This month's installment deals...
This month's installment deals with the short-block and its components. The NASCAR Winston Cup engine shown here benefits from meticulous parts selection and has excellent repeatability from engine to engine. Your short-track engine (below) can benefit from the same meticulousness.
Editor's Note: This month we dig into the short-block and its major companion parts. Included will be crankshafts, connecting rods, pistons, and rings. Let's begin with the block itself.
The Block The block is the foundation for the engine, so inspection of the casting is critical. Sonic-checking is a service available at most machine shops today and is critical to ensure stability of the block. The small-block Chevy and small-block Ford each have areas where the castings are traditionally thin. You should rely on someone with experience with your type of engine to check the block to make sure it is acceptable. For example, a small-block Chevy is traditionally thin at the base of cylinder No. 7 and should never be any thinner than 0.140 inch for Late Model Stock applications. Engines with less power can afford to be 0.125 inches at the minimum.
Lifter bores must be checked for proper position since "misplacement" or poor "indexing" is common in as-cast conditions, thereby affecting valve timing and valvetrain geometry. Blocks that were not specifically designed for motorsports applications are particularly problematic. Line bore and hone the mains to proper size and make sure the deck is at a true 90-degrees-to-the-crank centerline. If the block you have is of unknown history, it is also a good idea to check the distance from the crank centerline to the cam centerline because this can move (or change) with repeated line boring.
Cylinder deck height should also be checked. The deck height can be adjusted within a small window to optimize a given engine configuration. For example, when hood height severely restricts the induction system performance, running the minimum deck can create some room. This is usually not an option for budget engine builds. Deburr the block and lighten it as much as the rules allow.
Crankshaft Buy the best parts that fit into your budget. Higher engine speeds or an engine that must run a full season without a rebuild require better parts. Remember, when working on a limited budget, the money should go into valvetrain, cylinder heads, and intakes for power. Lighter cranks will allow the engine to accelerate faster and can ease the load on the bearings, but they tend to cost more and may have a shorter service life.
How Rotational And Reciprocating Mass Affect Engine Acceleration
Think of rotating crankshaft and reciprocating piston/rod assemblies as flywheels. The greater the total mass, the more resistance offered to changes in system acceleration (or deceleration). Also, the shorter the stroke, the less the mass moment of inertia or resistance to a change in rpm. In the so-called "slider-crank" mechanism of the crankshaft (weight and stroke length), connecting rod (length and weight), and piston (weight and pin position), changing the energy state of this system is a function of weight and leverages. Keep in mind that changes of rotational or reciprocating speed requires energy that might otherwise be available to accelerate a race car.
Journal Sizes The trend in most forms of motorsports is to run smaller and smaller journal sizes where rules permit. This not for budget builds but will benefit high-speed engines the most. The reduction in journal radius reduces the speed of the journal at any engine speed and can reduce friction. NASCAR has limited rod journals to a minimum of 1.847 inches. If this size will work to support an 800hp Cup engine, it may be suitable in yours. Only forged cranks specifically designed for these journals can handle the loads, so don't try cutting down your cast crank to run Honda bearings.
Note: Take a moment to study...
Note: Take a moment to study this illustration. Typically, peak cylinder pressure occurs slightly past TDC piston position, delaying further with increases in rpm. As ring seal deteriorates, both peak cylinder pressure (P1, P2) and the crank angle (CA1, CA2) at which it occurs is also decreased. See story for additional comments.
Journal Size And Bearing Speed
It's a complex subject. Simply stated, there is a trade-off among features of strength, durability, weight, heat, and friction. Small journals reduce friction horsepower losses present, an opportunity to run smaller journal and lighter weight rods. They accelerate more quickly but also offer less strength, higher unit loadings, and increased oil temperature (at the bearing surface interfaces). Typically, the highest unit loading occurs at or just past TDC (depending upon combustion conditions), and the greatest rate of change in surface friction appears at or near maximum piston velocity (in the range of 60-75 degrees past TDC during the power stroke). Generally, engines required to operate over a wide range of rpm seem to benefit more from smaller connecting rod journals than those in a narrow span of engine speed. In making decisions about journal size, the larger the diameter, the greater the bearing speed (relative motion between journal surface and bearing surface during any interval of rpm). As journal diameter is decreased, unit loading (psi) increases with bearing surface area, oil temperature can rise (cooling ability decreases) while the rotational weight reduction aids crankshaft acceleration. Consider these and related effects before making a change.
