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Connecting Rods: So Many Choices

When building an engine, of all the decisions you need to make regarding parts specification, connecting rods are the easiest, right? After all, how tough can it be? There’s only a couple of different choices, so you can just pick one and go…or can you?

By Larry Carley

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Choosing a set of connecting rods for a performance engine is not as simple as it sounds. The rods you ultimately choose to use in an engine will depend on a number of factors, each of which can be critical to the life of the motor and the success of your customer.

Your decision will be shaped by:

• The type of engine you are building (drag, circle track, street, endurance, diesel or marine);

• If rod selection is limited by rules (which is often the case in many circle track classes);

• The desired torque and horsepower curves the engine will produce (are you building a low rpm, long stroke, large displacement torque motor, or a high revving, peak horsepower motor?);

• The maximum rpms it will turn;

• The physical dimensions of the engine itself (stroke, rod ratio, piston height, deck height, standard block or tall block, crankshaft journal diameter and wrist pin size);

• The relative importance of weight, strength and reliability;

• The type of rods you or your customer want, or the style of rods that are available to fit the engine you are building (I-Beam, H-Beam, A-Beam or other variants);

• The type of rod material you or your customer want, or is available to fit the engine you are building (4340 or 300M forged or billet steel, aluminum, powder metal or titanium);

• You or your customer’s brand preference (which includes the brand’s reputation, your experience with that brand, and your relationship to the rod supplier);

• Whether you can use ready-made, off-the-shelf rods in your motor, or you need rods custom-made to your exact specifications;

• How much your customer can afford to spend on a set of rods (which often over-rides everything else!).
Every one of these factors must be considered carefully when choosing a set of rods because the rods ultimately affect engine performance, reliability and how much profit you make on the job. The rods you choose can also affect your reputation. If the rods you ultimately decide to use in a customer’s motor fail, your customer may blame you for putting the “wrong” rods in his engine.

Horsepower vs. RPM
When it comes to rod selection, which is more important: horsepower or rpm? Higher power levels increase the compressive force on the connecting rods while higher rpms increase the tensile strain on the rods. As it turns out, most rods don’t bend and fail on the compression stroke but are pulled apart at high rpm and break on the exhaust stroke. Consequently, rods need additional compression strength and stiffness to handle higher horsepower loads. But in hig- revving engines, increased tensile strength is an absolute must for the rods to survive at high rpm.

The stock rods in most V8s are stout enough to handle upwards of 400 to 450 horsepower, and 5,500 to 6,500 rpm. Beyond that, reliability begins to suffer. Upgrading to stronger rods becomes increasing necessary as horsepower and/or rpms go up. Now you can start to compare the relative merits of various rod configurations and materials.

The two basic styles of connecting rods are I-Beam and H-Beam. Some rod suppliers only make I-Beams, others only make H-Beams, and some offer both types or variants of the I-Beam design. The I-Beam design is used for most stock connecting rods because it provides a good combination of light weight and strength.

An I-Beam rod can handle high compressive loads while also providing good tensile strength. But the thickness and strength of the steel in the rod limit what it can safely handle. So performance I-Beam rods are typically made of a higher grade of steel (4340 or 300M), and often have a thicker cross-section in critical areas to increase strength.

H-Beam rods have a completely different design. An H-Beam rod has two large, flat sides that are perpendicular to the piston pin and crankshaft journal, with a thin center section in the middle. This makes the H-Beam design very stiff so it can handle higher compressive loads without bending.

Which is stronger, I-Beams or H-Beams? It depends whom you ask, and the relative weights and cross-sections of the rods. I-Beams can be just as strong as H-Beams, but H-Beams can often handle higher compressive loads than I-Beams with less overall weight.

Consequently, H-Beam connecting rods are often recommended for high torque motors that produce a lot of power at low rpm (under 6,000 rpm). Some rod suppliers offer H-Beams as their “entry level” or less expensive line of performance rods, and offer I-Beams for all of their high end racing applications. Other suppliers only sell I-Beams, and some only sell H-Beams.

Quality Issues
Whether I-Beams, H-Beams or something else, the alloy used in a set of rods and the subsequent heat treatment the metal undergoes during the manufacturing process are extremely important for both strength and reliability.

Most forged and billet steel rods are made from 4340 steel. This is often referred to as an “aircraft” grade alloy because of its superior strength and durability. Steel that meets American Iron and Steel Institute (AISI) 4340 standards contains 1.65 to 2.0% nickel, 0.70 to 0.90% Chromium, 0.60 to 0.80% manganese, 0.20 to 0.35% silicon, and 0.20 to 0.30% molybdenum. The addition of these elements to the steel give it hardness, toughness, ductility and fatigue resistance.

The ultimate tensile strength, yield strength and hardness of 4340 steel also depends on the temperature at which the steel is forged into a connecting rod blank or billet, and how the steel is heat treated. Variations in the tempering temperature and quenching procedure can produce widely different results, with tensile strength, yield strength and even hardness varying as much as 2X!

Many rod suppliers are making steel connecting rods out of 300M alloy. This alloy has a higher level of silicon (1.45 to 1.80%) and a little more carbon and molybdenum for added strength. This allows the thickness and cross-sectional area of the rod to be reduced so the rod can be 10 to 20% lighter than a comparable rod made of 4340 steel.

One of the biggest issues facing both rod suppliers and engine builders today is steel quality. Those who still use domestic-made forgings say some of the offshore product identified as 4340 steel does not meet AISI 4340 standards. Said one disgruntled rod supplier, “They do not meet specifications and they do not hold up in high horsepower or high rpm applications.”

When steel is produced from recycled scrap, it’s not as easy to control the makeup of the alloy that pours out of the furnace. This concern regarding overseas manufacturing has become a hot button of debate, and while it is certainly inaccurate to label ALL foreign-made product as inferior, rod suppliers who are concerned about the quality of their products are testing the forgings to make sure the steel meets specifications. Those who don’t test their forgings may be taking a big chance, say suppliers.

In recent years, the U.S. aftermarket has been flooded with dirt-cheap connecting rods. Many of these are H-Beam style rods, which are either raw forgings (that are final machined here) or fully finished rods. According to several rod manufacturers we interviewed for this article, rods from China or India reportedly may cost as little as $10 apiece when purchased in bulk quantities – which is far less than what forgings made in the USA cost. This creates a huge profit opportunity for distributors and rod suppliers who can resell them to end-users for $600 to $800 a set.

To make matters worse, there has also been a reported epidemic of knock-off products being sold as brand name connecting rods. One manufacturer of high-end connecting rods said, “Probably 60% of the rods you see listed for sale on eBay are not our rods. Unless the rods come in our box and have our name on them, they are not our rods. If the price is unusually cheap, there’s a reason why. We get three or four calls a day from people who have bought these phony rods and have had problems with them. It’s tarnishing our reputation as a supplier of quality products.”