Dyno Disaster! 120 Pounds of Boost = Kaboom!!

High boost levels are great for building cylinder pressure and generating power. But when the exhaust valve has trouble opening, that pressure gets absorbed in the valvetrain, sometimes leading to broken parts.

That’s what happened to this 5.9 Cummins that had been outfitted with a pair of turbos in anticipation of hitting 2,000 horsepower, but the billet steel camshaft and rockers had other ideas.

“We think the cam didn’t have enough overlap,” owner Chase Lunsford tell EngineLabs. “It originally was built as a single charger motor.”

The standard (4.02-inch bore) block was filled before receiving a 6.7-liter crankshaft (4.88-inch stroke), Ross pistons (14.5:1 compression ratio) and Carrillo rods. The cylinder head was treated to a full port and polish, oversized valves and dual valve springs.

“It made 1,200 horsepower last year with the single turbo,” says Lunsford, noting the engine will be used in a Pro Street drag truck with expectations to run in the 8s.

The new compound turbo arrangement includes a 75mm Comp Turbo unit mounted to the exhaust manifold, and a Holset HX82 serves as the atmosphere turbo. Pressurized air then flows into a Taterbuilt intake manifold. The fuel system comprises Hart’s Diesel 13mm P7100 pump and billet body 5x.025 injectors. A Keating Machine water injection kit cools down the air charge. The build also included a custom front cover from Kenny’s Pulling Parts.

“It was making around 120 pounds of boost when it broke,” says Lunsford. “As boost came up, it had some crazy high cylinder pressure. The pushrods were strong enough to where it broke the cam at the hub by the gear. Then four or five rockers were sheared off.”

The next version will have different cam timing with a goal of reaching 150 pounds of boost, this time, hopefully, without incident.

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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