Connecting Rods Again, buy the best parts that fit into your budget. Higher engine speeds require the best parts. Even budget builds should use premium rod bolts. Rod lengths and their affects on the engine cycle and performance will be discussed in a future article.
Pin Oiling Some rod manufacturers offer a modification where an oil passage is bored by electrical discharge machining (EDM) from the journal though the beam of the rod to oil the pin. The oil passage in the rod supplies pressurized oil to lubricate the pin. This type of rod should only be considered if pin wear or galling is observed in previous builds or is a concern. If the EDM operation is not performed with care, the process may initiate cracks. Adding pressurized pin oil will increase windage losses.
Wristpin problems are common in high-rpm, high-horsepower engines. The loads placed on the pin tend to cause two primary types of distortion: pure bending and hoop bending. Pure bending, as the name implies, tries to bend the pin along its length. Hoop bending tries to distort the pin radially in the pin bores of the piston.
Pins are an example of where modulus is more important than mass. If the pin is not stiff enough, at best it will deform and cause power to be consumed by friction. In a worst-case scenario, hoop bending will cause the pin to stick in the pin bore or pure bending will cause the pin to seize in the small end of the rod.
The key is to put the mass in the right place. Increasing the overall diameter will improve the pure bending performance of a pin. Reducing the overall diameter while increasing wall thickness will improve the hoop bending performance of a pin. Obviously, it is not possible to do both, so it is important to understand how the pin is being loaded to pick the best part. Work with your piston and rod supplier to better understand these issues and find the best pin for your application. This can be a critical part.
What is Modulus and Why Is It Important?
Here, this term refers to a modulus of elasticity. By one definition, it is a mathematical ratio of unit stress to unit deformation of a material and is a constant so long as the material is not loaded into permanent deformation (proportional limit). What does this mean to an engine builder? It means that parts (not always metal) under repetitious loading will return to their original shape once the load is removed. Sometimes, this is called "memory." Comparing an example sample of steel to aluminum, the value of E (modulus of elasticity) for steel might be 29,000,000 psi and only 10,300,000 for aluminum.
In engine building (particularly parts selection), there are times when the ability of a given part to be repeatedly loaded within its limit of elasticity is more important than the static (or dynamic) weight of the part. For example, lightweight piston pins that are operated outside their modulus of elasticity have a higher cyclic failure probability than, perhaps, a heavier pin that is continually loaded within its modulus of elasticity.
Pin Coatings Diamond-like coatings (DLCs) can be very effective in preventing pins from galling the pin bores. When coated with lubricant, these coatings have an extremely low coefficient of friction. Not all DLCs are the same and coating retention can become an issue. It is best to purchase the pins already coated. Coatings are a last line of defense and should be used as such, not relied upon to fix an insufficient design problem.
What is the cylinder kit? The cylinder kit is the system of parts, including the piston, rings, and cylinder bore, which serve as components working together to seal the combustion gasses from the sump (crankcase) and transfer the chemical energy (combustion) into mechanical energy (torque) with as few losses as possible. The pin and connecting rod are generally included in this system.
In order to minimize friction and maximize power we must look at the piston, rings, and cylinder bore and their interaction as one system, since a change in one component can influence the entire system. Friction reduction in the cylinder kit area can lead to significant power gains. The ring/piston/cylinder wall interface is the single largest source of friction in the engine. Even small reductions in friction in this area can lead to measurable gains in power.
Cylinder Bore The bottom line here is that you can't get the bore round and straight enough. Improving concentricity (roundness) of the bore itself will not reduce friction or improve ring seal but it will enable the rings to seal more efficiently and allow a piston skirt design for skirt-to-wall friction reduction.
