The first part of this Series laid out the importance of carefully planning your engine build. In this month's segment, we will further discuss the planning process and begin addressing bottom-end component selection. The information should help you understand more about current techniques in circle track engines or, if you purchase your engines, is intended to make you a more educated buyer.
When planning a build, the first place to start is a careful review of the rules you'll face. You need to understand what is permitted in the class you'll be competing in. It is also important to evaluate what the engine will be required to do. Answers to the following questions will help guide you:1. On average, what is the minimum engine speed where wide open throttle (WOT) occurs?
2. What is the minimum WOT engine speed under worst possible cases (slippery track, ill-handling car, hot humid air with low barometric pressure)?
3. On average, what is the maximum engine speed?
4. What is the maximum speed (rpm) the engine should ever see (good track, good-handling car, cold dry air with a high barometer)?
5. How many miles will the engine run before a rebuild?
6. Will the engine be maintained or must it race without any regular maintenance?
7. What is the engine-building budget?
Questions one and two will help define the desired torque peak of the engine.
Questions three and four help define the maximum speed (rpm) for the engine and where power peak should occur.
Questions five and six will define the type of build (conservative or aggressive). If the engine will be required to survive many races with little or no maintenance, the bottom-end and valvetrain must be overly-strong for the power level produced. The engine should also be run at lower speeds.
Question seven is the all important one. How much do I have to spend? This simple phrase will make your engine build more successful: "Plan what you buy and buy what you plan." Also remember when planning your build that the greater the engine speed, the greater the cost. Don't forget that.
Power curve In many types of racing where gear changes (shifts) occur, the engine needs to operate at WOT an equal number of rpm both below and above the power peak rpm. This is the plan when rules are open and gearing choices are unlimited. However, most circle track racing requires a different approach. For example, if the track where you race generally has good traction, then the torque peak should occur just after corner exit with power peak occurring at a point about 75 percent of the longest straightaway. This will allow the car to have the greatest rate of acceleration when it is possible to pass.
When tracks are generally slippery, we would like to increase the rpm where peak torque and power occur so that the power can still be used. It is pointless to have the torque peak occur at an rpm where the engine never sees WOT. If the torque peak occurs at the point where the driver is just getting back to WOT, the car may be more difficult to drive.
Graph A will help illustrate this point.
The Short Block When building a house, you start with the foundation. Similarly, when planning an engine build, you start with the bottom of the engine. This is your foundation.
Selecting a block is of critical importance. Assuming that you are running an iron block, the block should be sonic checked to verify that the casting is solid at the thinnest sections. A good machine shop will have the equipment to check a block by this method.
Block mass Remember, in circle track racing, chassis performance is almost always more important than engine performance. If you spend time grinding or milling 5 or 10 pounds of material from the block, you will be able to place more ballast where you need it. Total mass (weight, in this instance) reduction often costs more than $100 per pound. The engine block is one of the easiest and cheapest places to remove weight.
Two-bolt vs. Four-bolt Mains If your engine is not run at sustained speeds greater than 7,500 rpm and horsepower output is less than 500, a properly set-up, two-bolt main block may be sufficient. Always use studs to fasten the mains. As maximum engine speed increases, you may wish to consider replacing the cast main caps with billet steel parts. In fact, it's good insurance.
Graph ANNC Car at a 1.5 Mile Track
Big Bore vs. Long Stroke An over-simplification of bigger bore vs. longer stroke comparison is that a big bore engine will run better as engine speeds increase, and a longer stroke engine will run better at lower rpm. Graph B shows the piston velocity of a 4-inch stroke vs. a 3.25-inch stroke.
Mean (average) piston speed is traditionally calculated with the formula: 2 x stroke x rpm. Graph C shows how stroke affects mean piston speed as engine speed increases: As mean piston speed increases, so does friction (and power loss). Ducting losses (due to more rapid descent) also increase as mean piston speed increases, which negatively impacts cylinder filling (volumetric efficiency or torque output). Overall, a long stroke engine is a cost-effective way to make power at lower engine speeds but is one not likely to be efficient or reliable at higher engine speeds.
Connecting Rod Length Much has been written about the effects of rod length on engine performance. The following data in Graph D shows a rod length comparison on a NNC-type engine. In this case, the shorter rod made better low speed power and the longer rod made more power at higher engine speeds. Graph D shows the results of the test. For this test, pistons and rods were changed as a unit, so the CD (compression height) of the piston did change with rod length. The results are presented as the change in power from the 6.200-inch rod to the 6.080-inch rod (bhp of 6.200 to bhp of 6.080) plotted against engine speed in rpm.
Graph E shows how rod length affects piston position. Only to make the difference appear obvious, the plot compares a 4-inch rod to an 8-inch rod. While this is an absurd difference, it illustrates how the shorter rod moves the piston away from TDC more quickly. Long rod lengths reduce piston side-loading and, therefore, friction. Valve timing (camshaft design) should be considered when contemplating connecting rod length. Primarily, variations in piston motion away from TDC on the intake stroke (as rod length changes) should be linked to intake valve events. If your camshaft provider of choice is unable to respond to this concern, we suggest you look for another supplier.
