functionally reflect the mechanical and design intentions of the camshaft. In fact, valvetrain performance is often a limiting factor in circle track engine power output.
These statements are as true for engines in the NASCAR Sprint Cup (NSC) as they are for those at your local Saturday night track. Small-block V-8 engines that are the foundation for the majority of circle track racing today were originally designed in the mid-50s to the early-60s, even though aftermarket parts designed for racing purposes have provided much-needed improvements. Keep in mind that the primary objective of engine designers at the time was to develop a low-cost package with competitive power, at the production engine level.
In reality, the small-block Chevrolet and small-block Ford V-8 engines are outstanding designs that have stood the test of time. But, by today's standards, they are not state-of-the-art in terms of what constitutes a competitive circle track engine. To a large measure, these are the same engines that Smokey worked with when he was racing in NASCAR, during the mid-to-late-50s part of his career.
A cross-section of a NASCAR Sprint Cup engine's valvetrain.
Today, newer pushrod V-8 racing engine design features include raised cams which shorten, and therefore allow for, stiffening of the pushrods. This provides for larger diameter camshafts which enable more aggressive cam designs. Current NSC engines have significantly raised centerline camshafts that are 60 mm in diameter. NASCAR now regulates both the height of the cam above the crankshaft centerline and the diameter of the camshaft journals themselves. The newest NASCAR engines (the GM R07.2 and Toyota Cup engines) are presently at the limit of these regulations. As you would expect and without regulation, cylinder block and camshaft designers would likely continue incrementally increasing the cam diameter and height (in the block) until packaging limits were reached.
Pushrod engines have inherent design compromises which were made to produce cost-effective power for production vehicles. This is a clear result of the ongoing challenge facing OEM engine designers; e.g., production costs vs. potential power output. Such compromise designs have significantly more component mass and less overall stiffness than overhead cam engines. These aforementioned compromises all combine to cause valves to follow the cam profile less precisely at high speed, certainly when compared to valvetrains characterized by higher stiffness.
One way to mathematically model a valvetrain is to consider it as an engineering problem that addresses vibrations. In this type of model, each component is represented as a spring, a mass and a damper. Because each component is essentially a "spring" (based on the fact it is compressible and elastic with regard to compression loads), this is an important concept to keep in mind.
The spring portion of the model represents the stiffness of the component. Theoretically, you would like to keep the mass of all the components as low as possible, while increasing stiffness. This would result in a valvetrain which follows the cam precisely (absent of any "flex" or distortion and loss of effectiveness). The pushrod valvetrain, as raced in most circle track engines, cannot accomplish this. Over the years, specialty parts manufacturers and engine builders have optimized the small-block valvetrain to perform exceptionally well in racing. For example, engine builders use the (flexing or compressible) pushrod to store energy like a spring, which effectively extends over the nose of the cam. Some have called this "loft."
Most rocker manufacturers now offer steel body rockers as an alternative to aluminum.
In reality, the purpose of valvetrain development is to make more power and accomplish this reliably. As we discussed in last month's series segment, improvements in gas exchange (cylinder filling or volumetric efficiency) is at the heart of power production. One way to improve power with the valvetrain is to open and close the valves in such a way to maximize volumetric efficiency and airflow, as rpm increases (particularly in the higher ranges).
Many Cup teams have taken a "high stiffness" approach to valvetrain development. The trend has been to increase the stiffness of the pushrod and rocker arm while using very aggressive lobe profiles to improve gas exchange and overall perform-ance.
For example, a current trend with Cup engines is to change from aluminum body rocker arms to steel body rockers, principally to improve stiffness. Most rocker manufacturers now offer steel body rockers as an alternative to aluminum.
However, stiff parts tend to have a bit more mass. As a consequence, more aggressive cams and increased mass of the stiffer rockers and pushrods require increased spring force (pressure) to keep everything under control. Generally speaking, increased weight leads to more "stored" energy at high rpm and greater difficulty in maintaining a given valvetrain's design parameters. While low mass (weight) and stiffness may seem diametrically opposed, you may want to err on the side of stiffness, especially when higher rpm comes into play.
Some Cup engine departments have been inspired by Formula One engine developments, focusing on power gains through valvetrain development. Instead of maximizing stiffness, teams maximize "specific" stiffness, or the stiffness-to-mass ratio. This tends to result in lighter components and the requirement for somewhat less spring force. Accordingly, a spring force reduction can result in lower frictional losses and net gains in power. Don't overlook this aspect in your Saturday night engine package.
