Connecting Rod Guide for Building Big-Inch LS Engines

Few components in an engine are as underappreciated as the connecting rods. While they’re not much to look at, connecting rods attach the pistons to the crankshaft, and they are, therefore, burdened with the responsibility of converting the reciprocating motion of the pistons into rotating motion at the crankshaft. As such, the connecting rods are some of the most highly stressed components in an engine, and the loads placed upon them increase dramatically as cylinder pressure, RPM, and horsepower increase. In extreme racing applications, the loads on a rod can exceed 12,000 pounds as the piston is pulled back down the bore from TDC. Potential rod failure isn’t something to take lightly, because a rod that cracks in half can catapult past the block deck and destroy a piston, taking the cylinder head along with it. In other words, it pays to make sure that the connecting rods you select for your stroker project are up to the task of handling the abuse you plan on throwing at them.

 

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As with crankshafts, the past 15 years have produced an influx of affordable aftermarket connecting rods. Consequently, enthusiasts have more choices than ever, and it’s no longer costprohibitive to step to some aftermarket forgings. Quality aftermarket rods are now so affordable, in fact, that tried-andtrue practices, such as shot-peening and reconditioning stock connecting rods, are a thing of the past. Rugged 5140 forged steel aftermarket rods can be had for under $300, and $600 buys a set of H-beams that will handle up to 1,400 hp. Connecting rods come in a variety of shapes and sizes, and they are made from several different materials, so it pays to study before laying down cash on a set of rods for your stroker project.

 

 

Stock Rods

Factory Gen III/IV connecting rods incorporate a very durable design that has proven to be reliable up to 500 hp. They’re built from powdered metal, a process that involves packing powdered steel into a mold, heating it, and then forging it into the shape of a rod. A parting line is then machined onto the big end of the rod before cracking off the cap. The result is a cap that fits perfectly into the grooves of the rod when bolted down. Stock LS rods are actually stronger than the coveted “pink” rods used in select Gen I small-blocks.

 

 

GM manufactured LS connecting rods in three different lengths. Rod length is measured from the center of the big-end bore to the center of the smallend bore. The vast majority of Gen III/IV small-blocks—including 5.3L, 5.7L, 6.0L, and 6.2L car and truck motors—utilize 6.098-inch rods. Of these, most feature press-fit piston wrist pins. The exceptions are the rods used in 6.0L Vortec truck motors, which have floating pins and a slightly thicker beam. With the launch of the Gen IV small-block in 2005, GM began phasing in rods with bushed smallends in order to support full-floating wrist pins. Otherwise, all 6.098-inch factory LS rods are very similar.

 

   
 

To minimize production costs, GM also manufactures a longer 6.275-inch rod for 4.8L Vortec truck motors. Because these smaller engines are equipped with a shorter 3.267-inch stroke, a longer rod allows GM to use the same piston casting for the 4.8L as for the 5.3L. One of the latest iterations of factory GM rods is also the most extreme. To keep reciprocating mass to a minimum, GM developed a brand-new 6.067-inch titanium connecting rod for the LS7. These ultra-lightweight rods tip the scale at just 464 grams, about 30 percent lighter than stock powdered-metal rods, which is part of the reason why the LS7 revs freely to 7,000 rpm. At more than $400 for each rod, however, the LS7 forgings are extremely expensive for anyone considering bolting them to a stroker motor.

 

 

 

 

Because stock connecting rods are adequate to the 500-hp mark, they are a viable option for a budget-oriented stroker build. However, the factory rod bolts become marginal once engine speeds approach 6,500 rpm. This is because every time the piston is pulled back down the bore after it reaches TDC, tremendous loads are placed upon the rod bolts. In fact, rod bolts are subjected to the greatest amount of stress in the entire engine. Consequently, in any performance application where the stock rods are re-used, the bolts must be replaced with quality replacements from a company, such as ARP. Doing so requires machining the inside housing diameter of the big end of the rods, because new fasteners may change their shape.

 

 

The biggest drawback to re-using stock connecting rods is labor costs for resizing; they can come close to the price of new aftermarket forgings. Re-sizing a set of eight connecting rods costs about $80. Most stock LS rods must be press-fit onto the pistons, which racks up another $80 in labor. Throw in the $100 that quality rod bolts will set you back, and the total cost to recondition a stock rod is almost enough to buy a set of forged I-beam aftermarket rods.

