When talking connecting rods, H- and I-beam pieces each have unique traits as applied to performance and racing duty. If you look at a cross-section of the rod in relation to the crankshaft centerline, you see the apparent H and I shapes.
In many performance and racing engines, I-beam rods are used for general applications. The use of the H-beam style connecting rod may offer an increase in strength properties, but with that rod design generally comes with additional weight added to the engine’s rotating assembly.
A Better Connection
“It looks like an H-beam rod on steroids,” describes Alex McCormick, Lunati Power lead engineer for the newly designed X-beam rod. “At first, we were trying to experiment with some different connecting rod designs with our goal being to optimize the strength while also reducing weight.”
Reducing weight, they did accomplish. The X-beam connecting rods currently being produced for General Motors LS and GM Gen-V LT engines are tipping the engine builder’s scales at an average weight of 660-grams for the boosted X-beam rod design.
Lunati also offers a lightweight, high RPM version of the X-beam rod, which scales in at 575-grams. That enhanced rod is rated for use up to a naturally aspirated 850-horsepower and 9,000 rpm.
Set side-by-side with the Lunati 4340 H-beam rod (680-grams), the lightweight rod can save you more than 100-grams in weight with each connecting rod. Compared to the heavier X-beam rod design intended for boost applications, the lightweight X-beam rods are comparable in weight to the H-beam rod, but feature far greater strength according to Lunati.
As I was working with the rod design in the FEA software and computer stress testing, we found that with most connecting rods, the weakest locations are either in the pin or journal area. The shank of many rods is stronger than it needs to be because it’s going to fail somewhere else first. – Alex McCormick, Lunati Power
McCormick adds, “We wanted to be sure that this was going to be a robust rod, so we began from scratch. We allowed the computer stress analysis software to help guide our designs step-by-step towards an entirely new rod.”
FEA computer software is a big buzzword in engineering circles. This allows for strength experiments in a very early stage of the design before the first connecting rod is actually created for physical testing.
According to McCormick, “Compared to many previous rod designs that were developed around countless man-hours of R&D testing, here, we saw no surprises between the FEA computer results and live testing of these new rods in our dyno room.”
Proof Under Pressure
“We did a lot of follow-up testing of the lightweight X-beam rod in performance and racing LS-engine packages on our engine dyno,” McCormick adds. “Our R&D department wanted to be sure it could handle what we threw at it. I think we tested those with as much as 950-horsepower. We also tested them for endurance by running them for hundreds of dyno pulls in engine designs producing over 800-pound-feet of torque.”
As far as the material composition of these new rods, they are forged from 4340-steel with very close tolerances and strict quality-control parameters to ensure durability in high-stress performance and racing environments. All X-beam connecting rods are weight-matched to plus or minus 1-gram.
In addition to strength-versus-weight benefits, the Lunati engineering team accomplished another goal with the new rod. These first two rod designs for the LS-family engines are described as being easier to manufacture compared to some other rod designs. This will provide an advantage when it comes to cost.
A typical I-beam rod relies on a relief area designed into the face of the rod while the H-beam rod removes material on the sides of the rod shaft. “What we accomplished with these X-rods is to relieve material on both the sides and the face,” defines McCormick. “We then allowed the FEA computer analysis to indicate where any of our reliefs might have gone too far.”
The lightweight X-beam rod uses a 3/8-inch rod bolt to save as much weight as possible from the journal end of the rod for performance engines running in the higher RPM ranges.
For any engines using forced induction, this weight isn’t as much of an issue. The need for strength related to loads caused by higher cylinder pressures is the priority. These boosted X-beam rods use a 7/16-inch rod bolt. All Lunati X-beam rods utilize Automotive Racing Products’ 2000-series rod bolts.
“When we went after the boosted X-beam rod design, we applied much more pressure against the rod from the piston pin,” McCormick explains. “With the FEA computer software, we were more concerned with analyzing the rod’s ability to handle compression loads and less with the rod’s ability to handle tension. The boosted rods are obviously going to see a lot more compressive loading from the piston compared to a naturally aspirated engine.”
He continues, “We then changed the stress parameters for increased tension in the lightweight rod. The tension loading is higher there because of inertia as the engine is spinning at higher RPM. As the rod travels up and then back down, all that mass from the piston, wrist pin, and the rod itself, stretches the rod. So, in high-revving applications, you get a ton of tensile loading.”
More To Come
As of today, the X-beam rod is only manufactured for the LS-engine, with 6.125-inch rod length, 2.100-inch crankshaft journal diameter, and a rod width at the journal of .940-inches.
We asked McCormick about future engines that will also see the X-beam rod offering. He answered, “Our long-term goal is to continue to add applications. LS engines are one of our biggest movers, so we started there. We will continue to offer it for more engines as we see a need for them in the marketplace.”
Lunati Power is also known for its Voodoo line of engine rotating assembly kits. McCormick notes that these rods will be offered within soon-t0-be-released-kits in the future as well.
With all the amazing engine development in the racing and hot rod world today, creating big horsepower is far more accessible than ever before. What is critical, is the development of individual components with greater strength to reliably live with those power levels.
The short block and connecting rods, in particular, are the front line in defending against failure against these bigger and broader torque and horsepower numbers. Lunati is using a mix of traditional engineering and new FEA computer technology to win the longevity wars.