This block was lightened using CNC equipment, although there are much cheaper ways to drop
Main and Rod Bearing Sizes and Clearances Oil film thickness increases as bearing clearances are reduced, provided everything is perfectly round and straight. This trend continues until the clearance becomes too tight and the oil film goes away. As the oil film thickness increases, friction is reduced.
Smaller bearing diameters reduce the surface speed of the bearing. A reduction of bearing speed will reduce friction. This effect will be most noticeable at high engine speeds.
Small crank and rod journal sizes with relatively large strokes will reduce the stiffness of the crankshaft and cause bending loads to increase. This will also increase loads on the bearings. As a result, the best time to try smaller journal sizes is when using shorter strokes and big bores.
The trend in Cup today is to have bearing sizes at the minimum with very tight bearing clearances. This is very difficult and expensive to achieve. The crankshafts, connecting rods, and bearing shells must be ultra-precision components which turn out to be extraordinarily expensive. Plus, this combination leaves no room for error. Small errors in a component or during assembly will most often result in a spun bearing.
What you can apply from this information: Exotic parts may not fit your budget. However, with research, you may be able to find an older configuration of your engine which had smaller main and rod journals. When properly set up, this will reduce friction (and power loss) at higher engine speeds. Carefully measure your crankshaft. If it is very straight and has consistent journal sizes, try setting up clearances at the low limit of their traditional (recommended) tolerance. This should increase oil film thickness and reduce friction. However, you must be very careful when using this strategy. If there is any error, you will experience a spun bearing.
Crankshafts In the last five years, significant developments have occurred in crank design and supply, at least at the Cup level. For example, five years ago, finished Cup cranks cost less than $3,000. Today, caused by some of the exotic materials being used, the billet of steel alone will cost this much. In fact, a finished crank can cost significantly more than $10,000. What has changed that has driven up these costs? In short, just about everything. What has been gained by using these exotic crankshafts? The improvements include reduced friction and a low moment of inertia (MOI), which helps the vehicle accelerate and improves mechanical efficiency via increased stiffness.
Most crankshafts are ground and then nitrided with a relatively thin layer of nitride. The current trend in Cup crankshafts is to have a very deep nitride layer with grinding after nitiriding. The nitride layer is very hard but brittle. A thin nitride layer provides a hard, wear-resistant surface with a ductile (flexible) core underneath. Nitriding also improves the fatigue performance of the steel. Deep nitiriding can significantly improve the fatigue performance of the crankshaft, which allows the design to optimize the section for reduced mass or MOI.
Any imperfections in the steel can cause stress and cracking, if the nitride layer surrounds them. For this reason, deep nitriding requires much "cleaner" steel. During smelting, imperfections are removed by re-melting the steel and removing the imperfections from the top layer of molten metal. The more frequently this process is repeated, the "cleaner" the steel becomes, but the smaller the yield (total metal produced). As a direct result, ultra-clean steel is very, very expensive. This explains why an ultra-clean billet may cost as much as a finished crank, five years ago.
Like most forms of heat treatment, nitriding can cause distortions in the part. Crank manufacturers go to great lengths to minimize these problems, even though they cannot be completely eliminated. Also, to improve the dimensional accuracy of a crankshaft, a deep nitride layer may be applied before removing some material by a finish-grinding process. By finish-grinding, following nitriding, significant improvements may be made in the following areas:* Concentricity of all bearing journals* Alignment of all bearing journals* Accuracy of the stroke (from cylinder to cylinder)
Improvements in the bearing journal size and alignment, when coupled with tighter bearing clearances, will reduce friction. An improvement in stroke accuracy will allow engines built closer to their compression ratio limit or to simply have more compression, if no limit exists.
Today, crankshaft designs are separated into two configurations:* Low inertia to help the engine and car accelerate* High stiffness to improve oil film thickness and reduce friction
Counterweights may be designed in different ways to achieve the same balance. State-of-the-art designs have the tip radius (distance from crank axis to "tip" of the crank's throw) for a given balance mass. This may require extensive use of "heavy metal" which is extremely expensive. If an engine is raced on short-tracks requiring rapid rates of acceleration, a low MOI crank may provide a bit less power but should help the car accelerate. Short-tracks, such as Martinsville, will have 10 times the maximum acceleration rate required than (for example) at the Brickyard. If an engine is raced on fast tracks where the maximum rate of acceleration is low, due to high aerodynamic loads (again, as an example), a stiff crank will provide more power but not accelerate the engine/car as well.
What you can apply from this information: All crankshafts are not created equal. Pick the type of crank that best suits your form of racing. Low MOI cranks are more important when required rates of engine/car acceleration are high. If the engine speed does not change significantly and you are on the throttle most of the time, a "stiff" crank may be the best. Do not confuse low MOI with low mass. Mass and moment of inertia are not the same (see Cranks & Rotational Moments of Inertia, page 26). Ask your crank supplier to what tolerances the cranks are supplied. Thoroughly inspect cranks for journal size (and concentricity) and stroke accuracy. Make sure the crank you receive matches the manufacturer's specifications. When building your engine, set clearances accordingly.
