The three most important areas...
The three most important areas for engine performance improvement are the cylinder heads, valvetrain, and intake manifold.
Remember, heads that flow...
Remember, heads that flow the most are not necessarily the best for your application.
As referenced in the story,...
As referenced in the story, this chart compared a typical trace for an intake valves motion and the multiple excursions of pressure (spikes) that occur in an intake path (cylinder head and intake manifold runner) prior to and during an inlet cycle. Note not only range of pressure (minimum and maximum values) but also nature of pressure history during intake cycle (crank angle from about 300-620 degrees as shown in illustration).
This illustration compares...
This illustration compares a typical intake valve motion and mass flow during filling event. Particularly note reduction in volumetric efficiency around peak valve lift, suggesting flow gains in this area are of lesser significance than at low-lift values.
Largely because of blow-down...
Largely because of blow-down pressure (residual combustion pressure at the time of exhaust valve opening), this illustration shows period of a typical exhaust valve cycle when exhaust flow is the greatest. Compared to the closing side of an intake valve event, note how little flow occurs during exhaust closing.
NOTE: The purpose of this...
NOTE: The purpose of this illustration is to show how valve loft changes as engine speed increases. It can be seen that the point of peak loft, compared to actual maximum lobe lift, varies not only in amount but at points not symmetrical with the design lobe lift. While some cam manufacturers assert benefits from some amount of loft, this sort of valve motion cannot be considered consistent with lobe design and an indication of varying valve motion.
Note here that peak cylinder...
Note here that peak cylinder pressure occurs past TDC and (not shown) that this point is developed farther past TDC as engine speed increases ... something to keep in mind when either tailoring ignition spark curves, setting initial timing, or both.
From Street Stock to Winston Cup, the three most important areas for engine performance improvements are cylinder heads, valvetrains, and intake manifolds. In particular, cylinder heads affect most aspects of power. Ports, valve size, and placement affect engine breathing or volumetric efficiency. Thermodynamic and combustion efficiencies are affected by the volume and shape of the combustion chamber. Valvetrain performance is affected by placement of valves relative to the rockers. And the water jacket design can simultaneously affect cooling performance, power, and overall engine durability.
First, its important to understand cylinder heads that flow the most air on the flow bench may not make the most power or perform best on the track. Cylinder head flow gains are not an end unto themselves. For circle-track racing, a primary goal is to pack as much mass of air and fuel into the cylinder as possible while maintaining the highest port velocities. This will not necessarily occur with a head that flows the most air on a bench.
Process of Evolution
Upon evolution of an economical airflow stand or bench, engine builders were quick to see the tool was an effective way to quantify airflow performance of one port relative to another. The flow bench is most effective when used to develop cylinder heads that are closely related to production parts. The reason is that airflow performance of a production part is so restricted that any gains in flow will most likely increase engine power.
Most flow bench testing is performed at a depression of 28 inches of water. At best, this is accurate only twice during a full inlet cycle (See Chart #1) This chart shows an inlet port that sees negative pressures up to 2.6 psi (73 inches of water), much higher than the 28 inches of water used on an airflow bench. A true representation of cylinder head flow performance would be using a three-dimensional plot of flow at varying lifts and depressions. Flow benches become less effective as cylinder head design is less restricted and mass flow increases. For example, heads developed for CART, IRL, Pro Stock drag racing, or Outlaw sprinter engines are less likely to be successfully developed on a standard airflow bench.
Further, an intake port must flow fuel and air. Engines running methanol operate at air/fuel ratios more than twice the richness of those on gasoline. The large quantity of fuel, much heavier than air, clearly impacts the performance of an intake port. Wet flow benches that can operate at much higher flow depressions make a better tool for such engines. Some engine builders working on these engines simply use the dynamometer to test cylinder head improvements. The flow bench becomes a quality control tool, much like a go/no-go gauge, to verify a finished head compares to its prototype. The point here is that the Saturday-night racer should not become overly dependent on airflow benches. The dynamometer is a superior engine evaluation tool.
On The Intake Side
Where in the valve event does flow take place? Study the chart, noting intake mass flow vs. crankshaft angle. In this case, one area on which to work is at low valve lifts. Since the piston has started moving downward on the intake stroke, there is already negative cylinder pressure as the intake valve cracks open, causing a jump in mass flow.
Note that the rate of mass flow increase flattens at high lifts, so increasing valve lift does not appear a good option for more flow. Also note that some reversion takes place. (This example was taken from an engine speed near peak torque.) A compromise in the valve events must occur to have good power at higher engine speeds. The reversion will disappear at power peak.
