Engine Building Blocks: Selecting the Right Connecting Rod

23 Dec

With winter knocking on the door, many use the next few months as a welcomed opportunity to plan and build their engines in preparation for the warm spring and summer weather. In order to build an engine, the customer is left with a wide range of choices as to what parts to use. Scanning around, you are inundated with a tremendous variety of components, and each one sounds absolutely terrific. But differences do exist, and costs do vary significantly. In this new series all about engine builds, we’ll explore what these differences are, and why they matter to you and your engine.

One thing we want to make clear from the beginning. There are lots and lots of options for your rods. Tons of manufacturers. You’ll see guys who swear by one type only, and others who curse that same brand as spawned from the depths of hell. While component selection is important, the MOST important thing is selecting who is going to do the job of assembling the jigsaw puzzle together. The success or failure of an engine is often directly tied to the person doing the grunt work. Are the components being balanced, or are they taking them out of the box and tossing them in? Are they using the correct torquing sequence and method for securing the rod bolts? Are they using a high end, calibrated torque wrench? Anyone can bolt 2 parts together, but few can assemble something correctly.

The connecting rod is a simple, yet essential part of any conventional engine. It’s job is fairly simple – and as the name implies, it connects the crankshaft and the pistons together. The rods rotate around each end, with the pistons connecting to the “small end” of the rod (top), and the crankshaft connected to the “big end” of the rod (bottom).

Internal Combustion Engine Visual Aid

As you can see from the diagram, the connecting rod is subject to a tremendous amount of stress through it’s movement cycle. As such, it’s important to select the right type of rod design in order to maximize engine efficiency.

In looking around at various manufacturers, you will see the alphabet soup of designs mentioned. What is important to note, is at their core, 2 basic types exist: H Beam and I beam. The others are variations on these 2 general designs.

An H Beam is the most common, cost effective aftermarket rod type. Brands such as Eagle, Scat, Manley, Wiseco (via their K1 brand) all manufacturer these popular designs. They tend to be lighter than the I beams, but they do not quite have the torsional rigidity of the more expensive I beam, assuming the same materials are used. One thing to keep in mind when evaluating the rod choice is the net weight, and hardware being used. Like many aftermarket parts, this is an area where if you ask 10 people, you will get 11 answers. Each machinist has their favorite, and least favorites, and many are prone to push what they sell. Rod choice can, and should be, very application specific. Just because a manufacturer makes a rod that works well in one engine type, does not mean it’s appropriate for all engine types.

An I beam, by design, tends to take up less overall area, and depending on material and engine application, can also be lighter. Again, it’s important to remember this is a general rule, and not one that is etched in stone. This lighter weight often makes them best suited for very high rpm use. What constitutes ‘high rpm’ is of course, very application specific.

One of our own 350Z’s has a redline currently set at a cam/header-limited 8450 RPM. It’s seen that rpm nearly every day of it’s life since break-in was completed in April 2007. It uses a very conventional, off the shelf Eagle H Beam rod. Now 8450 is certainly up there as far as rev limits go for most automotive engines. So why is this H beam still an appropriate choice? It’s light weight, combined with the supplied ARP hardware, torqued appropriately. People often state that one type of rod is best for high rpm, and another is best for high rpm. The biggest source of a connecting rods demise, and what causes them to see daylight above performing their assigned task is the forces they see when asked to change direction (in other words, at Bottom Dead Center, or BDC). This is where the 2 piece section of a connecting rod (aka the Big End) is under it’s highest load, and where the hardware is really being asked to perform it’s task. Use wimpy hardware (as many stock engines do), or torque them inappropriately, and that joint will fail…sending your rod on a wild ride clear through the side of the block. Use a rod that’s too heavy at too high of an rpm, and it will fail. Take rods out of the box and try to just dump them in, without balancing them, and it often will fail. Doesn’t matter if you spent $400 a set, or $4000 a set. These same rules hold true. The amount you spent isn’t what makes the rod stay in place or seek escape from the confines of the block.

You’ll also see other types of rods out there, such as A Beam, or X Beams. These are variations on the existing H and I beam designs.

Again, the above is just an overview. A cursory look at what goes into a connecting rod. In the next installment, we will begin examining material differences (steel, billet, titanium), and how they are an essential part of proper connecting rod selection. In a later installment, we will also examine piston choices as well. The rod and piston are the building blocks of your engine. They are tasked with a simple, yet essential job: to stay together.

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