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Understanding Oval Track Cranks & Rods: There's more to it than you think
Getting across the finish line first takes different techniques depending on the type of racing it is.
By John Carollo
Racing's racing, right? The first one is the winner and second place is just the first loser. While that may be true as far as it goes, getting across the finish line first takes different techniques depending on the type of racing it is. Racing engines are not immune to specialization. Where one style of essential components can be used in one type of racing, the same parts wouldn't even be considered in another type of racing. Such is the case with crankshafts and connecting rods in oval track engines. Or is it?
Sure, the same principal perfect triangle is desired for rotating assemblies: the goal is to reduce the weight, increase performance and lengthen the service life of the components and motor. But that in and of itself demands certain trade offs. Where is the line between using lighter rotating components such as rods and the crank and just how long those pieces will live in such a harsh environment? The answer isn't simple. There are many factors that come to bear, such as rules on minimum weights or sizes of components in crate and spec engines and even on down to claimer engines.
The primary function of an oval track engine is to last, as it's the final lap that pays out the purse. So while builders have that mission first and foremost in their agenda, they are also looking at making those pieces lighter, if nothing other than a little bit at a time.
In drag racing, the norm is aluminum and even titanium rods to save weight for a decidedly quicker rev-up. But even they have their shortcomings. Those trick rods often have a super short service life of as little as a few passes before they end up as souvenirs on someone's tool box or find themselves recycled into paper towel holders, clocks or steering column and license plate brackets on hot rods. The key to success is knowing the limits of those pieces.
But as builders, we have an ally. Some of the rods and cranks on the market today offer the same traits, features and performance levels. We would never think of putting drag rods on a stock car engine but when we research the types of metal used for those disciplines, we find that they can often be the same. And it's not only the metal but heat treating and other secondary processes and procedures that yield the same part.
And, as we all know, when more parts are manufactured, the price drops as opposed to lesser quantity lots. So it is in our best interest to know what makes a crank and rod usable for one type of racing and not another - and how to read that interchangeability.
High end NASCAR engines went through a cycle that was decidedly different and unique to them. Way back when, NASCAR teams had qualifying motors complete with the lightest and often most expensive components not always available to the common man. They even had special "qualifying oil" that was good for about four laps before it started to break down. The life of the oil didn't matter as the engine itself was like a hand grenade with the pin pulled - it was only a matter of time before it exploded.
Even if the engine made it back to the garage after qualifying, it was yanked for a "practice motor" and later changed again for the actual race motor, often referred to as a "million miler." This process showed that builders knew the limits of their engines and they used that knowledge to gain an advantage in qualifying and earning the bonus money winning a high visibility pole award brings. And that's why NASCAR eventually banned the use of such motors.
The high tech - and often high dollar - components used in such engines expanded the chasm between the "have and have not" teams. Nowadays, the rule requires teams to start the race with the same engine that qualified the car. When an engine has to be replaced before the race, for whatever reason, starting at the rear of the field is the automatic penalty.
Even after NASCAR banned the use of qualifying motors, the sanctioning body eventually had to come up with some way to limit the use of ultra high-end pieces. The reasons were the same: the lighter pieces created an upset in the balance of what some teams could afford and others couldn't. Now, there are minimum weights and sizes used on engine parts. For connecting rods and cranks, the diameter of the journal is limited to 1.850? and pistons are limited to 440 grams in weight.
But in the real world, it still comes down to what your customer has in mind. If he or she wants the very latest and greatest, from a material or technology standpoint, the builder has to reach for what are usually the most expensive parts. If the customer wants his engine to be a "million miler" the more reliable parts are the obvious choice. Race engine builder Joe Rhyne says "If you're building a cutting edge engine you would want to use the lightest parts you can. If money is no object, you can use throw-away parts. If you want to save money, you go with larger, heavier parts that may have more longevity."
Crate and spec motors, a relatively new aspect of racing, mostly fall into the "million miler" category, as their primary mission is to be reliable and consistent. We all learned early on that "reliable" is not always a trait that is mentioned in the same breath as "high-horsepower motor."
When racing sanctioning bodies stipulate spec or crate engines to save costs - both for the cost of the engines themselves to competitors and the cost of policing those engines during competition for the racing organization. When all the engines are the same - and often even coming from the same builders - any slight deviation in parts can be spotted faster and with a less complicated (read that less costly) inspection procedure that often requires fewer officials. For their competitors, the goal of the racing organization is to keep all those engine costs contained.