 

Forging Materials

Thanks to the efficiency and low cost with which aftermarket connecting rods can be manufactured in today’s market, cast rods are virtually nonexistent. As with crankshafts, forged steel alloys are most common, and extremely exotic and expensive materials, such as titanium and aluminum, are also available. There are tradeoffs in strength, weight, and cost with each, so it’s important to know the difference between each material in order to select a rod that’s both durable and affordable.

The three most popular grades of steel used to manufacture aftermarket connecting rods—in ascending order of strength—are 5140, 4130, and 4340 alloys. Just as with crankshafts, highergrade alloys offer advantages in tensile strength and ductility. This is due to their higher concentration of carbon, nickel, and chrome content. Typically, entrylevel aftermarket rods are forged from 5140 or 4130 steel, and pricier high-end rods are built from a premium 4340 alloy. After the forging process, aftermarket rods are heat-treated, shot-peened, and stress-relieved to further enhance durability. They’re also fitted with rod bolts or cap screws that offer up to 280,000 psi of tensile strength.

 

 

 

For mild street applications up to 600 hp, 5140 and 4130 steel rods are sufficient, but 4340 rods are advisable at anything beyond that point. With the increasing affordability of 4340 steel, however, 5140 and 4130 alloys are becoming less prevalent, even in entry-level rods. Interestingly, the number of entrylevel connecting rods in the $300 range on the LS market is extremely limited compared to their higher-end alternatives, with Scat and Eagle being the primary players this arena. This is probably due to the fact that the factory Gen III/IV rods are very strong, and most performance LS stroker builds can easily approach or exceed the 600-hp mark where premium rods are a necessity. Not surprisingly, Scat and Eagle sell more of their premiumgrade rods than their entry-level rods.

Two of the more exotic materials used to manufacture connecting rods are aluminum and titanium. Available from aftermarket companies, such as GRP and Howards Cams & Racing Components, and popular in racing applications where low mass takes precedence over ultimate strength and long-term durability, aluminum rods can be forged or cut from blocks of billet. Because they weigh 25 percent less than a steel forging, aluminum rods reduce reciprocating mass, and, therefore, improve power output. However, they only offer half the tensile strength of steel, and they have a much shorter fatigue life. That means that as aluminum rods accumulate mileage and are subjected to repeated heat cycles, they tend to stretch, harden, and weaken over time. Additionally, aluminum rods must be made bulkier than their steel counterparts to compensate for their lower tensile strength. Consequently, they are unsuitable for street motors, and they must frequently be checked for stretching in race motors. According to some rod manufacturers, the fatigue life of an aluminum rod is 90 percent lower than that of a steel rod. For these reasons, aluminum rods are most frequently used in high-dollar drag race engines that are rebuilt multiple times per season.

Striking a balance between steel and aluminum is titanium, which offers the highest strength-to-weight ratio of all materials used to build connecting rods. The most commonly used material for automotive applications is 6AL-4V titanium, an alloy that contains 6 percent aluminum and 4 percent vanadium for improved machinability. Although titanium’s tensile strength is 15 percent lower than that of steel, it’s 30 percent lighter. As a result, titanium rods are often used in applications that operate at extremely high RPM, such as 9,500-rpm NASCAR Sprint Cup motors, 18,000-rpm Formula One engines, and even 7,000- rpm LS7s. Offered by aftermarket companies, such as Crower and Cunningham, the biggest drawback of titanium rods is price. A set of eight titanium rods usually costs twice as much as a set of comparable steel rods. Furthermore, despite their extra weight, steel rods are more than up to the task of enduring sustained engine speeds above 8,000 rpm.

 

   
 

Although the term “billet” doesn’t refer to any specific material and can more accurately be described as a manufacturing process, billet is worth discussing to determine where it falls in the hierarchy of connecting rods. A billet rod starts out as a single ingot of forged steel, aluminum, or titanium and is machined into the final shape of the rod. The primary advantage of this manufacturing technique is customizability. Because they don’t rely on very expensive forging dies and presses to pound them into shape, billet rods can be manufactured in custom lengths much more easily. With a forging, building a custom-length rod requires building a new set of dies, which is cost-prohibitive. Billet steel rods are machined from a purer, more highly refined alloy than most forgings. Additionally, they feature a longitudinal grain flow with excellent molecular bonding properties for enhanced strength. Although billet steel rods aren’t as strong around the big end as forged steel rods, due to the absence of a circular grain flow, they’re much more resistant to the formation of surface cracks. Overall, billet steel rods are stronger than forgings, but also cost twice as much.