So, now how do you improve the cylinder head? Even if you dont have a flow bench, its possible to improve performance of the basic head. The easiest way to improve low-lift flow is to work in the valve seat area. Blending from the seat to the bowl (without damaging the seat) will increase low-lift flow. Using a radius-type valve seat cutter with a live pilot can produce better flow than traditional types of valve seat machines. If youre having a shop cut seats for you, find personnel with experience in this type of equipment.
Back-cutting valves is usually another low-lift flow gain method. If you have access to a valve seat re-facer, start with a 30-degree back-cut for Street Stock-type applications. Position the back-cut so it narrows the 45-degree seat to match the valve seats width.
In addition, most production cylinder heads have excess material around the valve guide, due to casting (draft) requirements. Faring the guide boss into an airfoil cross section and removing material in the bowl area immediately adjacent to the guide is usually productive. When removing material, go slowly.
Along the intake and exhaust paths, developing and maintaining optimum flow energy (momentum) is important to net volumetric efficiency. This is an important consideration, and heres why ... along with some food for thought.
It takes energy to accelerate air. It also requires a given amount of energy to move air along a constant cross-section duct. If the duct (passage) expands, the air will slow. If the duct then contracts after the expansion, the air will accelerate. This acceleration robs the air stream of energy. And on the intake side, this alternatively changing rate of flow can hamper air and fuel ability to stay mixed (homogeneity). Many novice porters make the head opening larger on the pushrod side of the port. This allows the intake charge to expand and then contract to clear the pushrod, exacerbating the problem.
Remember, the goal of intake port development is to fill the cylinder as full and as efficiently as possible. Energy wasted in re-accelerating the intake charge after a duct area expansion is not helping fill the cylinder. This issue also speaks to the subject of maintaining passages of fairly equal cross-section, or at least those that dont vary in much area along a given flow path.
What Is Momentum?
Simplified, momentum is a measure of energy. Mathematically, it is a product of a body (or system) mass and linear velocity. In an inlet path, were dealing with a system (air and fuel) of different mass trying to move at the same velocity. Changes in either mass or flow rate will change this systems momentum, thereby causing a companion change in the systems energy level.
More simply stated, if air/fuel charges moving along an inlet path experience a change in momentum (energy) by alternatively accelerating or decelerating their velocity (by companion changes in cross-section area), either of two primary conditions may occur: (1) Air and fuel may mechanically separate and/or (2) net energy may be lost to optimizing cylinder filling or volumetric efficiency. Both are undesirable.
While it may not be possible to maintain zero changes in momentum along an inlet path, particularly since were dealing with bi-directional and unsteady flow, keeping these changes to a minimum will help boost cylinder filling and net torque.
On The Exhaust Side
In similar fashion, the exhaust side of a cylinder head responds much like the intake. For production-based engines, the exhaust valve responds well to a radius from the face to the edge. Back-cuts should also be tried. Work the bowl and guide areas similar to the intake. A generous radius from the seat to the short turn is best. Try not to remove metal from the floor of most production heads.
Reverse flow Many anti-reversion devices have been tried to keep the direction of the charge (or flow) constant. This theory would be interesting if the anti-reversion device could be timed to prevent reverse flow only when it was undesirable. (This statement assumes reverse flow is desirable.)
We do know that scavenging and sonic tuning must have an effect since racing engines routinely have volumetric efficiency in excess of 100 percent. If scavenging or sonic tuning does occur, then the flow is pulsing in the intake and exhaust tract and going in both directions (not simultaneously) for benefit. If reverse flow can assist scavenging and sonic tuning, then the reverse flow of the port must be of some significance.
In most cases, it turns out that modifications that improve forward flow also improve reverse flow, and it would not be a good idea to sacrifice forward flow for reverse flow. Nevertheless, it is good practice to flow ports in both directions. Any modification that can be made to improve reverse without affecting forward flow would be a good idea, but tuning changes may be required for optimization of any change.
What Are Overlap And Scavenging?
For a variety of reasons, including improved volumetric efficiency (v.e.) at high rpm, camshafts may be designed to delay exhaust valve closing to overlap when its corresponding intake valve begins to open. During such time, incoming air/fuel charges may pass directly into a cylinder and out through its exhaust. In one sense, fresh air/fuel charges may be lost from combustion. In another, unburnable exhaust gas from the preceding firing cycle may be removed from the cylinder. If we define scavenging as the process enabled by intake and exhaust event overlap, its possible to examine how volumetric efficiency greater than 100 percent can be achieved.
