Engines may seem complex. They consist of hundreds of parts, but the main goal is to produce power to turn the wheels. This is done by burning a fuel and air mixture at the right ratio. In petrol engines, the mixture is ignited by a spark plug, whereas in diesels higher compression does the trick. All this happens in the combustion chamber. As the air-fuel mix combusts, the resulting energy is thrust against pistons and piston rods and onto the crank. While most car enthusiasts know exactly how important the role of crankshafts and pistons is, it’s the connecting parts, the conrods, that are little understood.
Conrod Basics
Connecting rods, or conrods for short, are engine components that transfer the linear motion of pistons in the cylinders to the crankshaft. They are attached to the pistons via a piston or wrist pin, with the bottom end attached to crank pins. Their main function is power transfer in each engine cycle. The parts are some of the most stressed engine components, being pushed and pulled in each engine cycle, and facing the extreme heat and pressure generated during combustion.
Anatomy of Connecting Rods
A conrod consists of several parts. This is to ensure easier manufacturing, and strengthening sections exposed to higher stresses. The small or top end is connected to the piston, and the pistons themselves include the conrod small-end bushings that are grooved for optimal lubrication to endure the forces of accelerating and decelerating pistons as engine speeds change.
Balancing the connection of pistons and piston rods is done with pin inserts. The conrod then elongates into a flat shaft section and attaches to crank pins via a connecting rod cap. This is a pre-assembled part that is held together with a high-strength bolt and nut combo. To prevent lower wear as cranks spin and twist, the caps are lined with two conrod semi-circular bearings. These also include lubrication holes and recessed portions that deal with excess friction.
Main Types
There are several considerations when selecting conrods for your car. Different designs are meant for different applications. There are two main options in this respect. I-beams, with shafts resembling the capitalised letter when viewed in cross-section, are found in stock and unmodified engines. They’re lightweight, popular and cheaper options, and with recent designs now being used for high-performance applications with high engine speeds and where tensile strength is important. They’re also preferred in turbocharged engines.
H-beams have higher compression strength (meaning less bending), so geared to larger aspirated engines pushing out more power and torque. The intended uses have been blurred in recent times, but as a rule of thumb, most auto enthusiasts will chose I-beams for higher revving engines, whereas H-beams, due to thicker shafts are chosen for racing applications with high torque figures.
Material Options
Materials largely determine compressive and tensile strength (preventing conrods from snapping or bending), overall weight and general resistance to wear from heat and oil. Again, different materials will be suited to different engines. As an illustration, the high compressive forces acting on diesel internals require choosing piston rods able to cope with increased pressures, or in this case, forged alloyed steel.
Steel is also found in different grades in conrods in both petrol and diesel cars. The material is suited to production cars and everyday driving applications, with high resistance to fatigue and wear. Somewhat more expensive is carbon steel, found in exposed sections like the caps due to the higher strength.
For high-revving cars, choose lightweight materials like aluminium. While short on overall strength compared to steel, the material has more flex in the constant elongation and contraction rods face as they move up and down. Aluminium is also preferred in stroker setups, with longer rods, and to offset the weight gains.
Lastly, if money is no issue, there’s a slew of exotic materials, specifically titanium blended with trace materials like molybdenum and vanadium. These can handle higher torque and revs required in racing, and also come at a fraction of the weight of steel conrods. But with the engineering involved, you’ll need to shell out quite a bit of cash, especially for one-off custom engines.
How They’re Made
Cast conrods are the most common and the cheapest type, regardless of materials. Cast aluminium and steel connecting rods are offered as standard gear in production models, are well adjusted to moderate power and torque applications, but will also be happy with shorter high-speed runs. For performance applications, turn to forged or billet conrods. Forged types offer a good balance of weight, strength, performance and price, and are chosen in modifications from stock. Billet types have the highest strength, but the complex production methods using a single piece of metal (then machining it to tight tolerances) keep these well beyond the reach of most street applications.
Final Thoughts
Consider that you’ll be buying kits with the number of connecting rods the same as the cylinder count. If you’re completely rebuilding your engine, pair conrods with pistons of the same strength, and consider crankshaft treatment or upgrades. With all mods in place, the engine has the potential to handle power upgrades such as forced induction with turbo or supercharger add-ons and reworked timing and fuelling extras (performance cams and high-flow injectors) without mechanical risks.
