Tech Talk #63 – Spare the Rod and Spoil the Engine

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Connecting rods tend to be taken for granted – until they break.”

Imagine riding an elevator that makes a 10-story round trip 7,000 times a minute, alternately stretching and compressing its occupants with every cycle. That’s exactly the kind of punishing treatment a connecting rod endures. A connecting rod must bear the compression force of thousands of pounds of cylinder pressure, withstand the tension loads produced by the piston’s inertia at TDC, and survive the bending loads that try to push the piston through the cylinder wall.

The connecting rods are vital links in every reciprocating engine. They tend to be taken for granted until they break – and when a rod lets go, it will spoil your day and ruin your engine.

In drag racing, the choice of material for connecting rods comes down to steel and aluminum. I’m not privy to the inner workings of Formula 1 racing engines, but we did experiment with titanium connecting rods in our Pro Stock engines a few years ago. While titanium has some appealing attributes, it also has some shortcomings when used as a connecting rod material. The necessity to coat the thrust surfaces, the expense of machining and tooling, and the problems with galling fasteners in titanium convinced me that aluminum was a more practical choice.

Aftermarket steel connecting rods have become popular in mid-level sportsman racing. It’s tough to beat a set of affordable steel rods in a bracket or Super-style racing engine. One of our customers has had a set of relatively inexpensive steel rods in his big-block for 11 years. The engine turns 7600 rpm and makes 1,000 horsepower, so this is not a weak motor. We’ve replaced the bolts during regular rebuilds, but the rods just go back in after every overhaul.

Steel rods have limitations, of course. They’re seldom suitable for a big-inch, go-fast engine. The chief problem is weight. A steel rod for a large displacement motor might weigh 1200 grams, versus 850 grams for a typical big-block bracket engine. Like a valve spring, a connecting rod is subject to its own mass, so a portion of the load on the bolts and cap is produced by the weight of the beam and the small end of the rod. As the rod becomes longer and heavier, the stress on the fasteners and cap increases dramatically.

Heavy steel connecting rods are also tough on pistons. As the crankshaft turns, the rod’s reciprocating motion is controlled by the piston. If it weren’t for the restraint of the piston moving up and down in its cylinder, the rod would sling around in a circle. I often see the telltale evidence of the thrust loads generated by heavy connecting rods on pistons. The pistons are more susceptible to cracks where the pin boss joins the skirt, and the skirts are also more likely to collapse when a heavyweight rod is used.

This brings us to the chief advantage of an aluminum connecting rod: weight. Aluminum weighs approximately 1/3 as much as steel, and because it is so light, connecting rod manufacturers can use thick cross-sections in their rods without incurring a weight penalty. The tensile strength of steel is approximately 200,000 psi; the tensile strength of aluminum is about 95,000 psi. Consequently an aluminum rod can equal the strength of a steel rod at two-thirds of its weight.

Aluminum rods also are a little friendlier to the crankshaft and pistons than steel rods. The aluminum seems to cushion the peak loads, and that becomes apparent in the condition of the bearings and piston pins when an engine is torn down.

The downside of aluminum is its fatigue life. Aluminum loses strength with heat and load cycles, so it has a relatively short lifespan in a highly stressed application such as a connecting rod. Steel, on the other hand, does not fatigue as long as it isn’t stressed to its yield point. Think of a steel paperclip; if the wire is bent back and forth until it reaches its yield point, the wire will break. But as long as the metal isn’t stretched to it’s yielding point, the paperclip will last a lifetime.

Aluminum loses strength when it is subjected to heat cycles. Fortunately in a drag racing engine we have the ability to control engine heat to a great extent. I’ve written previously about the importance of keeping a racing engine’s temperature under control. Now I’ll add the effects of heat cycles on aluminum rods to the list of reasons why it’s advantageous to keep an engine cool.

Aluminum is also highly notch sensitive. A stress riser produced by an exposed bolt thread, a sharp radius or a tool mark is likely to be the point where the rod fails. If a lifter breaks and its needle bearings leave dozens of tiny notches in a set of aluminum rods, it’s an excellent idea to replace the rods. Even though the rods may otherwise be in good condition, the stress risers left by the lifter bearings have compromised the aluminum’s strength. In contrast, steel has relatively little notch sensitivity – although it’s a really bad idea to run a steel connecting rod that’s been cut with a hacksaw just to see how long it will last.

It’s very unlikely that aluminum rods will fail as long as they are replaced at regular intervals. I don’t put new bolts in aluminum rods simply because we install new aluminum rods with every rebuild. If I’m using steel rods in an application where they’re not being stressed to their yield point, then I replace the bolts periodically.

Steel connecting rods are available in two different styles: a conventional “I” beam rod (similar to a factory forged rod) and an “H” beam rod (often referred to as a Carrillo-style rod). I’ve had good success with both styles, so I really don’t have a strong preference for one over the other. In an endurance racing application, the H-beam rod is more suitable for a pressure-oiled piston pin, but that’s not a consideration for a drag racing engine.

Steel connecting rods will provide good longevity at an affordable price in an engine that has a reasonable rod length and doesn’t turn extremely high rpm. As horsepower and engine speed go up, and as the components get bigger, then aluminum rods become a more practical choice.