On the subject of cam followers, flat tappets and roller tappets often pose different challenges which can result in the necessity for different valvetrain components. Here are some examples:
A flat-tappet valvetrain with a non-mushroom tappet and a tappet bore of fixed diameter has a geometric limit to how fast the cam (lobe) can move across the face of the tappet. If the velocity is too high, the cam lobe can run off the edge of the tappet.
For flat tappet engines, always use the largest tappet bore that packaging or regulations allow. Have the cam designed specifically for the tappet diameter you're using.
The equation for the maximum velocity of the lobe for a given tappet diameter is:
Vmax (inches/degree) = diameter (inches)/114.43
For a Cup tappet, NASCAR allows a maximum diameter of 0.875 inches. The maximum velocity is:
Vmax = 0.007654 inches/degree
Improving Maximum Valve Velocity with Flat-Tappet Valvetrains Traditional NSC valvetrains use the deflection of the system as a spring to store energy which is then used to accelerate the movement of the valve. (Think of this deflection motion as what you would see in a flexible "pole vaulting" pole, as viewed at a track meet. In the case of an engine, energy is stored in the overall valvetrain, not just the pushrod.) As the overall stiffness of the valvetrain is increased, other methods must be used to increase valve velocity and acceleration to achieve equivalent engine performance.
As an example, increasing rocker ratio is one way to increase valve velocity without breaking the speed limit of the flat-tappet. Some Cup engines have used rocker ratios as high as 2.5:1. Most teams are running ratios in the 2.1:1 to 2.3:1 range.
Increased Rocker Ratio has the following effects:
* Increase valve velocity and acceleration
* More aggressive valve motion
* Lower rocker Moment of Inertia (MOI)
* Higher pushrod loads
* Higher rocker arm loads
* Less camshaft lobe lift
Increasing the rocker ratio 25 percent increases the maximum velocity of the valve by 25 percent (for the same lobe) as shown in Graph B (page 38).
The net effect of increasing rocker ratio is as follows: As the rocker ratio is increased, the pushrod lever arm that opens the valve is shortened. This increases pushrod loads and may cause problems of galling at the rocker arm end of the pushrod. As a result, stiffer pushrods may be required.
In addition (when using high ratio rocker arms as shown in Graphs A and B), lobe design and cam grinding becomes much more important (critical for power). For a given valve lift, as rocker ratio increases, lobe lift decreases. As a result, any errors in camshaft grinding will be amplified by the higher rocker ratio. Finally, with high pushrod loads, there is a benefit to have the pushrods as straight as possible, thereby reducing bending loads.
Roller-tappet valvetrains do not have the same geometric limits as those using flat-tappets. For example, the cam cannot run off the edge of the roller face as it can with a flat tappet.
Without having to worry about the speed limit at the tappet, the correct approach is to put as much lift at the lobe as possible. In this regard, the diameter of the cam journals then becomes the limiting factor. So, use the largest diameter cam core that the rules will allow. Have the lobe designed specifically for this tappet wheel size and core. Larger base circles also increase the maximum radius of the curvature for the lobe.
As the lifter roller wheel diameter is increased, the lobe designer can design a more aggressive cam.
Roller cams will require significantly higher spring loads than flat-tappet cams, so start your considerations with the valve.
Make the valve as light as the manufacturer recommends for your application. Every gram of mass removed from the valve makes the valvetrain easier to be controlled by the spring. Less force (load) means that the components don't have to be as strong, which therefore means they can be lightened for another cycle of force reductions.
Intake valves with head sizes of 2.200 inches have been successfully run in Cup, using both 7mm (hollow) and 6mm (solid) stems. The large diameter hollow stem provides improved bending stiffness, when compared to the solid stem, but is more complex and can be more expensive. At extreme (high) airflow rates, the smaller diameters can have an incremental improvement in airflow. Be careful not to split hairs on this issue.
Cup engines have run 7mm hollow exhaust valves. And, as you would expect, sodium filling the exhaust valves improves their ability to survive at extremely high temperatures. When leaner air/fuel ratios are run (to reduce fuel consumption), sodium-filled valves are more likely to survive. This is especially (and obviously) true in longer races.
Don't forget the retainers, when looking to improve performance or durability. Depending on your application, appropriate choices include steel, titanium, or even aluminum. Consultation with your chosen retainer manufacturer never hurts. At some point, you can assume they know more about their product than you-although not always. Lightweight metal may need a wear coating to survive, although some coatings can add significantly to retainer mass. Be sure to factor this in when evaluating the overall mass (weight) among different types of retainers.
Today, many Cup teams are using steel rockers because of their improved stiffness. Teams are also removing the adjusters from the rocker for a 10- to 20-percent reduction in MOI. Lash adjustment is accomplished either with a cam at the pivot point or by changing lash-cap thickness.
Here is a rendering of an F1-Inspired Steel Rocker from Del West.