Benefits of stress plate honing are well documented (see Circle Track, June '02). This is the first step in improving concentricity. Hot honing is an extension of this process that attempts to duplicate the running state of the block as it is honed with coolant passing through the deck plates and water passages. The cutting fluid is also heated. In Winston Cup applications, bore distortions of 0.0015 inch have been documented when increasing blocks from room temperature to 220 degrees F.
With the hot-honing process, bore distortion (at temperature) can be reduced to 0.0003-0.0005 inch, a significant reduction. When the cylinder bores are hot honed, they are round at operating temperature but return to an out-of-round condition at room temperature. Sufficient piston-to-wall clearance is maintained at room temperature to allow for engine start-up clearances.
Hot honing itself may reduce friction on production-type cylinder blocks where the at-temperature distortions are outrageous. Aluminum blocks can benefit significantly from this process. Most well-prepared racing engines may not be able to measure the benefit from hot honing itself. However, when combined with an optimized piston, this process can provide measurable gains.
Nikasil is an extremely hard coating that has been used on cylinder bores in two-cycle engines for years. It can be applied directly to the cylinder block or applied to sleeves, which are then installed and honed to finish. Both parent metal coating (applied directly to the bore) and sleeves have their problems, so this technology is not for Saturday nighters on a budget or the faint of heart. Engine builders have been experimenting with Nikasil for years. While claims have been made for friction reduction, it is my (Jenckes) opinion that Nikasil's primary benefit is the hardness of the coating it produces.
Note: Crank angle "A" represents...
Note: Crank angle "A" represents the period of highest connecting rod loading. Since detonation typically begins just prior to TDC, abnormal bearing pressure can occur nearer TDC than when normal (maximum) cylinder pressure develops over the span indicated. Maximum relative motion of connecting rod journal and bearing surface (bearing "speed") typically occurs during span "B" (see story).
The hard face of a Nikasil bore will allow different types of ring face coatings and ring materials to be used, all of which can lead to measurable friction reduction. Nikasil is tricky to hone, and a power loss can result from reduced ring seal if the honing process is not correct. Hot honing is a good process to use with this type of coating. With the appropriate ring material and ring face coating, it is likely that the bore and ring package can be re-used from build to build just by replacing the pistons.
Pistons In recent years, piston technology has undergone a revolution with almost all major manufacturers bringing new designs intended to reduce mass, friction, and improve ring seal. Much of this technology comes from Formula 1 and CART. These lightweight piston designs include shorter skirt slippers and can have significantly less inertia and friction than more traditional designs.
The piston is also a key in developing ring seal. Ultra-flat top ring lands can reduce "micro welding" or the transfer of material from the piston to the ring. Micro welding will usually cause a complete loss of ring seal and scuff the bore. Any machining operations that are performed to the piston after the top ring groove has been cut can affect the flatness of the land. This should be avoided if possible.
Winston Cup teams and local racers can both benefit from pistons designed specifically for their particular application. The more a piston design departs from the traditional full-skirt design, the more important all of the details in the cylinder kit become. For example, with less skirt comes higher area loads. As the skirt length is reduced, piston contact areas move upward on the piston. As material is removed for mass reduction, pistons can become susceptible to over temping (excessive heat) from lean air/fuel mixture operation, possibly making it brittle and prone to failure.
The rule of thumb for pistons is to work with a good supplier experienced with your type of racing to determine if these new designs are correct for your particular application. New technology is normally good. Just make certain it fits into your program. Never be reluctant to ask questions.
Gas Porting Gas porting can improve both ring seal and durability by allowing the combustion pressure to push the top ring against the cylinder wall. Circle-track gas ports should be lateral, not vertical.
Piston Rings Ring seal is a key issue to optimizing engine performance, regardless of the parts used. The most basic race engines will benefit from attention to detail in this area. It is the job of the ring pack to keep the cylinder pressure from bleeding into the oil pan and to keep oil from getting into the combustion space.