Not too far from that, at least in concept, the "claimer motor" brings just as many concerns, most of them centered on the cost of the engine. When a motor can be claimed by a competitor for a relatively small amount, there's just no sense in putting more money into any part of the engine, much less the rotating assembly, than the claiming price. Even if the parts were bought at a great savings, it becomes a matter of the value of the engine more than the cost. Builders of claiming engines are usually experts about finding and using the best parts at the best price.
We talked to a few companies that build rods and cranks to see what is out there that we need to know about. What they told us is food for thought on the versatility of rods and cranks.
Manufacturers say they offer a variety of rods and cranks specifically designed for the various levels and needs of oval track racers. The product's characteristics are usually tied directly to material and manufacturing variables - what you get is probably proportional to what you pay, but what you need may be different than you believe.
Most stock cranks and even some street performance cranks are made of cast iron and steel alloys. A 1053 high-carbon alloy steel typically has around 100,000 psi tensile strength. This is adequate for a stock or even mild performance engine, but is not strong enough to withstand the rigors of serious racing. Stock cast cranks can safely handle up to 350 horsepower in a small block V8, or up to 400 horsepower in a big block V8 with a redline of 5,000 to 5,500 rpm. Beyond these limits, a better grade of material is needed.
A forged crankshaft made of 5140 grade steel alloy is a good upgrade and has around 115,000 psi tensile strength. A 5140 alloy contains less carbon (0.38 to 0.43 percent) than 1053 alloy (0.47 to 0.55 percent), plus it has chrome (0.9 to 1.7 percent) and silicon (0.2 to 0.35 percent) to improve durability. The difference in carbon between 1053 and 5140 is not as important as the chrome, which adds strength and toughness. With the proper heat treating, a 5140 forged crank is an economical choice for many Sportsman class racers or the street/strip Saturday night racer.
If you're building a small block V8 that's capable of making more than 450 horsepower, a big block that's making upwards of 550 horsepower or you're pushing the engine's redline beyond 7,000 rpm, you should upgrade to a performance crank - which usually means a forged or billet 4340 steel crank. Forged cranks made of 4340 alloy typically have a tensile strength of 140,000 to 145,000 psi and are much more resistant to fatigue than 5140 or 1053. A forged 4340 billet crank will typically have a fatigue strength of 160,000 to 165,000 psi. The increased strength is due to the addition of nickel (1.65 to 2.0 percent) and molybdenum (0.2 to 0.3 percent).
Something else to keep in mind is that not all "4340" alloys are the same. ASM (American Society for Metals) grading standards allow a certain range for the amount of each element in the steel. If the concentration of a particular element such as chrome, nickel or molybdenum is at the upper or lower limit, it can affect the ultimate strength of the alloy. Also, we've heard reports that some 4340 alloys from offshore suppliers don't necessarily conform to ASM standards. Some contain contaminants that have an adverse effect on strength and other properties. That's one reason why some offshore forgings are priced much less than their domestic counterparts. The alloys are not the same.
The strength of the crank not only depends on the base alloy, therefore, but also on how it was made (whether it was drop-forged or machined from solid chunk of billet steel). A welded up stroker crank may be okay for a street engine but probably isn't strong enough for racing.
Forgings generally produce a flowed grain structure, which is stronger than a casting. Even so, the forging process stretches, pulls and deforms the grain structure, and subsequent machining cuts through the grain structure. The strength of the forging also depends on the metallurgy of the alloy used, and the heat treatment that is applied to it after it has been shaped. Forgings require a die to shape the metal. Dies and forging presses are expensive (which adds to the cost of the crankshaft), so the availability of forgings for various applications depends on their popularity and how much people are willing to pay for a forged crank.
Billet crankshafts, by comparison, are CNC machined from a solid chunk of forged steel. The grain structure is not stretched or deformed, and machining leaves fewer residual stresses in the metal. Consequently, some crank manufacturers say billet cranks are the strongest cranks available. Most Top Fuel drag racers run billet cranks, as do many circle track racers. Another advantage with billet cranks is that CNC machining allows a crank to be custom made with virtually any stroke, journal diameter, configuration or countershaft placement that will fit the engine. A billet crank can be one-of-a-kind or mass produced.