Del West Engineering has been showcasing rockers with a Diamond-Like Coating (DLC), cam-shaped tip. Under the correct conditions, a DLC coating (when run on an oil film) can have similar friction characteristics when compared to a conventional roller-tip. From a dynamics perspective, the roller can add significant weight to the valve side of the rocker arm. Non-roller tips can materially reduce rocker MOI. Removing mass from the valve side of the rocker has an effect similar to reducing overall valve mass.
The three highest stressed components in a racing engine are the valve springs, the connecting rods and the pistons. Improvements in spring materials or technology usually result in improved engine performance. In order to optimize valve spring function and durability, spring wire requires the cleanest possible steels to prevent internal defects called inclusions. Advanced heat-treating processes are also required to achieve optimal performance. At the end of the day, the valve spring should be one of the most carefully selected components in the engine build. Simply put, do not cut corners on valve springs.
As engine speeds increase, higher "natural frequency" springs are needed in order to maintain valvetrain stability. Higher natural frequency generally comes from high spring rates. The spring must have enough open pressure to keep the valvetrain under control and sufficient spring pressure to prevent valve bounce at high speed. However, there is a balance between spring pressure and frictional losses, simply because running too much pressure will increase the forces a valvetrain must handle and increase net friction. Running a spring with less than optimal open spring force may reduce the maximum speed the engine will turn safely but could lead to parts failures.
Selecting the correct spring is one of the most critical aspects of valvetrain development. Without a Spintron (or comparable spin fixture) to develop your valvetrain, it is best to seek advice from the cam designer and spring supplier. Once again, don't overlook the fact they designed the part.
Changing one part in the "system" may affect the performance of the valvetrain as a whole. Here's an example: Improving pushrod stiffness will reduce pushrod deflection, thereby increasing the duration of the valve events in the same way achieved by installing a cam with longer duration as shown in Graph C on page 40. If the cam was optimized for the original valvetrain and stiffer pushrods were installed in the engine, it would behave as if the cam was too large.
Do not assume that the lift or duration of your valvetrain (as measured statically) stays constant with engine speed which is in a dynamic environment. In fact, valve motion for pushrod valvetrains changes with engine speed. Graphs D & E show how both lift and duration change with engine speed, for a NSC-type valvetrain. The changes you'll note in duration and lift are due to the deflection in the valvetrain. And, although Saturday night engine packages don't compare directly with these data, you can expect similar tendencies to exist and should consider how "flexible" valvetrains can impact your particular engine.
At the outset of this series, we promised to provoke some thoughts, along with providing you a few ways to think about (or re-think) your approach to building weekly racing engines. Obviously, not all aspects of Cup engines apply to making power with mainstream circle track applications. But if we have been able to shed some useful tidbits that'll save some money and time while putting you nearer the checkered flag, mission accomplished.
It would be foolish to assume we've addressed all your concerns or interests. In an "extended classroom" like CT's forum, you can't raise your hand with a question or comment. But in this case, let e-mail to CT become your channel of communication, and we'll do our best to either respond directly or build a piece of editorial that expands on the information you're trying to nail down. And don't forget the quote from Smokey we previously shared, "To finish first, you damn well first have to finish." Meanwhile, don't spin out.
A Spintron is a fixture which spins the valvetrain while measuring valve motion. This Spin
With advent of "spin machines" and dynamic analysis of valve motion, much more is known today about valvetrain technology than in years past. While a considerable amount of this technology has been advanced by the engineering staff at COMP Cams, there are some fundamental issues the Saturday Night engine builder (and racer) should come to recognize. If for no other reason than participants in these types of racing classes don't always follow the directions provided by experienced valvetrain and related component providers, it may be helpful to discuss some basic topics.
For whatever reason (improper valve spring pressure, incorrect match of cam lobe profile to spring combination, wrong camshaft break-in procedure, excessive engine speed, etc.), the inability of a valve to seat and remain seated upon initial closing contact is one concern. Separation of lifter and lobe at maximum lift is another.
In the first case, it's critical that a valve remain in contact with its seat the first time they come into contact, during a closing event. On the intake side, failure to accomplish this leads to a reduction in net inlet cylinder and combustion pressures, leading to lost power. This is a consequence aside from any mechanical damage that can occur, particularly to valve springs that already operate in a dynamically hostile environment. During improper exhaust valve closing, both volumetric efficiency and reduced combustion pressure (power again) stand to suffer. When lifter/lobe separation occurs over the nose of the camshaft lobe, the resulting "loft" can materially alter closing-side dynamics and valve timing, neither of which is good for power or parts longevity-particularly valve spring life.