 

 

 

 

Rod Shape

The shape of a connecting rod is just as important as the material it’s made from. Like most factory rods, Gen III/IV small-block units utilize an I-beam design; aftermarket rods are shaped into both I-beams and H-beams. Making a blanket statement as to which design is stronger wouldn’t be accurate, as each offers distinct advantages. Most rod manufacturers agree that I-beams are lighter and stronger in compression, because they distribute stress more evenly throughout the rod. Conversely, H-beam rods are heavier and can withstand greater tensile loads, due to their bulkier design. In theory, these attributes would indicate that I-beams are better suited for forced-induction and nitrous applications, and that H-beams are more durable in high-RPM race motors. In practice, however, this isn’t the case. Most companies that sell connecting rods manufactured overseas market their I-beams as entry-level offerings and their H-beams as premium offerings rated at higher horsepower limits. As a result, many hot rodders have concluded that H-beam rods are a stronger overall design. However, this isn’t always the case, as manufacturers, such as Manley and Lunati, have recently introduced heavy-duty I-beam rods that are rated at a higher horsepower level than their own H-beam rods.

For instance, Manley rates its standardweight 4340 steel H-beam rods at 800 hp and 8,000 rpm and its standard-weight Pro Series I-beam rods at 850 hp and 8,500 rpm. Furthermore, companies, such as Oliver, have been manufacturing some of the highest-quality and most durable connecting rods for decades. They’re used in the most demanding racing applications, including everything from circle track cars to monster trucks to sprint cars. Interestingly, Oliver utilizes an I-beam design exclusively in all of its rods. Consequently, both I-beam and H-beam rods can be designed to handle serious abuse, and generalizing as to which is stronger is futile.

 

 

 

Although “I-beam” and “H-beam” refer to the general design of a rod, that doesn’t mean that all H-beams and I-beams are shaped the same. Some offer more clearance than others. Due to the tight clearance between the rods and camshaft in stroker motors, most aftermarket connecting rods have profiled shoulders to buy some extra space. Likewise, the rod bolts can also compromise clearance. As a piston approaches TDC, the top of the rod bolt nears the cam, and as a piston approaches BDC, the bottom of the rod bolt nears the crankcase and oil pan. To combat this problem, aftermarket rods are often fitted with low-profile bolts. Some companies take it one step further by using cap screws in lieu of bolts, which thread directly into the big end of the rod instead of relying on a separate nut.

Regardless of the particular style of fastener that is used, aftermarket rods come equipped with heavy-duty bolts or cap screws. ARP offers 8740 chromoly fasteners rated at 220,000 psi of tensile strength, and some rod manufacturers offer their own proprietary fasteners rated at up to 280,000 psi. Considering that they’re the most highly stressed fastener in an entire engine, quality rod bolts are cheap insurance.

 

Rod Length

In stroker builds utilizing a standarddeck block, 6.100-, 6.125-, 6.200-, and 6.250-inch rods are most common. Longer 6.460- and 6.560-inch rods are used more frequently in tall-deck motors with longer strokes. Like those in the Gen I small-block, LS crankshafts incorporate 2.100-inch rod journals. This means that Gen I and Gen III/IV connecting rods are interchangeable, because they share the same big-end housing diameter. One important difference is that Gen I rods have a smaller .927-inch small-end bore than the .945- inch bore on rods used in stock LS motors. Consequently, using Gen I rods in a Gen III/IV motor requires matching them with pistons that have a smaller .927-inch small-end diameter. Although most aftermarket LS rods are built with the smaller Gen I small-end, some are offered in a .945-inch housing bore to make them compatible with pistons that use stock-size LS wrist pins.

 

 

One of the most controversial topics in engine building is the supposed benefits of maximizing connecting rod length. Most people tend to overgeneralize this issue, and legions of hot rodders firmly believe that using a longer connecting rod yields dividends in horsepower over a shorter rod. An entire chapter can be written on this topic alone, but suffice it to say that the benefits of using a longer connecting rod over a shorter rod are two-fold.