Durability depends on the material the crank is made from, the method used to make the crank (forged or billet), the size of the rod journals, and the radius of the journal fillets. Bigger journals are stronger, but many racers want smaller journals to reduce friction. Consequently, the crank itself must be stronger.
Most racing cranks are heat treated and case hardened to provide additional strength and durability. The journal surfaces may be hard chromed, nitrided or induction hardened. Nitriding is often used, and can be done by several methods. Some crank manufacturers use a "plasma nitriding" process that vacuum-deposits ionized nitrogen on the surface of the crank inside a high temperature oven. Others use a process called Tufftriding that soaks the crank in a hot "ferric nitrocarburizing" salt bath, or heats the crank to 950° F in an oven filled with nitrogen. Nitrogen penetrates the surface of the metal and changes the microstructure of the steel. This roughly doubles the hardness of the surface from about 30 to 35 Rockwell C to 60 Rockwell C, and increases fatigue life up to 25 percent or more.
The connecting rods are a vital link between the pistons and crankshaft. They connect the reciprocal motion of the pistons to the rotational motion of the crank. The weight of the rods is important because it affects the reciprocating forces inside the engine. Lighter is usually better because less weight means faster throttle response and acceleration. But strength is even more important.
Basically, you want a set of rods that are as light as possible, but are also capable of handling all the forces the engine can generate (rpm and horsepower).
If you are building an engine for a sprint car that is constantly on and off the throttle, an ultra light crankshaft with the lightest possible rods and pistons will deliver the kind of performance that works best in this application. But other racing styles need different approaches.
The best advice when selecting a particular set of rods is to talk to your parts suppliers and ask them what they would recommend. Every rod supplier we interviewed for this article said rod selection depends on a number of things. First and foremost is the application. In other words, what kind of engine are you building and how will it be used?
If you choose a set of rods based strictly on a catalog or Web site description, or you choose a set based solely on length, weight or price, you may not be making the best choice. That's why a few minutes spent on the phone with your rod supplier can be so valuable. They may recommend a particular type of rod you hadn't considered, or they may have some new product offerings that have not yet been added to their catalog or Web site. Catalogs get out of date very quickly, and many Web sites are not updated as frequently as they should be.
Rods essentially come in two basic types: I-Beam and H-Beam. Some rod suppliers only make I-Beams, others only make H-Beams, and some offer both types. I-Beam rods are the most common and are used for most stock connecting rods as well as performance rods. I-Beam rods have a large flat area that is perpendicular (90 degrees) to the side beams. The side beams of the rod are parallel to the holes in the ends for the piston pin and crank journal, and provide a good combination of light weight, and tensile and compressive strength. I-Beam rods can handle high rpm tension forces well, but the rod may bend and fail if the compressive forces are too great. So to handle higher horsepower loads, the I-Beam can be made thicker, wider and/or machined in special ways to improve strength.
Rod suppliers produce a number of variants on the basic I-Beam design. The center area may be machined to create a scalloped effect between the beams, leaving a rounded area next to both beams that increases strength and rigidity much like the filets on a crankshaft journal. These kind of rods may be marketed as having an "oval-beam", "radial-beam" or "parabolic beam" design.
H-Beam rods, by comparison, are typically designed for engines that produce a lot of low rpm torque. This type of rod has two large, flat side beams that are perpendicular to the piston pin and crankshaft journal bores. The center area that connects the two sides of the "H" together provides lateral (sideways) stiffness. This type of design can provide higher compressive strength with less weight than a comparable I-Beam, according to the suppliers who make H-Beam rods. That's why H-Beam connecting rods are often recommended for high torque motors that produce a lot of power at low rpm (under 6,000 rpm). Of course, an I-Beam rod can also be strengthened to handle almost any load but it usually involves increasing the thickness and weight of the rod and/or using a stronger alloy.
Most aftermarket performance rods are made using 4340 billet or forged steel. This is a chrome moly alloy with high tensile and compressive strength. A word of caution, though, is that all "4340" steel alloys are not necessarily the same. Heat treatments can vary, and this will affect the properties of the steel. Some rod manufacturers also tweak the alloy by adding their own proprietary ingredients to improve strength and fatigue resistance. Several rod suppliers said the 4340 steel that some offshore rod manufacturers use falls short of American Society of Metals quality standards, and is not as good a steel as they claim it is.