In the application of a circle track engine, there are times when engines are subjected to over-speed conditions. Knowledgeable engine builders will tell you valve spring pressure (and life) diminish very rapidly after a condition of valve "bounce" occurs only once. In fact, the rpm at which valve control instability happens is reduced each time the condition is caused. What first occurred at 7,500 rpm, for example, may next occur at 7,300, then 7,000 and then even less.
Also critical is valve spring selection and installation. These seemingly-simple components are subjected to some of the most severe treatment in a racing engine that includes high operating temperatures and structural issues. Consequently, it's important to follow not only camshaft manufacturer recommendations regarding specific spring selection, but also making certain all recommended installation procedures are in compliance is equally vital to proper spring operation and longevity.
If you have any doubt an engine is or has been experiencing valve control stability problems, take the time to examine the participating components. Look for areas of abnormal wear on pushrod ends, rocker arm pushrod cups, interference marks on spring coils, "hammered" spring pockets and/or shims, damaged valve faces and seats, excessive guide wear-even interference between piston crowns and valve heads. Recheck seated and open valve spring pressures to make certain they're within recommended specifications. And, if any of these abnormalities are detected, identify the cause and take corrective steps. In this case, an ounce of prevention could help avoid pounds of damaged engine parts.
According to COMP Cams engineer Billy Godbold, "There are several fundamental valvetrain topics we feel the Sportsman racer should know about and understand that relate to our Cup engine research. These pertain to valvetrain mass and stiffness, valve spring weight, component material selection, the criticality of coil bind and the matching of system components.
"With respect to valvetrain mass and stiffness, most Sportsman racers don't own a gram weight scale. Not only should they have one, but be in the practice of weighing every component in the valvetrain. This includes valves, springs, retainers and locks. Actually, every gram that can be removed from these parts will allow faster lobe profiles, more engine speed and lower valve spring loads. All of these will reduce friction horsepower.
"We equate the importance of valvetrain stiffness with valvetrain component weight reduction. Our testing has revealed component deflection can cause up to 20 degrees of valve duration loss. Even over a range of only 2,000 rpm, duration losses from deflection of parts can amount to 5-10 degrees. Because of this deflection, cams need to become 'too big' at low rpm and then (because of component deflection) are 'too small' at higher engine speeds. By increasing push-rod o.d., cam journal size and rocker arm stiffness, engines will perform better at both low and high rpm since cams will then run closer to their design specifications.
"Cup engine development has really provided something here for the Sportsman racer. Despite what we've previously believed about heavy valve springs, it's now apparent (when the issue of valvetrain stiffness is properly addressed) the smallest spring that will control the system is the best choice. Excessive spring loads result in heavier springs (or the opposite is true), and the latter aggravates valvetrain deflection and friction horsepower losses.
"With respect to valvetrain component material selection, the Sportsman racer now has an opportunity (based on Cup engine investigations) to select from tool steel spring retainers that simply perform comparably to aluminum or titanium parts, at a penalty of only about 1-2 grams of weight. These are well suited to Sportsman-type engines because they're more affordable and increase valvetrain performance over time, owing primarily to the fact they help maintain the distance to coil bind longer.
"And speaking of coil bind, we've found this to be almost (if not more) important than spring load. This is because much of a spring's internal dampening occurs at maximum valve lift, near coil bind. If excessive clearance is provided (short of coil bind), the spring is not able to provide adequate harmonics damping. In Sportsman engine applications, a clearance range from 0.050-0.100-inch from coil bind seems to work best. The more controlled a valvetrain becomes, the closer to spring coil bind it likes to be run.
"Sportsman racers cannot individually afford the amount of dynamic testing required for Cup engines. However, they can benefit from some of the results these tests produce. Whenever possible, it's best to work with a camshaft (valvetrain component) manufacturer who is involved in these type test programs. At COMP, we've spent an inordinate amount of time and resources in an ongoing effort to be involved with such activities. And, wherever possible, we've allowed the knowledge thus gained to trickle down into our products that relate directly to the Sportsman racer community.
"I also think that it's important to note how easily a given valve spring assembly (springs, retainers and locks) can be taken apart. If it takes a sharp rap with a hammer to separate retainers from locks, suspect there're valve control issues afoot. But if it only requires a light tap, chances are good no problems exist. Sportsman racers should pay particular attention to wear at the interface between spring and retainer. For these engine packages, this is one of the first areas to exhibit abnormal wear."
As mentioned in a previous part of this series, knowledge continues to be power-including horsepower. The breadth of an understanding about virtually any topic creates advantages. In the case of this series of stories, you have an opportunity to benefit from the investments Cup teams have and continue to make in their respective programs. Learn, enjoy and use to your racing advantage.