Connecting rods are a major part of your internal combustion engine. It’s the part that connects the piston to the crankshaft. The connecting rod, along with the crank, converts the reciprocating motion of the piston into the rotation of the crankshaft. The rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.
Any internal combustion engine the pistons withstand tremendous heat and pressure of combustion. Your connecting rods must persevere through the pounding as they transform the pistons reciprocating motion into the rotational movement of the crankshaft. Vehicles that are producing higher horsepowers, typically have higher compression. These vehicles that operate at higher RPMS, have power adders like nitrous, turbochargers or super chargers the need for stronger internals is imperative. It’s important to have pistons and rods that are able to endure this extreme strain and temperature from these types of engines. Additionally it’s important for these parts to be as light as possible, allowing your engine to rev freely, while creating very little frictional power loss.
When upgrading your connecting rods there are a lot of factors that go into choosing the right ones for your engine. Proper rod selection is one of the most important decisions you can make when building your engine. They play a huge role in engine performance and longevity. With this article we hope to make upgrading your rods a little bit easier by breaking down the differences.
Your factory rods are typically made of cast iron or powdered metal. These rods are not designed for performance use. When you have a performance vehicle the connecting rods have to be strong enough to handle all the horsepower the engine can make. As well as and 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. Luckily, performance rods are precision made. They feature uniform center-to-center dimensions, bore concentricity, and weights. Performance rods are also commonly bushed for floating pins. They have stronger and larger diameter rods bolts. They are also available in varied lengths to suit the desired rod ratio.
Most performance rods are manufactured from forged or billet steel, titanium, and aluminum rods. These materials are generally lighter in weight vs. factory rods. The weight of the rods is important because it affects the reciprocating forces inside the engine. The lighter the better because less weight means faster throttle response. As a result you will experience faster acceleration.
Steel vs. Aluminum vs. Titanium
When we’re talking about Steel connecting rods theres one word that immediately comes to mind, strength. Steel rods offer the highest strength capabilities. Steel rods can have a tensile strength of approximately 200,000 psi. However all this strength comes with a price. Steel connecting rods tend to be heavier than other materials such as aluminum. They also tend to be tougher on pistons.
Aluminum rods can be as much as 25% lighter than steel rods. This makes them a popular choice amongst racers. Aluminum may not be as strong as steel, but is often used in very high power engines. This is due to the fact that aluminum ‘gives’ slightly under violent combustion and acts like a ‘shock absorber’. Therefor the harmful shock of combustion does not propane to the road bearing and cause a bearing failure. The downside to installing aluminum rods is their fatigue life. Under highly stressed applications aluminum rods will begin to lose their strength.
Titanium connecting rods combine the lighter weight of aluminum with a strength comparable to steel. That makes them a great option for track cars that require quick throttle response. However Titanium rods tend to be very expensive.
I-Beam vs. H-Beam
Rods essentially come in two types: I-Beam and H-Beam.
I-Beam rods are the most common style used. This style is used amongst most stock connecting rods, as well was performance rods. They have a large flat area that is perpendicular (90 digress) 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. They provide good compressive strength. I-Beams can handle high horsepower loads. Additionally they can be made thicker, and machined in special ways to improve strength.
Depending on what manufacturer you decide to go with the I-Beam design may vary. Some may have a machined scolloped effect between the beams, leaving a rounded area next to both beams. This helps increase strength and rigidity. These rods are often marked as having an “oval-beam” or a “radial-beam”.
H-beam rods features 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 stiffness. This design can provide higher compressive strength than a comparable I-Beam. H-Beam rods are often recommended for high torque motors that produce a lot of power at low rpms.
Overall H-Beam rods are a stronger design when bending stress is considered. H-Beam rods are more difficult to machine, so they tend to have a higher ticket price. In contrast, I-Beam rods are easier to produce allowing for a slightly better cost. I-Beam rods can sometimes be lighter than H-Beam rods. I-Beam rods can also be manufactured to be slightly thicker, to help increase strength. Specific designs across manufacturers and materials used will also vary. These designs and materials will change weights, and strength factors.
Depending on your intended use of your vehicle, the rod that works best for you will vary. A truck which does heavy duty towing will need the reliability of a heavier crank and the strongest possible rods. While a track car who is looking to break into new records will benefit from an ultra light crankshaft with the highest possible rods and pistons. Overall, you want a set of rods that can handle all the forces your engine will generate, as well as help your vehicle perform its best for your intended use. Its always advised to talk to your mechanic or an experienced representative when deciding on upgrading your rods. They can help you reach your goals for the engine your building, and recommend the best set up.