Proponents contend that a longer connecting rod increases high-RPM horsepower, because it forces the piston to move more slowly away from TDC during the power stroke. In theory, this allows for a greater buildup of cylinder pressure and increases power. Longer rods also reduce the angle of the connecting rods in relation to the pistons. This relieves side loading and friction from the piston skirts, which is said to improve both durability and horsepower. Unfortunately, very few engine builders and hot rodders have gone through the painstaking efforts necessary to test this theory. Interestingly, those who have—including GM engineers and the Pro Stock engine builders at Reher-Morrison—aren’t convinced that longer connecting rods provide any performance benefits at all.

A more accurate way to compare connecting rod length between different engine combinations is by looking at their rod-to-stroke ratios. For example, most LS small-blocks that have a 3.622- inch stroke use a 6.098-inch connecting rod, and this equates to a rod-to-stroke ratio of 1.68:1. From a mathematical standpoint, a couple thousandths of an inch of rod length doesn’t impact the ratio much at all. In an exhaustive series of dyno tests that Reher-Morrison performed on NASCAR engines for GM, the shop varied the rod-to-stroke ratio from 1.48 to 1.85:1. In the test, mean piston speeds were in the 4,500- to 4,800-fps range, and painstaking measures were taken to minimize variables. The result was zero difference in average power and no difference in the shape of the horsepower curves. According to Reher-Morrison, one could have laid the curves over each other without being able to distinguish the difference between the different rod-to-stroke ratios on paper.

Although the effects of varying rodto-stroke ratios might not always be evident on the dyno, there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is extreme enough that it increases the side loading of the piston by a noticeable margin, increases piston rock, and increases skirt load. Although this doesn’t necessarily change the actual power an engine makes, it does accelerate wear. Conversely, above a 1.80:1- or 1.85:1 ratio, there’s so little piston movement at TDC that it hurts the ability of the pistons to draw in air on the intake stroke. Compensating for this requires advancing the cam, or decreasing the cross-sectional area of the cylinder head ports or intake manifold to increase air velocity. When it comes to rod length, the biggest mistake a hot rodder can make is compromising an entire engine combination by trying to achieve a target rod-to-stroke ratio. Consequently, according to Reher-Morrison, from a performance standpoint, connecting rods are nothing more than pieces of metal that connect the pistons to the crankshaft. It’s as simple as that.

 

Manufacturer Choices

Trying to distinguish one manufacturer’s connecting rods from another based on looks alone can be difficult. To help you sift through the dozens of aftermarket rods available, here are some of the most well-known LS rods on the market for both street and strip applications. With such a broad selection of quality aftermarket rods now available, reconditioning stock rods is a thing of the past.

Callies

As with its premium crankshafts, Callies’ Ultra connecting rods are 100 percent American made. Forged from a proprietary 4340 Timken steel alloy, the units are premium rods with a premium price of roughly $1,300 per set. That investment buys hardware replete with innovative design features capable of withstanding more than 1,000 hp. The Ultra rods have gussets around the cap screws to help fortify the big end of the rod. Profiled shoulders improve clearance in long-stroke engines, and the cap screws are rated at 260,000 psi. Additionally, the Ultra rods’ I-beam design reduces mass over a comparable H-beam. Covering a wide range of stroke lengths and block deck heights, they’re available in 6.100- to 6.560-inch lengths.

Compstar

Although Compstar was conceived as Callies’ Sportsman line in 2004, the company puts tremendous effort into ensuring top-notch quality in its entry-level products. Forged overseas to keep costs down, Compstar rods start at $400, making them an excellent value. All Compstar rods are forged from 4340 steel, and they are offered in I-beam, H-beam, and H/I-beam configurations. Common features throughout the Compstar line include premium ARP 2000 rod bolts, chamfered and honed pin bushings, and stress-relieved surfaces. According to Compstar, its rods clear 4.000-inch-stroke cranks and cams with up to .660-inch lift without any modifications.

