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. Connecting rods have to be stout enough to handle all the horsepower the engine can make, and be strong enough to withstand the tension forces that try to pull the rod apart when the piston hits top dead center on the exhaust stroke. If a rod is going to break, more often than not it will fail at TDC on the exhaust stroke than at any other point in its travels. Consequently, piston weight and maximum engine rpm are more important factors to consider than how much power the engine will make when selecting a set of rods.
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 if you are building a large displacement, relatively low rpm, high load drag motor, truck pulling motor or a marine engine, you need the reliability of a heavier crank and the strongest possible rods.
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? The rods that work best in an all-out drag motor probably wouldn’t be the best choice for a street performance engine. Nor would rods designed for a circle track sprint car be the best choice for a NASCAR engine or a marine endurance engine.
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 site are not updated as frequently as they should be.
The engineers who design connecting rods know how to analyze the forces that act on rods. Years ago, the design process involves a lot of trial-and-error testing. An engineer would design a rod configuration, test it until it broke, then try to beef up the areas of the rod he felt were weak. Today, most of the development work is done with computers and sophisticated software. Engineers nowadays use finite element analysis (FEA) to analyze the compression and tension forces on a rod. The software creates 3-D images with color coding that indicates the areas of highest and lowest stress. This allows the engineer to visualize what’s actually happening to a rod at various loads and speeds. He can then tweak the design on his computer screen to add metal where extra strength is needed, and to remove metal from lightly loaded areas where it isn’t needed. By repeating the FEA process over and over with each design change, he can optimize the rod to deliver the best possible combination of weight, strength and reliability — in theory anyway. It still takes real world testing to validate the design. But with today’s design and analysis software, most of the work is done before a prototype part is manufactured.
One rod supplier said using FEA on their current rods allowed them to increase strength 12 to 15 percent with less than a 2 percent increase in overall rod weight.
Computer controlled numeric (CNC) machining also allows manufacturers to machine billets and forgings in ways that were previously too difficult, too time-consuming or too expensive. This allows manufacturers to offer a wider variety of rods in terms of rod length and beam construction. It also allows them to produce custom made-to-order rods very quickly. In fact, some rod suppliers say the majority of the rods they sell today are custom order rods rather than standard dimension rods from off the shelf stock.
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.
STOCK CONNECTING RODS
Over 60 percent of late model connecting rods are powder metal I-beam rods. Powder metal (PM) rods are made by compressing powered steel in a mold and then heating the mold to a temperature where the powder melts and fuses into a solid part. This method of manufacturing allows parts to be cast to very close tolerances. This reduces the amount of machining needed to finish the rod, which lowers its cost. Powder metal casting also allows the ingredients in the steel alloy to be combined in ways that are impossible with conventional metal casting techniques, and the finished parts have less internal stress as a result of the fusing process. PM rods can also be up to 20 percent lighter than a comparable rod made of forged steel. Only one aftermarket rod supplier (Howards Cams) currently offers performance rods made of powder metal.
The special alloys that are used to make powder metal rods allows the rod caps to be “cracked” (separated) from the rod rather than cut. Score marks are cast into the part along the rod parting line, and the cap is then sheared off in a large press. The cracking process leaves a slightly irregular surface along the parting line between the cap and the rod that is like a jigsaw puzzle and only goes together one way. The result is better cap alignment and a stronger rod when the cap is bolted to the rod.
One of the drawbacks of powder metal rods is that the caps can’t be reground to compensate for bore distortion or stretch. Consequently, if the rod bore is out-of-round or worn, the rod usually has to be replaced unless a replacement bearing with an oversized outside diameter is available.
Stock rods are typically designed for 5,500 to 6,500 rpm, and 300 to 350 horsepower in a V8 engine. In an overhead cam four or six cylinder engine, the rods may be designed to handle up to 7,000 rpm but probably only about 200 to 250 horsepower. As a rule, most stock connecting rods can handle up to 25 to 40 percent more horsepower than an unmodified engine was originally designed to produce. So for a typical budget street performance engine or a Saturday night dirt track racer, the stock rods may work just fine.
