Many late model engines are factory equipped with powder metal rods. The vehicle manufacturers like powder metal rods because they are much less expensive to manufacture. The rods can be cast to very close final tolerances and require less machining than a cast iron or forged steel connecting rod. There is no grain structure in a powder metal rod so the rods can be cracked to separate the cap from the rod. This is faster, easier and some say better than cutting and machining rod caps because cracking leaves a slightly jagged surface on the cap and rod which will only mate together one way. The advantage is that it provides perfect alignment between the cap and rod but the trade-off is that the cap and rod cannot be machined to correct for any bore distortion or wear that has occurred over time. Because of this, powder metal rods are essentially throwaways if the big end is worn or the cap has loosened up over time (which they do).
If you’re doing a performance build, therefore, one of the first parts that will have to be upgraded is the rods. Replace the original equipment powder metal rods with some type of aftermarket performance rod (I-beam or H-beam).
There are a lot of choices when it comes to aftermarket rods. Choosing a rod depends again on the application. You want a rod that’s strong enough to handle the power and RPMs the engine is capable of producing. You also have to match the rod length with the stroke of the crank, the pistons and pin location, and the engine’s deck height.
Strength is critical in a connecting rod. The forces that stretch and compress the rods exert tremendous stress on the beam section of the rod. If a rod is going to fail, it will most often pull apart on a piston upstroke rather than bend during a piston downstroke. A rod can also fail if the bearing starves for oil, seizes and rips the rod apart.
The stock rods in most V8s are stout enough to handle upwards of 400 to 450 horsepower, and 5,500 to 6,500 RPM. Exceed these limits and you’ll need stronger rods for reliability. One aftermarket rod supplier we interviewed for this article said his entry level rods can handle 800 to 990 horsepower, and his top end rods are running in engines producing upwards of 2000 horsepower. H-beam rods made of 4340 or 300M steel are commonly used in many circle track engines, while I-beam rods are popular with drag racers and marine engine builders. The debate of I-Beam versus H-Beam often boils down to a matter of personal preference.
Most Top Fuel dragsters and funny cars use aluminum rods in their motors. So do many ProStock racers. Aluminum rods have a limited service life because they can stretch and fatigue in high stress engines like these. Even so, they work well enough because the typical Top Fuel racer replaces the rods after 8 to 10 runs. ProStock racers may replace the rods after 20 or 30 runs. In the lower drag racing classes, a set of aluminum rods may last 100 to 200 runs or longer.
Aluminum rods can work on the street, but it requires a high quality alloy. For this reason, many engine builders prefer to stick with steel rods for their proven longevity.
Titanium rods are another option for those with deep pockets. Titanium is light weight (about 24% lighter than a comparable steel rod) and has about the same durability as a steel rod, but titanium is expensive and tricky to manufacture. If you can afford them, great! Otherwise, they are probably too expensive for the average street performance customer or weekend racer.
Rod length is another choice you will have to make when selecting a set of rods. Rod ratio is the length of a connecting rod (center to center) divided by the stroke of the crankshaft. Many performance engine builders say a rod ratio of 1.57 to 1.67 works best. A longer rod ratio can make an engine’s torque output peak more sharply. Lower rod ratios work well with lower RPM stroker motors while higher rod ratios are better suited for high revving engines.
Another option is to use rods that do not have wrist pin bushings. This leaves more metal around the wrist pin for added strength at high RPM. But it requires a highly polished pin hole and a low friction coating on the wrist pin to prevent the pin from galling.
The type of bolts used to attach the rod cap to the rod is also important. The stronger the bolts, the better. Poor quality bolts can stretch and allow the cap to wander, leading to rod and bearing failure.