Reducing ring tension can lead to significant power gains from friction reduction, but only if ring seal is not compromised. Running smaller ring widths, such as 0.43 inch, 0.43 inch, 3mm (as a stack), will help reduce loss, but the margin for error in the remainder of the cylinder kit is reduced. In particular, top ring land flatness becomes more critical and bore finish must be absolutely correct.
Smaller rings have less area to transfer heat and tend to run at a higher temperature. Such increases in temperature can lead to micro welding. Small rings usually will not have the longevity of a larger ring, due to the reduction in contact area and increased operating temperature.
Producing a vacuum in the sump, either from a dry-sump system or an external vacuum pump (where legal), can dramatically increase ring seal and allow for a more aggressive ring pack.
Piston Ring Materials Most rings today are made from iron. The ring face has a channel or groove cut into it with a sprayed-in moly filler. The ring is then finish-ground. This combination is very forgiving and works well on most cylinder bore surfaces.
Steel rings with Physical Vapor Deposition (PVD), Titanium Nitride (TiN), or ceramic coatings are being used successfully in other forms of motorsports. These rings and coatings are hypersensitive to the type of bore material and finish required. In some instances, these coatings may offer some advantages with a Nikasil-coated bore. Such combinations may yield the lowest friction yet but can be extremely difficult to achieve ring seal.
Measuring Piston Ring Tension The traditional way to measure ring tension is to load (install) the ring into the bore and use a spring scale to determine the force required to pull the ring. This is a crude but effective way of measuring ring tension, more commonly called ring "drag." Properly performed, this method requires a great deal of engine builder experience to be truly effective.
OEMs measure ring tension with a machine that loads the ring tangentially. This type of machine is costly but is extremely accurate and repeatable. A tangential ring tension gauge usually loads the ring into a dummy ring grove while a steel band is wrapped around it. Bearings allow the band to apply a true tangential load to the ring. One end of the band is attached to a load cell to measure force. The other is pulled. Load is applied until the proper ring gap is found and the force is read. Eventually, as the practice of low-tension rings continues, more engine builders will use this type of gauge. It will be the only way to effectively measure the ring tension, with ultra low-tension rings.
Cylinder Pressure Vs. Crank Angle Where The Ring Needs To Seal
Study the first illustration showing cylinder pressure history as a function of crankshaft angle. Note that peak cylinder pressure during "normal" combustion occurs just past TDC. While piston ring seal is important during the compression stroke and prior to ignition, it is especially critical to create and maintain pressure seal in upper portions of the cylinder in this post-TDC area. The higher the rpm, the longer the delay before peak pressure is developed.
The measurement of engine output often includes a comparison of negative and positive work performed on a piston. Simply stated, pre-combustion pressure ("work") is applied to a piston on its upward stroke during compression. Depending upon the crankshaft angle at which ignition occurs (prior to TDC), this amount of "negative" work will vary. In other words, the earlier the spark, the greater the negative work applied, all else being equal. In fact, again noting the illustration, the nearer the point of ignition (A) and the earlier the point of peak cylinder pressure (P1 or P2), the greater the amount of position work applied to the piston. Once a piston has passed TDC on its power stroke, positive work is applied.
At the risk of oversimplification, the difference between positive and negative work is a measure of IMEP (indicated mean effective pressure). As an aside, the faster the combustion rate (burn), the later can be the initiation of spark ... resulting in decreased negative work and increased positive work on the piston. Technically, indicated work is the difference between work done on a piston during its compression stroke and work done on it during expansion. Graphically, this relationship is often expressed in the form of P-V (pressure-volume) diagrams or so-called pressure "loops."
Based on decreasing combustion space and rising combustion temperature, controlled cylinder pressure (absent any pre-ignition or detonation) will peak past TDC, owning in part to delays from gas dynamics. Ordinarily, it is within the first 30 crankshaft degrees past TDC where ring seal is particularly critical. This leads to the necessity (particularly at this point) for pistons being square in their bores, rings that are in proper contact with cylinder walls, bore concentricity at temperature, and skirt drag minimized for friction horsepower loss reduction.
We'll be supplying Cup engine information applicable to Saturday-night cylinder heads, intake manifolds, and valvetrains. Some surprises are guaranteed.