Over time, numerous terms have been used to describe this process. Whether called sonic tuning, wave tuning, ram tuning, or some other applicable term, the objective is to achieve a greater volume of air/fuel charge filling of a cylinder (at the time of ignition) than could be achieved under the singular influence of atmospheric pressure. While the term scavenging often applies to two-stroke-cycle engines (particularly scavenging ratio and scavenging efficiency), the ability of a four-stroke-cycle engine to utilize some form of pressure excursions/dynamics to enhance v.e. involves valve timing and dimensional properties of the intake and exhaust systems.
Keep in mind that the point at which the intake valve opens is critical to how much residual cylinder pressure (and exhaust gas) is present. As opposed to the suggested benefits of bi-directional flow (in the passage tuning process), this initial reverse flow or reversion pulse can dilute fresh air/fuel charges subsequently flowing into the cylinder. Exhaust flow efficiency also affects the residual volume and pressure of unburnable gas available to the intake system at inlet valve opening. It is beyond this initial point that bi-directional flow may produce benefits to the tuning process.
Ideally, the scavenging process would enable replacement of exhaust gases with fresh air/fuel charges without any loss of such charges to the exhaust system. In reality, due to multiple and highly complex factors, optimum scavenging needs to be timed with engine speed closely associated to the rpm most often used. Even the most comprehensive of contemporary computer modeling systems require final analysis on an engine dynamometer (often of inertia type) and on-track evaluation. To further discuss specifics of the issue is beyond the scope of this particular story. But it is fundamental to camshaft, intake, and exhaust system parts selection that components be chosen that perform in a pre-determined range of engine speed (and as a package), else the benefits of both overlap and scavenging will not be achieved.
The Multi-Functions Of Cylinder Heads
Too often, it seems cylinder heads are viewed only for their ability to help achieve high or increased airflow. If fact, although these parts can contribute significantly to total airflow and specific flow patterns, it is in the combustion space that any quantities of air (or air/fuel charges) must be converted into useful heat. So in the design or modification of intake and exhaust ports and combustion chambers, it is particularly important to note how inlet air is being delivered to the combustion space.
Various terms have been used to describe the quality of inlet air. Based on the relationship between intake port flow path and cylinder bore, there will be certain airflow characteristics dictated by this relationship. For example, an inlet port may be positioned (or modified) to either increase or decrease both the circular and downward motion of flow as it enters a given cylinder bore ... somewhat like twisting or screwing the air column down into the cylinder.
If an intake port is created that causes air/fuel charges to be forced against the inside diameter (id) of a cylinder bore, mechanical separation of air and fuel may occur, thereby leading to reduced mixture quality and combustion efficiency. In a sense, intake and exhaust ports need to enhance flow into and out of the combustion space, not just show substantial quantities of flow on an air bench.
Top motorsports teams often utilize computer simulations of air movement along an inlet path and into the combustion space. In the design or investigation stage, multi-dimensional computer programs such as Computational Fluid Dynamics (CFD) can be used to construct three-dimensional models enabling rapid analysis of various changes to portions or functions of an engine. Even though some of this technology is still in a developmental state, sufficient precision exists to include this method in developing cylinder heads, intake manifolds, and related components.
What this says to the Saturday-night engine builder is there is more to airflow than quantity. In previous Circle Track tech articles, the importance of airflow quality has been discussed and stressed. Beyond the benefits of CNC-machining processes and the ability to duplicate ports and combustion chambers designed or modified by hand, its important to determine port and combustion shapes optimal to combustion efficiency and power before applying the techniques of CNC-machining.
As crude as the process may appear on the surface, the use of spray patterns during air bench port and chamber analysis and/or modification remains a powerful tool to the Saturday-night engine builder seeking to derive some of the benefits of CFD from a toolbox or non-computerized approach. In the process of using this method, creating ports whose airflow quality helps reduce air/fuel mixture separation in the combustion space (as evidenced by dye patterns showing streaks of liquid fuel on chamber or cylinder walls) while still optimizing net flow will typically lead to solid power gains.
Valve un-shroudingOne of the easiest ways to improve airflow performance of a cylinder head is to un-shroud the valves. The simplest way to do this is place the head on as big a bore as practical for your particular engine. It is safe to say the more a valve is lifted from its seat, the greater the distance it should be from the chamber and cylinder walls. Canted valve heads and hemispherical heads naturally have this feature. Typically, the intake side is more sensitive than the exhaust, and the long side is more important than the short side. Obviously, there are physical limitations to what can be done, but compromising the combustion performance of the chamber for greater flow is not an acceptable trade-off.
Other Valvetrain Issues
In Winston Cup racing today, a key to improving engine performance is to improve performance of the valvetrain. A significant portion of Cup team engine development effort is spent working with valvetrain spin fixtures. These devices allow gross valve motion to be measured at different simulated engine speeds.
