Rod stretch and rod bolt fatigue: Beerhorst talks boosted engines

Rod stretch and rod bolt fatigue: Beerhorst talks boosted engines

Written by Moore Good Ink:

When the crankshaft pulls the connecting rod downward on the induction stroke of a naturally aspirated engine, a stretching load is exerted on the con rod because the piston area is so much greater than the column of air being drawn into the cylinder.

In contrast, when the inlet valve is open on a boosted engine, the rod is always under pressure, not a stretching load. Therefore the life of all rotating parts in the boosted engine is significantly prolonged.

Of course, there is also a stretch on the rods when the throttles are closed and the engine is decelerating. It’s under these conditions where most con rods fail on oval track cars: when the throttles are closed as the car approaches the corner. Consider that situation for a moment: the piston is sealed to the bore by the rings and the crankshaft is pulling it down against huge resistance. Remember, the throttle plates are closed so there’s not much air pressure in the combustion chamber to assist. 

Xtreme Fords, Connecting Rods Story

Read 24 things you need to know about competition connecting rods.

A streetable naturally aspirated engine producing 2,000hp doesn’t exist. But if it did you’d be lucky if the engine’s rotating parts survived for more than a few quarter-mile passes. There are, however, large displacement maximum-effort engines operating near this power range, but they are not streetable and only the most durable could complete 50 quarter-mile passes without a rebuild. In contrast, when you boost the engine, it will make this vast amount of power and you could run it on the street. Using lower engine speeds and with less radical valve train it will run for thousands of miles, as proven on HOT ROD Drag Week. 

In our next News Brief Norm discusses air flow and camshaft specifications combined with piston and piston pin choices, combustion chamber temperatures, and fuel-burn rates when related to different fuels.

See lasts month’s issue on the differences of the induction and combustion processes between the turbocharger and the naturally aspirated engine

Norm Beerhorst, engine builderSource:
Norm Beerhorst
Ultra Tech Racing Engines
417 S. Byrkit Street,
Mishawaka, IN.
Tel: (574) 256-9462

READ 24 THINGS YOU NEED TO KNOW ABOUT COMPETITION CONNECTING RODS →

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