STEEL vs ALUMINUM vs TITANIUM Connecting Rods
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Publicado 2020-07-12
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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Todos los comentarios (21)
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Let's hang out: superpeer.com/driving4answers Support d4a: driving-4-answers-shop.fourthwall.com/ Forged steel rods Subaru: amzn.to/2Eb8LoP AEM wideband AFR gauge: bit.ly/D4Axserieswb AEM ECU: bit.ly/D4Ainfinity5 D4A merch: teespring.com/en-GB/d4a-merch Patreon: www.patreon.com/d4a
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I'm a PhD metallurgist who worked in the jet engine business. You'll hear a lot of nonsense about materials in popular media, but this video was excellent! I especially liked hearing the rarely spoken truth about "billet" parts - better than cast, not as good as forged.
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Materials scientist / metallurgist here. You did a good job at explaining usefulness for the different alloys! You made one mistake though: When comparing steel against aluminium and titanium, for the application of connecting rods, you have to compare the yield strength of the material, not the tensile strength. The yield strength is the load per unit of area (be that psi or ksi, or MPa) where the material starts to deform permanently. Often you see a value for "0,2% yield strenght", which is the load where your piece will have been loaded so that, after release, it has permanently deformed by 0,2%. Tensile strength (ultimate tensile strength) is the highest load the material can take before it breaks/fails/snaps, which is typically quite a bit higher than the yield stress. So in the application of a connecting rod, you have to avoid getting stretched or bend rods, so the 0,2% yield strength is the value to compare, not the ultimate tensile strength. You don't need to know when it breaks but you do need to know the point where it would start to deform permanently . With regard to the shock absorbing capabilities, what you need to look at is the Young's modulus, or modulus of elasticity. It basically describes the "springiness" of a material in the load range below where it gets to deform permanently. Typical steel E-modulus is around 190 GPa (27500 ksi) where Ti-alloys are around 115 GPa (17000 ksi) and your average aluminum/aluminium sits around 70-80 GPa (11000 ksi'ish). So aluminium is indeed acting like a softer spring compared to steel. Minor detail: galling and fretting are not the same thing. Galling is local cold welding during direct metal-to-metal contact, where the formed weld is stronger than the base material so a tiny part gets ripped out of the counter part, which then causes more damage. Fretting is a form of wear where the sliding distance is really really small (micrometer or mils scale) such as when you have parts in vibrating contact.
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I am studying engineering but this video taught me a lot more than my college, thanks bro
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Jesus christ I learn more about metals in this 20min video then a year in weilding school. Once again your the best thx for your hard work benefiting all of us engine/engineering nuts!
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This is without a doubt the best explanation anyone could ask for. Huge appreciation for the work you put into this video
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One of the only channels you can hit 'like' before actually watching the video, and the most informative, and entertaining automotive channel on YouTube, in my opinion anyway.
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Didn't see pressure cast rods mentioned, well worth a mention as a lot of manufacturers used them in the transition period from mass market cast to forged rods. Manufacturers such as Saab and Suzuki were known for using this process.
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Next time I sell a car and someone points any issues, I'll come back with "But as a bonus, it has steel rods. Let me explain why".
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I can attest to titanium being 'difficult' years ago I was a partner in an engineering company, specialising in heat exchanger components for the petrochemical industry. We were approached by a customer who had tried to manufacture tube sheets from titanium and failed. I quoted the job @ 4x what we would ask for stainless steel + a big oops factor and very nearly came short. Eventually though we worked out how to do it and it became the most profitable job we ever did. I would describe is as trying to machine abrasive toffee to fine tolerances.
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Valve seats, valve guides and valves themselves. Types of which(shapes) and what materials are used for them would be a nice topic for the next video.
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main point of forged aluminum is the shock adsorber effect.. in our top level engines (12,000hp in 500CID) we run the rods until they shrink (get shorter) by our magic number.. This video is pretty spot on .. not sure where he got his info but it is really good... Now the work being done on carbon fibre rods.....
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Just want to say how much i respect your video quality and in depth explanations. Ive been trying to learn about cars specifically hondas and your k series motor break downs were awesome !
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Interesting info. Didnt know forged steel will still be the best of all applications. Your video saved us a lot of money if we just blindly choose the 'best' material for rods.
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As always, great and clear explanations on the subject. I love the way you manage to condense important parts of the "scientific" aspect of a topic into something a car enthusiast without an engineering degree can understand. Your videos give a good overview of what matters which enables me to quite easily dive deeper in the subject by my own. Just the simple fact that now I know which words to Google thanks to your vids is very helpful and helped me to gather quite good knowledge and understanding of particular subjects. Keep up the good work!
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Does anyone else have brain fluid leaking after watching his videos? So informative and entertaining, I accidentally learned something.
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LOVE your video's & I'm so glad I found your channel! You do a great job explaining things in a very clear, easy to follow & understand manner. I appreciate the level of depth you go into subjects without going too deep into "unnecessary" areas. Keep up the great work & I hope your channel continues to grow!
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Best explanation on connecting rod history I've ever seen. What a great job Sir! Thanks for the lessons.
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I love the concept of Titanium in everything. Such a great metal. However for rods, sigh, I can live with the cost, and selecting a good set of forged rods rather billet. But ! The galling and notching issues versus the now minimal weight offset leaves me thinking forged steel for now until someone comes up with a better Ti alloy. Great content.
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Man your vids are just the best. Excellent info, entertaining, well presented, easy to understand. Keep up the awesome work.
