Certainly one of the most critical components inside an engine is one of the most stressed. Connecting rods take the power from the combustion process and convert it to crankshaft rotation, which, of course, is what makes things go.
The more power you want to make, the more rpm you require, the more stressed the rods become, and it’s not in just one way. Rods undergo tensile, compression and bending stresses, and getting them to survive all three is a matter of design, materials and manufacturing.
“There’s a lot of force in tensile, compression and a little bit of bending, so those are the main things we look at,” said Clayton Stothers, engineering lead for Wiseco. “The inertial case is pretty hard on connecting rods as well, because 10,000 rpm is a lot of load, so we look at the tensile loading a lot.
When a piston rises to top dead center and the crankshaft begins the second half its rotation, pulling the big end of the rod, the two ends of the rod are essentially being pulled apart from each other. That’s tensile load, and it also occurs when you lift off the gas and decelerate. That puts stress on the rod cap and rod bolts, which also transfer their load to the big end of the rod.
Compression load occurs when the piston transfers load from the combustion to the wrist pin and to the rod and crankshaft. Compression load also occurs on the compression stroke, albeit to a lesser extent. A connecting rod also undergoes bending stresses, which rise commensurately with rpm and power output.
“From what I’ve seen, the transitions between the beam and the big end and where the beam transitions to the small end, those two radii there definitely see the most stress in most of the cases I’ve looked at,” Stothers said.
That’s where design, materials and manufacturing come into play. When designing its new BoostLine rods, Wiseco was looking to exceed the specifications of the marketplace. Ideal for boosted applications of up to 2,000 horsepower or for overbuilt engines for endurance racing, BoostLine rods are reported to have a 60 percent increase in bending strength compared with H-beam rods. The company performed significant testing of the finished product, but also initially through finite element analysis.
“Finite element analysis is basically a way of numerically calculating the stresses throughout a connecting rod and we’re able to do that on a computer and that means we can run through lots of different pieces,” Stothers said. “We can change the design. We can change the loading, to simulate what we thing the rod is going to see. Obviously there’s no complete replacement for physical testing, and simulating what a rod is going to see inside an engine is extremely difficult because there’s a lot going on, but we like to simplify and look at what I call each loading case separately.”
To withstand severe duty, Wiseco engineers designed the BoostLine rods using a Japanese spec and sourced 4340 chrome moly steel. It’s forged, which can add weight, but the strength of the material is first rate.
“In forging, you take the material and heat it up whatever temperature that material needs to be to become malleable, and then you press it with a large forging machine into a mold,” said Nick DiBlasi, Global Automotive Product Manager and manager of engineering for Wiseco. “It’s not pouring it. You basically take a very hot piece of material and you smash it into the shape of a mold and what that does is align the grain structure to become stronger. So a forged piece of material will always be stronger than it’s billet counterpart.”
In terms of design, the three pockets at the big end of a BoostLine rod subtract a little weight at the stress points where the rod has been beefed up. Wiseco also uses ARP rod bolts, and includes instructions for installation and grease for the threads with each set.
Wiseco recommends using a rod bolt stretch gauge rather than a torque setting for assembling the engine. That stretch, Stothers pointed out, is key to maintaining the clamping force. All told, it’s about .004- to .006 inches of stretch — about as thick as a piece of paper.
“That’s extremely important to making sure that the friction between the bolts, the lubrication and the rod itself is consistent with the testing that we’ve done,” Stothers said.
The rods end up weighing more than stock in some cases, but less in other cases, likely where factory turbos are used. BoostLine rods are overkill for many of the engine applications commonly seen on a NASA weekend. However, for high-horsepower applications or in cases where reliability is paramount, they might fit the bill. Boostline has part numbers for big- and small-block Chevrolets, GM LS engines, Ford modular and Coyote engines, and for popular Honda, Mitsubishi, Nissan, Subaru, Toyota and VW engines.
“If you’re going to run an engine for 24 hours, this would be the rod for you,” DiBlasi said. “If you really wanted to put something in there that you didn’t want to worry about, this is what you’d want to put in there.”