Let’s start with good old common steel. Steel is a great material, it’s strong and it’s plentiful and has been the material of choice for connecting rods for many many decades. Steel rods can be cast, forged or billet. Cast ones are ok for stock applications, but are usually a bad idea if you’re interested in significantly increasing the power and torque output of your engine. Forged rods offer a superior grain structure within the metal, which becomes far more coherent thanks to the large pressures exerted on the rod during the forging process. Billet steel rods start out by having their rough shape cut out from a plate of forged steel and then finish machined on a CNC machine. Billet rods don’t have the surface degradation that occurs during forging, which means a fully machined billet rod has the same kind of material with the same carbon content and quality both in it’s core and on its surface, making billet rods better at resisting the formation of cracks. The majority of steel connecting rods found in OEM applications use steels from the 51XX series, so stuff like 5130 or 5140. When it comes to aftermarket forged connecting rods you will typically see the 4340 alloy, which in addition to having a high carbon content also has other elements (nickel and molybdenum) which make it a superior connecting rod material. Steel has a tensile strength of approximately 200.000 psi and excellent fatigue life, the material doesn’t get tired unless you push it to it’s yielding point.
Now let’s take a detailed look at aluminum connecting rods. We know that aluminum is a much weaker material than steel. While high carbon steel typically has a tensile strength of around 200.00 psi, aluminum only manages around 95.000 psi. So why in heaven’s name would you put something that’s twice as weak inside an engine and expose it to all the extreme loads of engine operation? Because aluminum is a lot lighter than steel! And when it comes to performance engines light is right! The lighter the rotating assembly of your engine – the better! Aluminum rods also have the ability to act as shock absorbers. Because they sort of give in a bit to the peak loads present in an engine, they help absorb these loads and transfer less of the stress onto your bearings and crankshaft. But there’s a price to be paid for all the benefits. Aluminum has a much shorter fatigue life compared to steel and engines with aluminum rods must be warmed slowly and fully before you can beat on them, and once you beat on them you have to let them cool of a bit. Something else you need to consider when installing aluminum rods into your engine is clearance. Sometimes they don’t clear stuff in your crankcase, like girdles or the bases of the cylinders, and you need to adapt these to suit the rods.
And now for exotic guy in the bunch! Titanium. Many describe titanium as an incredibly strong material, often stating that it is stronger than steel. This is a bit misleading. The reality is that titanium is impressively strong compared to it’s density. Titanium is significantly less dense than steel while maintaining comparable strength which means it’s lighter than steel but with pretty similar strength. This is why titanium rods don’t need to be thick like aluminum rods. In fact titanium rods will usually look similar to steel rods. Aluminum rods are typically machined out from billets of high quality aluminum alloys and are rarely forged. When it comes to titanium the opposite is true because the forging process greatly benefits titanium and helps increase it’s strength. Titanium is less dense and it also has smaller grains compared to steel, so the forging process does a lot to help improve the grain flow and increase the strength of titanium. The downside is that titanium rods are prohibitively expensive and that’s not jut because the alloy itself is expensive but because titanium is very difficult to machine. Titanium is also very susceptible to galling, or friction welding but the galling issue has been largely solved with coatings such as chromium nitride or titanium nitride which is why you can find titanium rods in mass production vehicles like the Honda (Acura) NSX or the Corvette Z06 with the LS7 engine. By the way, the first ever application of titanium rods in a production vehicle was Honda’s amazing RC30 motorcycle. Titanium is also very notch sensitive so you have to be careful not to scratch titanium rods when handling them.
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