Tech: Connecting Rod Failure
This Integrated Engineering tech article is meant to take the mystery out of connecting rod failure in engines with horsepower outputs far greater then the original design specification, and explain the ways in which an aftermarket connecting rod is different from a factory piece, to prevent this failure.This article is written with special emphasis placed on VW / Audi engines such as the 1.8t 20v, VR6, 16v, and other engines.
When a connecting rod is designed, the engineers take into account the expected duty of the rod, that is, the loads which will be placed on it by the mass of the piston accelerating and decelerating from the stroking motion, loads generated by friction, and of course the load placed upon it by cylinder pressure during the combustion stroke. Once these loads have all been calculated and summed, it is possible to specify a material. Using published material data from their suppliers, the engineers pick a strength at which the material can be subjected to an infinite number of cycles without failing, and then design the beam of the rod with sufficient cross sectional area so that this chosen strength is not exceeded. This effectively gives you a beam design which will last for well beyond the life span of the motor provided that design levels of RPM, piston mass, and cylinder pressure are not exceeded.
Clearly, one can run into problems quite quickly when we make the choice to increase the rev limit of the motor, add a larger turbocharger and higher boost levels, thus increasing the cylinder pressure, and causing higher friction from the ring pack. Between these two we have effectively increased all loads on the connecting rod well beyond their specifications, the extent of this depends on the modifications. Once the OEM safety factors have been exceeded, we move into a portion of the fatigue strength to cycles curve at which the number of cycles until failure are no longer infinite-- that is, your engine has become a ticking time bomb. The number of cycles a rod will survive in this state just depends on how far past OEM specifications you have pushed your rods.
Avoiding the perils of metal fatigue is where Integrated Engineering comes into play. We provide rods which feature significantly stronger base materials- SAE 4340 Steel in most cases, which has been forged into shape, then 100% machined to our exacting specifications. Care is taken to strengthen this material even further by applying a multi-stage heat treatment, and then the rods are externally shot peened, a process which helps alleviate any stress concentrations which can lead to premature fatigue failure. By contrast, the OEM VW 1.8t rods in 2002 and newer models are a pressed together form of iron called sintered iron, which is left unfinished on the beam of the rod, and merely machined to dimension on the two ends to make a workable piece. While some OEM VW rods are forged, they are left as rough forgings, and they are forged from a plain steel alloy, which has nowhere near the strength of SAE 4340. Furthermore, their beam designs are conservative in comparison- Even if the material we used was not as strong, our rods would be significantly stronger simply due to the strength inherent in the large H beam design of the beams themselves. These advantages allow our rods to operate safely in far more severe conditions then the OEM pieces they replace, allowing you to boost higher, rev faster, and operate squarely under the fatigue limit for strength, ensuring a long life for your engine without any chance of failure. In summary, connecting rod failure is avoided through careful design, manufacturing, and materials science treatments to extend the fatigue life of a connecting rod well out past the lifespan of the rest of the engine, even in the harshest environments. Only when all of these aspects merge can one achieve success, and this is what separates us from the pack.