What quickly becomes obvious is valve timing and lift effectively change with engine speed (Note Chart 4). Durations measured at 0.050 and 0.100 inch regularly vary 10-15 degrees from low to high rpm. Loft (additional lift from lobe-follower separation at maximum lift) can exceed 0.080 inch, on some roller profiles.
While most racers dont have access to a spin fixture, it is clear that comparing camshafts by looking at 0.050 inch duration numbers and lift is like comparing cylinder heads by valve size. What the Saturday-night racer should do is compare cams at multiple durations like 0.004, 0.020, 0.050, 0.100, 0.150, 0.200, and 0.250 inch. This type comparison will produce a better understanding of how the lobe is likely to perform.
What should you look for? Modern cam profiles have significantly higher low-lift acceleration rates. This means the duration on the seat and at 0.050 inch will actually be smaller, but the lobe will have the same duration at 0.250 inch. This type profile allows you to use a smaller cam for torque but will still make power where expected.
Talk to the cam manufacturer and ask about the dynamic performance of the lobe. You want to know its limit speed. The limit speed is the speed beyond which the valvetrain cannot be turned any faster. This should be much greater than the maximum intended engine speed. Ask at what engine speed does significant bounce occur. This speed should just exceed maximum anticipated engine speed. It is important to know exactly what components were used during the manufacturers spin fixture analysis. If your valvetrain varies from what was tested, the data may not relate to your particular engine.
How do components affect dynamic operation of the valvetrain? In general, you want to remove as much mass as possible from any part on the valve side of the rocker fulcrum (valve, retainer, and locks). This allows the spring to do a better job, helps reduce bounce, and increases limit speed. On the lifter side of the fulcrum, stiffness becomes more important than mass (weight). A stiffer pushrod will follow the lobe better. Reducing the mass of the lifter and/or pushrod will reduce loft, increasing mass will increase loft.
Most of us dont have access to a spin fixture, so work with your cam manufacturer to select the correct valvetrain components. Remember, the valvetrain is a system, and changing any component can adversely affect the performance of that system.
For serious circle track engines, valvesprings are life. A better spring will allow a more aggressive valvetrain. This, in turn, will make more power. Always use the minimum spring pressure that can be safely run. More spring pressure than required will generate heat and rob power. Keep track of the installed pressure of springs, before and after every race. Replace all the springs at the first sign of a significant decrease in pressure.
Treat valvesprings with care. Dont nick or scratch them because this may affect their durability. In high rpm operation, valvesprings generate tremendous heat. If spring durability becomes an issue, valvespring oilers can help but will increase oil flow that can cost power.
Deeper Into The Valvetrain
Knowing the basics is not sufficient. Equally important is understanding what areas in power development are affected by the valve function. Without this knowledge, power may be lost in the quest for simply doing whats necessary to prolong valvetrain life. Depending upon the class of engine, the length of time between planned valvetrain maintenance (particularly valvespring inspection and replacement) may range from after every race to not unless something fails. Regardless, the following information applies.
By their motion, valvesprings generate heat. It is therefore important that they receive adequate oil for cooling. Also, in the process of being compressed and extended, they never come to rest. Even when valves are seated, residual harmonics or compression and extension of spring coils within a given stack continue between times valves are being opened or closed. These movements are not always along the axis of a given spring, resulting in lateral distortion that accompanies axial motion. This produces components of spring motion that are counter to what might be called pure spring compression and extension.
Despite the efforts of camshaft designers to provide stable valve motion during low-lift opening and closing points, there remains a system of flexible parts between the valve tip and cam lobe (pushrods, springs, and rockers). These components contribute to valve motion that is not true to the cams lobe profiles, thereby producing lift patterns less than what is designed into a given camshaft. During valve seating, reducing the elastic collision between valve heads and seats is an ongoing problem. Engine builders can help the situation by making certain manufacturer-recommended spring installation and pressures are applied throughout the life of a given spring.
Springs should be sufficiently stiff to control valve motion approaching maximum lift as well as during its acceleration to closing. As engine speed is increased, the more important spring forces required to control valve motion are during maximum valve acceleration. Boiled down to its essentials, this points to the importance of maintaining sufficient spring pressure just prior to maximum valve lift in order to maintain lobe/follower contact at and just beyond peak lift. Even though multiple springs can add net pressure to a given spring package, this approach is also important to controlling surge by combining different natural frequencies of individual springs. Spring design that includes so-called beehive or tapered shapes can also provide desirable harmonic damping.
Technology Transfer, Part I
Technology Transfer, Part II