There is also a debate over the relative merits of "Made-in-USA" forgings versus foreign forgings that are machined in the USA or rods that are forged and finished overseas. Patriotic and international balance-of-payment issues aside, a connecting rod that meets metallurgical quality standards, is heat treated properly, and is accurately machined to specifications is the same no matter where it comes from or who made it. The engine won't know the difference. So as long as the rod supplier stands behind their product with their brand name and reputation, the "foreign versus domestic" rod debate shouldn't matter.
A growing number of rod suppliers are now offering lower cost performance rods as economical upgrades over stock rods for street engines and other entry level forms or racing. Consequently, these budget-priced rods allow engine builders to offer their customers more options and more affordable alternatives for upgrading an engine. For big buck racers or really demanding applications, though, these kind of rods probably aren't the right choice. You would want to use a set of top-of-the-line performance rods that are capable of handling the highest loads.
Over the past couple of years, the price of high quality steel as well as many other metals such as copper and titanium has shot up dramatically for a variety of reasons. Some rod suppliers are now having to add a steel "surcharge" to their current prices to help offset their higher cost of materials (which doesn't matter where they buy their steel because the higher prices are world-wide and affect everybody). The soaring cost of titanium has almost priced this metal out of the aftermarket. Some rod suppliers have discontinued making rods from titanium. Those who still offer titanium rods say the only people who are buying them today are the high end professional racing teams with deep pockets. One rod supplier said titanium has become "unobtanium" for the average racer. Connecting rods made of light-weight titanium rods can reduce the reciprocating mass of the engine significantly for faster throttle response and higher rpms, but at a cost of up to $1,000 or more per rod, the affordability factor comes into play.
Another lightweight material that has long been used for performance connecting rods is aluminum. Many drag racers run aluminum rods because they cost less than titanium and provide a good combination of lightness and strength. Most aluminum rods are fairly stout and typically much thicker than a comparable steel I-Beam rod. The added thickness may require additional crankcase clearance, and it increases windage and drag -- which at really high rpms may cost a few extra horsepower to overcome. The rods also require a dowel pin to keep the bearings from spinning because the bores stretch more than a steel rod. Also, the rod itself can stretch and grow in length at high rpm. This means extra clearance must be built into the engine so the pistons won't smack the heads.
Though aluminum rods are popular for drag racing and other high rpm forms of racing, most of the rod suppliers we spoke with do not recommend aluminum rods for street engines. Why? Because steel rods will hold up much better over the long run than aluminum rods. Aluminum rods are fine for a drag motor that will torn down after 200 runs and freshened up or rebuilt with a set of new or reconditioned rods. But for street applications or engines that have to run at sustained high speeds and loads for long period times, steel rods are usually better.
It's interesting to note that aluminum rods are only available from a few suppliers, and at least one supplier who used to offer aluminum rods has discontinued them. Another material that is used for many high performance rods is 300M, which is a modified 4340 steel with silicon and vanadium added, plus higher amounts of carbon and molybdenum. The 300M alloy is up to 20 percent stronger than common 4340 alloys, and was originally developed for aircraft landing gear. Now it is used for high end connecting rods.
The strength and fatigue resistance of most metals can also be improved by cryogenic processing after the rods have been heat treated. Heat treating causes changes in the grain structure of steel that increases strength and hardness, but it can also leave residual stresses that may lead to fatigue failure later on. By freezing parts down to minus 300 degrees below zero in special equipment that uses liquid nitrogen, the residual stresses are relieved. The super cold temperatures also cause additional changes to occur in the metal that help the parts last longer and run cooler. That's why cryogenic freezing is used on everything from engine parts to tool steels, aerospace hardware and even gun barrels.
The cryogenic process is a slow one, taking anywhere from 36 to 72 hours depending on the parts being frozen, and it must be carefully controlled to achieve the desired results. Most rod suppliers have their own cryogenic vendors who treat their rods for them. But you can also have ordinary untreated rods (even stock rods) frozen to achieve the same results.
So is "fat where it's at" or is "light right" when it comes to selecting reciprocating parts such as crankshafts and connecting rods for racers? There may never be a "right" answer. But we know there are more choices to make that decision today than ever before. Choose wisely…
For more information on racing crankshafts and connecting rods, read this expanded article on line at www.enginebuildermag.com. You'll find additional information about identifying and resolving crank failures, machining considerations, connecting rod designs and materials, as well as a buyers guide for crankshafts and rods.