Connecting rods are probably one of the easiest parts to understand in an engine — they have to make sure the pistons are moving up and down when the crankshaft tells them to. Where things start to get complicated is in examining the design of a connecting rod and determining the best option for your uses. There are two main choices when it comes to connecting rods: I-beam and H-beam, and in this article, we’re going to look at the difference between the two, along with the ideal engine application for each.
I-beam and H-beam connecting rods are the most common types of rods you’ll find in high-performance applications because they are so versatile. Every engine is going to be different, so having the option to use a connecting rod that best matches what its intended goals are is very important. When you drill down further and look at each rod type, you’ll see there are different design elements put into each that help optimize them for the build they’re used in.
Rod School: What You Need To Know
When you put an I-beam and H-beam connecting rod side by side, it becomes obvious how they got their names. The I-beam rod has a very robust appearance with a thick cross-section. Looking at the H-beam rod, you will notice that it has large sides that are flat with a thinner section in the middle that makes it appear like the letter ‘H’. These design differences are striking on the surface, and they each have an important purpose when it comes to how the connecting rod is used.
The design of the connecting rod is just part of what makes it the right part for the build; you also need to ensure the material it’s made from is of the highest quality. Nick Norris is part of the engineering team behind the design of connecting rods for Callies, and he explains why the material is the most important thing to look at when you select a connecting rod.
“By using a higher quality material it helps to limit the number of failures you will see. Over the years we have seen rods go through a hydraulic situation where the rod is basically turned into an S-shape. If the material wasn’t as good as it was the part would have broken. Now, it’s not ideal the rod is S-shaped, but that’s better than the rod completely failing and trashing everything inside the block. We use a 4330 material to make our rods — by using a better material we can take advantage of making parts a little bit lighter than a rod that’s just made of regular 4340 steel,” Norris says.
It’s easy to understand why you need a connecting rod to be made of the best material possible, but why do they come in different shapes? The answer to that question is actually pretty simple: it comes down to what you’re trying to accomplish with the engine. While both I-beam and H-beam connecting rods can function in the same settings, one will work much better than the other based on the application.
“The shape of the connecting rod has to do with the builder’s preference and what application the rod will be used in. The H- or I-beam design will work in most applications, however, if you’re building a high-boost application the I-beam is better. The reason being is the I-beam is a better rod when it’s under compression compared to an H-beam. If you overload an H-beam rod with compression, the blades along the H-beam shape will actually start to spread out,” Norris explains.
Breaking Down I-beam and H-beam Connecting Rods
We’ve given you some basic information about I-beam and H-beam connecting rods and now it’s time to take a closer look at each. As mentioned earlier, you can easily make either of these connecting rods work in any application, but if you’re trying to fully optimize a particular build you need to know which is going to function better. Using the right rod for your particular build will not only save you some headaches when you’re trying to select parts, but it will also save you from a costly parts failure that could do serious damage to your engine.
The I-beam rod lends itself better the higher compression loads, partially because of the shape up around the pin area. – Nick Norris
Let’s first take a look at the I-beam connecting rod. The I-beam design works best in a build where a power-adder is being used because the rod offers additional strength. You’ll see I-beam rods almost exclusively in engines that are designed for high amounts of boost because of its design. This is due to where the weight is added to these connecting rods among other reasons.
“The I-beam rod lends itself to higher compression loads partially because of the shape up around the pin area. We have a unique shape where the pin is located that’s almost like a gusset. Where the beam transitions on the pin end we create what I call a slope up that supports underneath the pin all the way to the outside edges of the pin bore,” Norris states.
When you need to lighten up the rotating assembly the H-beam connecting rod should be at the top of your list. These rods are a better fit for a naturally-aspirated engine where you’re turning a high level of rpm.
“When it comes to design and weight considerations it’s easier to make an H-beam rod lighter than an I-beam rod. Some of our H-beam rods are a straight beam, while others are a tapered beam. A tapered beam means you don’t have as much weight on the pin end of the rod. In a high-rpm application, you want to have less weight up on the pin end. When you combine this with a lighter piston, it helps to reduce the tensile stress on the rod itself,” Norris explains.
I-beam and H-beam connecting rods both vary in design features among manufacturers. You want to be sure when you’re looking at a set of rods to use in your build that the features they offer are what you need.
“With an I-beam, depending on the application, you would need to look at whether the rod was a straight up and down design or a tapered design. You would also want to look at the width of the beam itself. The bolt placement on the rod is also important. Bolt placement is key because the head of the bolt needs to be far enough away from the big end bore that it doesn’t have too thin of a cross-section there, because that would allow the bore to flex,” Norris says.
H-beam connecting rods have some additional things you want to look at before you pull the trigger on a set, according to Norris.
“With an H-beam rod you should look at the thickness of the blades; basically, the H-beam itself where the slot is cut out. Depending on the material, you would want at least .900-inch of thickness in that area. Now, you can reduce that thickness when you get into some of the higher-end materials. The thickness in the center of the rod between where the slots are cut out needs to have a certain minimum thickness as well based on your application.”
How Callies Approaches I-beam And H-beam Connecting Rods
Callies has always put a good amount of time into the design process of its connecting rods. There are many different factors that are considered before the pen is even put to paper, and this approach is used to ensure the final product performs at a high level. This process has led Callies to design its rods in a similar fashion, with similar cross-sectional thicknesses so the rods are as successful as possible.
“Between an H-beam and an I-beam, I would say there are areas of the rods that are basically identical. That would be between the big end bore and the inside edge where the alignment sleeve where the bolt goes through. Those thicknesses would be consistent between an H-beam and I-beam rod. Similarly, the thickness between the outside of the bolt alignment sleeve and the outside of the rod would be the same,” Norris explains.
When it comes to design and weight considerations it’s easier to make an H-beam rod lighter than an I-beam rod. -Nick Norris
Since I-beam and H-beam connecting rods are used in different engine applications they do need to have some different features added to their designs. These features have to be added based on the fundamental nature of the rods themselves, to ensure they won’t fail when they’re running hard inside a high-performance engine.
“Where things become different between the two connecting rods is when you get above the bolt from the big end bore to where the rod transitions into the beam leading up to the wrist pin. In this area, the I-beam will typically have a little bit less of a cross-section because it doesn’t have that H-beam intersection where the H-beam slot cuts through that area. With the I-beam, we still have the same minimum cross-section from the edge of the pocket to the edge of the beam itself; it would also have a minimum thickness on the pocket depth,” Norris says.
Having a connecting rod with a good design will only take you so far, the rod also has to be machined properly to avoid any issues. Callies uses a very intense set of processes to ensure all of its connecting rods are perfect. This includes spending a lot of time creating programs to machine the rods without humans touching them.
“We don’t do any handwork on our rods at all — the rods aren’t touched up by hand. We spend a lot of time on tooling and proper fixtures to prevent that. There are ceramic brushes we use to smooth out the hard-to-reach areas, plus shot-peening and other vibratory processes. When there’s handwork done it can take away from the consistency of the finish — by using machines we eliminate that,” Norris says.
Selecting the right connecting rods is one of the most important things you will do when setting up an engine build. Knowing the difference between I-beam and H-beam rods and their best uses will help you get the right rod for the job to get you through run after run, season after season.