Aluminum Vs Steel
Aluminum Connecting Rods
Aluminum, Steel, Titanium Connecting Rods
What’s the difference ?
There are two major categories for manufacture of connecting rods – forged and billet. When it comes down to material, there are a few options- steel, like most OEMs, titanium, and finally aluminum. Each has advantages and disadvantages. Use the table below for an overview, and read on below for more detail.
|Aluminum||• Flexibility in construction
• Low cost
|• Less durable in some cases|
|Steel||• OEM material||• Huge opportunity to improve design|
|Titanium||• Most durable
• Short fatigue life
Steel Connecting Rods
Most modern engines use some form of forged steel connecting rod for its balance of strength, durability, and weight. Most designs leave much room for improvement from their design to method of manufacture to even the bolts that hold the big end of the connecting rod onto the crankshaft. Most modern connecting rods are manufactured using “cracked cap” technology, making rebuilding these connecting rods nearly impossible. On the small end, better known as the side with the wrist pin, most rods are bushed with some sort of bushing, typically brass. Wrist pins vary in materials used, thicknesses, and style- some are thin-wall, some thick, and others tapered. In most cases, a tool steel, like H11, is used for the wrist pin. Now when it comes to holding the pin in floating pin setups, there are all kinds of keeper including wire-loc, spiro-loc, tru-arc, and even teflon and aluminum buttons, which are typically used in applications where the engine comes apart rather frequently, allowing for ease of disassembly.
Titanium Connecting Rods
Titanium connecting rods have been used for decades in high-end applications where the strength and longevity of steel is required, but the lightweight characteristics of aluminum are desired. That comes with a price; titanium rods typically are typically four times the cost of a billet steel rod. The other drawback is that titanium rods do have a fatigue life in race applications and are typically switched out every 40-80 hours. Cars like the Acura NSX, some Chevrolet Corvette models, and Porsche 911 GT3 favor titanium rods, but their cost along with the exotic nature of the vehicles they reside in is reflected in the overall cost of the vehicle.
Aluminum Connecting Rods
For decades, aluminum connecting rods have been made from forgings, plates, and extrusions, machined to suit. For sake of comparison, we’ll be talking about forgings and extrusions, since both have relatively equal grain structures and material characteristics. There are plusses and minuses to each process. With forgings, you have a die cost to factor into the price of each final forging, whereas with an extrusion you do not. But when it comes to machining, you have less of it to do with a forging than an extrusion, since you already have the general shape of a rod to start with. With a true billet rod made from an extrusion, you have lots more machining and more scrap, so there’s higher cost due to the material waste and extra machining time. For all intents and purposes, that cost is about equal to the die cost, unless you’re on a manufacturing scale like that of an OEM. Looking back at a forging, like we’ve already mentioned, you’re limited by two factors: dimensionally, a forging will only work for X number of different connecting rods and secondly, with a forging, you have to make a die for each new forging, limiting what connecting rods can be manufactured. Also, you have to make the forgings in large quantities, and typically, you have a large lead time associated with getting the aluminum to the forger, then having the forgings made. Then the forgings have to be heat treated. There’s lots of time involved and shipping of the parts even before machining can proceed. With a billet rod made from an extrusion, the final characteristics of the alloy are already determined, having been heat treated and in the case with R&R’s billet aluminum connecting rods, the alloy is pre-stress relieved, allowing for a more stable rod, at least dimensionally speaking. Extrusions are kept on hand in varying thicknesses in standard lengths, usually 10 to 12 feet, then cut to rough length for machining. With this process, any rod imaginable can be made. If the correct thickness is not available, it can be ordered from their aluminum supplier in as little as 2-3 weeks. In as little as a single day, a raw extrusion can be transformed into a set of billet rods through a multiple step process where the extrusion is cut, faced, serrated, drilled and tapped, then machined into what will become the final product, a custom billet aluminum connecting rod. Rods are then fitted with the proper ARP rod bolts then sent down the line for final honing and pin fitting. Most of the process is the same with a forged rod, but once again, it’s the machining that is different in that it just takes longer. With improvements in machining capabilities with faster, more rigid machining centers, this time is minimized. With these capabilities, any connecting rod can now be custom, at little or no cost over a forged rod that is limited in what applications they are available for. These same processes apply to steel and titanium connecting rods, as there are manufacturers that use both processes with equal results.
Now that the flexibility of the true billet process utilizing extrusions is explained, you can see why we prefer the process, which allows us to make almost any connecting rod. When it comes to what is typically considered a standard connecting rod, like a domestic small block Chevrolet or import Mitsubishi connecting rod, where some companies make them and stock them in standard configurations, which allows for same day shipping in the best case scenario, you get a generic rod, designed to do everything ok, but excel at not one thing. Each application has a different set of requirements, whether they are for added strength, weight, or even as simple as clearancing for fitment purposes. When you order an R&R Pro billet aluminum or R&R Pro billet steel connecting rod, before it’s even started, there’s a list of details that are poured over to ensure that the rod that is machined fits your needs precisely- that’s why we give you a choice of alloys, bolts, and machining options to suit your application and it’s exact requirements. Whether you order four, six, eight or two hundred connecting rods, each piece gets meticulously examined and strict quality control ensures that the first rod is the same as the last. So why are aluminum connecting rods the preferred choice for those running high RPMs, lots of boost, or making huge horsepower? Well, the answer is quite simple. Think of the connecting rod as a spring or shock absorber. It cushions the inherently rough nature of combustion and acts like a buffer between your more expensive pistons and crankshaft. In these applications, a steel rod is too rigid and is more like a sledge-hammer than shock absorber, often leading to damage to the crank and the rest of your engine due to failure of the rod itself or a part connected to it. Some manufacturers claim their rods are street-able and can outlive a steel rod. We’d be lying if we made the same claim. An aluminum connecting rod is a spring and there is only an X number of times a spring will compress and return to it’s rest state without deformation. Deformation in our case is a rod failure, as with any connecting rod. Aluminum has a fixed fatigue life, just like that of a titanium connecting rod. Well, what’s that you might ask? Every material can be deformed to a certain extent and only a certain number of times before it breaks. Think of a crushed soda can. You can bend it forwards and backwards in the middle a few times before it will split open or tear in half. It’s the same thing with a connecting rod. Depending on the material and the application, a steel connecting rod might last almost forever or an aluminum connecting rod may last just a handful of passes (if you are really making LOTS of horsepower with boost and nitrous at high rpms with a very heavy piston). It’s hard to say with an aluminum rod what is the useable life. Controlled deformation and its performance as a spring and shock absorber is the reason people seek an aluminum connecting rod. If its weight reduction that is needed with the strength and longevity of steel, say longevity of an OEM part, it’s a titanium connecting rod you need. But the price you pay is its extreme cost. The old axiom pick two holds true: price, strength, reliability- you can’t have all three.
Some racers change their aluminum connecting rods religiously every few runs or every season, since typically the cost of a set of rods is less than the cost of the rest of the parts in a race engine. Import racers whose cars double both as daily drivers and weekend warriors typically use their rods for 10 to 15 thousand miles and again, change them just as a precautionary measure depending on how hard they run their engines. We’ve even seen customers put hundreds upon hundreds of passes on their aluminum connecting rods without failures. Like we already said, we can’t guarantee how long an aluminum connecting rod will last but we do take the time and make the effort to build the best connecting rod possible, and do it affordably.