TECH QUICKIE: What Makes a Top Fuel Connecting Rod Special?

The Top Fuel rod is on the left. Note the extra material on the little end and lack of any scalloping or efforts to lighten the rod, as seen on the alcohol rod.

Just about every component in a Top Fuel engine is punished beyond anyone’s cruelest of intentions, but the connecting rod is especially abused. So manufacturers such as GRP incorporate special design features that give its Top Fuel product a better survival rate and hopefully a more useful life inside the 9,000-horsepower environment.

Top Fuel cylinder pressures are incredibly high with one study predicting over 13,000 psi peak pressure. Shown in the accompanying photos is a comparison of GRP’s Top Fuel and Top Alcohol rods. Both run in similar-sized Hemi-based engines, but there are distinct design differences. The closeup of the big ends shows the closed-up bolt hole on the Top Fuel rod while the alcohol rod has a more conventional opening for the end of the rod bolt. Also, the piston-pin end on the Top Fuel rod is much beefier and fortified than its alcohol brother.

Note the additional material around the bolt hole in the Top Fuel rod, left. The alcohol rod has a larger hole to allow measuring bolt stretch.

“A fuel rod has really high compressive loads that try to fork the rod out at the parting line,” says GRP’s Brian Scollon, “so the extra material is there. On the alcohol rod, part of the reason for the bigger hole is allow access to the back of the bolt to check rod stretch for accurate torquing.”

GRP, which specializes in billet aluminum and titanium connecting rods, won’t disclose the exact grades of aluminum used in the construction of both rods, but says they’re not always the same because of different operating conditions.

“A fuel rod is typically shorter duration, high compressive load,” says Scollon. “Alcohol rods tend to be more tension load because they’re running almost 11,000 rpm.”

Top Fuel engines, on the other hand, are required to have a timing controller to keep the engine from exceeding 7,900 rpm. (for more on this innovation, see this EngineLabs story)

Shown is GRP’s radial serration design that helps keep the cap located to the rod.

GRP generally supplies L-19 bolts with both rods.

“But there are options,” adds Scollon, “such as the ARP Custom Age 625.”

Finally, GRP rods feature a unique radial serration pattern where the cap and rod meet.

“It locates the cap laterally and side-to-side,” says Scollon.

And some final trivia: Top Fuel connecting rods last about 8-10 runs or less. There are fatigue concerns with overusing rods, but also they will compress in length and change the compression ratio for that cylinder. Years ago engine builders could move the rods from cylinder to cylinder because three different compression ratios were often used to compensate for the uneven air distribution caused by the massive supercharger. But the set-back intake manifold and other advancements have started reducing the need to balance the engine with different compression ratios.

TJ Zizzo is one of the Top Fuel teams using GRP connecting rods. NHRA photo.

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About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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