Compstar’s most affordable rods are its I-beams, which can support 650 hp and cost less than $400 per set. They feature a fortified shoulder, a large parting line footprint, added material around the pin collar, a twin-rib cap, and ARP L19 cap screws. For engines that require a stronger rod, Compstar’s H-beam rods come in lengths ranging from 6.100 to 6.560 inches. Officially, they have no published horsepower rating, but engine builders routinely push them past 850 hp without a problem. For forced-induction and nitrous motors producing more than 1,000 hp, Compstar offers a unique H/I-beam hybrid rod design in a 6.125-inch configuration. The added strength comes from a 25-percent-thicker beam and triangulated big-end and pin bores.

Eagle

As with its crankshafts, Eagle offers a diverse lineup of connecting rods to suit virtually all power levels and engine combinations. The entry-level SIR I-beam rods are forged from 5140 steel, and they can handle up to 700 hp. They come in 6.125-, 6.200-, and 6.250-inch sizes. At $300 for a set, they pack some serious value. For not much more money, Eagle’s H-beam rods can handle much more horsepower. Equipped with standard rod bolts, Eagle H-beams are rated at 750 hp. Upgrading to ARP 2000 bolts increases that figure to 1,100, and stepping up to a set of ARP L19 bolts increases the horsepower rating to 1,400. Eagle’s premium H-beams cost just $550, which explains why overseas rods have become so popular in recent years. As far as strength per dollar is concerned, Eagle rods are hard to beat.

Lunati

Lunati’s connecting rods for LS-series small-blocks come in three different trim levels. Its entry-level H-beams are forged overseas from 4340 steel, and they are rated at 700 to 800 hp. The rods are finished in-house, and then they’re heattreated, shot-peened, and stress-relieved. They’re offered in just one size, 6.125 inches, and sell for $630. For just $30 more, Lunati’s Superlight H-beam rods have many of the same features as its standard-weight H-beams, but they are 75 grams lighter (680 vs. 605). The reduced reciprocating weight they offer makes them well suited for circle track and land-speed engines in which prolonged high-RPM operation is the norm. At the top of the Lunati totem pole are its Pro Series connecting rods. These Ibeams are made in America from aerospace-grade 4340 steel. At $1,300, they’re a bit on the pricey side, but the benefit is that they can handle well in excess of 1,000 hp. In fact, Lunati says that they’ll handle just about anything you can throw at them. Available in 6.125- and 6.300-inch lengths, the Pro Series rods are an excellent choice for extreme power adder applications.

Manley

Throwing a monkey wrench into the I-beam-versus-H-beam debate, Manley positions its I-beams at the very bottom and top of its connecting rod lineup. Manley’s Sportsmaster I-beam rods are forged overseas from 4340 steel and measure 6.100 inches in length. The rods are shot-peened, stress-relieved, and heattreated. Additionally, the Sportsmaster rods are profiled around the main cap area to remove stress risers and reduce mass to just under 600 grams. They’re rated at 550 hp and cost $650 per set.

Manley’s mid-level 4340 H-beam rods are also forged overseas. They measure 6.125 inches and are available with ARP 8740 or ARP 2000 bolts, for power ratings of 725 and 775, respectively. Interestingly, they’re priced cheaper than the Sportsmasters at $600.

At the top of the heap are Manley’s Pro Series 4340 I-beam rods, which are manufactured in the United States. Priced at $1,500 a set, these 6.125-inch rods can handle more than 1,000 hp. In addition to brute strength, the Pro Series rods are very light at just 609 grams.

Scat

Like many aftermarket companies, Scat forges its rods overseas, and then it performs final machine work in the United States. To ensure quality control and exacting tolerances, Scat rods are machined with modern diamond tooling, and they are heat-treated using temperature-controlled cooling.

Scat’s entry-level Pro Comp I-beam rods feature 4340 steel construction and are offered in 6.100- and 6.125-inch lengths. They’re lightweight at roughly 600 grams, and they are available with ARP rod bolts or cap screws. Rated at 550 to 650 hp, they’re best suited for mild naturally aspirated engine combinations, and they are budget priced at $325 per set.

Scat’s premium Gen III/IV rod offering is its Pro Sport 4340 H-beams. Available in 6.100- and 6.125-inch lengths, Scat’s H-beam rods are rated at 800-plus hp, making them ideal for forced-induction and nitrous motors. ARP 8740 cap screws come standard, and ARP 2000 fasteners are optional. With prices starting at $450, the Scat H-beams offer excellent durability for the money.

 

Written by Barry Kluczyk and Posted with Permission of CarTechBooks

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