Even so, to ensure reliability the rods should always be “Magnafluxed” to check for cracks. Any flashing, burrs, nicks or other defects along the sides of the rods should also be ground off (grind lengthwise, never sideways) to eliminate stress risers that could lead to cracks and rod failure later on. Shot peening is also recommended to improve fatigue resistance. When the shot strikes the surface, it compresses the metal slightly and actually relieves stresses that might lead to cracking and rod failure.
If an engine is being built to turn significantly higher rpms than the stock motor, or to produce significantly more power (more than 40 to 50 percent), the connecting rods will probably have to be upgraded to assure adequate reliability. For a high revving engine, some type of stronger aftermarket I-Beam rods would be a good choice. For a low rpm torquer motor, either H-Beam or heavier I-Beam rods would work well.
ROD MATERIALS & APPLICATIONS
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. Labor costs are far cheaper in China and other Third World countries, so there are cost advantages for suppliers who source their forgings and rods from offshore manufacturers. 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.
The mistake you don’t want to make, however, is to use low priced “economy” rods in an engine that really needs a set of top quality performance rods. 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. Most of these rods are made overseas (in China, primarily) and typically sell for less than $600 a set for a small block Chevy V8. The companies who sell these rods say their products are ideal for racers who otherwise might not be able to afford better rods for their engine. 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 would not be 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 (China’s exploding economy being one, the ongoing war in Iraq being another, and changes in the steel industry itself being a third reason). 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 $1000 or more per rod, who can really afford them?
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 rpm 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 (GRP is one), and at least one supplier who used to offer aluminum rods (Manley) 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.
BY THE NUMBERS
Other factors that affect the selection of rods are rod length and rod ratio. Rod length depends on the stroke of the crankshaft and the deck height of the block. If you are switching to crankshaft with a longer stroke, you are obviously going to need rods that have a shorter overall length. Even so, replacing the pistons with ones that have a higher wrist pin location can allow you to use longer rods.
Racing legend Smokey Yunick used to say that the longer the rods are, the better. His logic was based on the fact that a longer connecting rod for a given stroke allows the piston to dwell longer at TDC before it starts back down on the power stroke. This allows pressure to build longer in the combustion chamber before it starts to shove the piston down. The result is usually a broader, flatter torque curve than the same engine with shorter rods. An engine’s horsepower and torque curves depend on a lot of variables other than rod length alone. But if everything else is equal, many engine builders say longer rods produce a broader torque curve. Others disagree, and say it doesn’t really matter.
Rod suppliers say the only trend they see in rod lengths today is that there is no trend. Engine builders are buying just as many standard length rods as they are longer rods.
This brings us to rod ratio, which is the length of a connecting rod (center to center) divided by the stroke of the crankshaft. The range in engines today may be from 1.5 to 2.1, but most performance engine builders are going with ratios in the 1.57 to 1.67 range. Some say that going with a rod ratio over 1.7 makes engine torque too “peaky.” Lower rod ratio numbers are typically associated with lower rpm torque motors (a 383 Chevy street motor with a stroker crank and a rod ratio of 1.52, for example), while higher rod ratio numbers tend to be high revving high horsepower motors (a 302 high revving Chevy with a rod ratio of 1.9).
Another dimension to consider is the pin offset of the rod. On most rods (except Chevy “LS” engines), the pin bores are offset slightly. The change in pin geometry reduces the stress on the piston pin and small end of the rod when the piston reaches TDC and changes direction. It also reduces the rocking motion of the piston as it passes TDC to reduce piston slap and noise.
One new trend in this area is to run rods that do not have bronze bushings in the small end. Several racing teams are running bare rods with specially plated pins to improve durability. Eliminating the bushing, they say, leaves more meat in the small end of the rod for added strength. The only drawbacks are that the fit between the rod and pin has to be much more precise, and the wrist pin has to have a wear-resistant coating to prevent wear and galling. Also, if the pin bore becomes worn or out-of-round, the rod and piston will both have to be machined to accept a slight larger diameter pin.