Pistons and rings Full-round pistons have not been used in Cup for many years. The current style is the "box-in-box" design (see photo on page 38) with the skirts only on the major and minor thrust sides. The box-in-box design is generally a more mass-efficient design (strength vs. weight). Cup pistons have a minimum mass of 400 grams. Reducing reciprocating mass actually reduces bearing and crankshaft loads. A lower mass piston with an efficient structure may be more durable than comparably heavier components.
Most Cup pistons use a top ring coating to prevent ring welding, and hard anodizing is the most popular coating. Top rings are 0.8mm or smaller and should have a barrel face. Three rings are currently used on most Cup pistons. Remember that the higher the top ring, the hotter it runs and the greater the end-gap required. Also, the hotter the ring, the more it grows in length. Therefore, end-gaps must be adjusted to prevent gap "butting." Always start with more ring-gap than you think you will need, and inspect the ring ends carefully after running the engine to see if any signs of contact exist. Only close the gap when you are sure that the rings will never butt.
The top ring is typically moved up as high as possible. Open engines will have the top ring placed anywhere from 2.3mm to 3.8mm down from the crown. Second rings are usually 0.8 to 1mm and can be barrel or Napier style.
Oil rings are generally 3-piece and range from 1.5mm to 3mm. Oil ring tension relates directly to friction. Cup engines use rings of very low ring tension that are raced only one time. These rings are rated using a tangential gauge and open-class rings rate anywhere from four to eight pounds of tension.
Honing The final finish of cylinder bores is a key element in achieving optimum cylinder pressure sealing and highest power. Piston ring manufacturers are typically able to provide surface finish specifications for the best performance of a given ring package. If they cannot, perhaps you should investigate another supplier. In the same fashion, your machine shop should have a gauge to measure the bore surface finish they're providing. This gauge is important to ensure that each bore is honed correctly as specified.
Today, it is common to use or request the use of torque plates during the honing process. But, if your chosen machine shop does not use these, it's best to find another source. It never hurts to ask probing questions. And when it appears you're getting misinformation, you can always revert to the approach Smokey Yunick frequently took. That was, "Keep talkin' 'cause it ain't often I get to listen to a genius." Works just about every time.
This is a top of the line crankshaft from SP Crankshaft. The Irvine, Calif., based company
Hot Honing Hot honing is a technique where the block and honing fluid are heated to the operating temperature of the engine. The block is then honed at this temperature, providing a more cylindrical bore when the engine is running. To properly develop the correct technique for hot honing, a considerable amount of time and work is required. The block must be mapped at operating temperature and the hot honing technique adjusted to give a similar result. We recommend that you discuss this, somewhat at length, with the provider of the hot honing equipment or machine shop you elect to use. The results from this process may yield a few horsepower. This is very specialized work and may not be a good value except for the extremes of motorsport such as Cup racing. You must judge that for yourself, although the process has proven to be beneficial, correctly applied.
WristPins In today's circle track engines, the best solution for a wristpin is to run a high-quality steel pin with a DLC coating. The pin is run directly against the steel pin bore of the rod and without a coating. Cup pins for open class motors are as small as 19mm. Pin lengths tend to be short so the pin is as stiff and light as possible. As an example, a 20mm diameter by 45mm long pin with a 4.5mm wall is common in open Cup engines.
Top-Guided vs. Bottom-Guided Connecting Rods Most high-speed, circle track racing engines today guide the rod from the top, not the bottom as in stock production engines. The advantage of top-guiding is that the packaging of the pin towers can be moved in around the small end of the rod. This allows a shorter, lighter pin and a stiffer piston-rod-pin assembly. Top-guiding works most efficiently with a box-in-box-style piston and usually requires an integrated design of the piston, pin and rod. It will result in a lighter, stronger package which provides better stability to the piston, thus improving piston ring seal.
A NASCAR-style billet piston by JE. Je pistons
Top-guiding also requires that bore angle, bore spacing, and mains are all correctly located. The linear dimensions of the crank shaft must also be correct. For steel rods, the side-clearance for the small end of the rod to the piston should be no more than 0.125mm. The side clearance for the big end of the rod should be no more than 1.0mm. Typically, the small end of the rod will be smaller as well, and a 17.75mm small end width with a 20mm pin is comfortable.
Some Summary Thoughts By now in this Series, you've probably noted that we're not focusing on traditional subjects with equally traditional comments. Rather, we're digging into informational bites that skirt around what you've previously heard or read. That's not to say such prior information from other sources has not been useful. Certainly it has likely been. However, and from its beginning, the charter of this Series was to pull in fresh information you should probably consider or (best case) put into practice. For the most part, charts and graphs provided are actual case histories, or at least based upon them. So they're not hypothetical. That tends to bring reality and experience into play.
Our expectation is all this will become even more interesting when we get into discussing components keyed directly to the sensitive areas that make significant power and add durability to a given engine package. That's upcoming in the following parts of this Series. Don't spin out.