Offset rod beams in a mod motor

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  1. #1

    Default Offset rod beams in a mod motor

    Curious to know what the real world effects are of using an offset rod beam in a mod motor. There are some crank and piston combinations that the usage of an off the shelf small chevy rod fills the void. Small block chevy rods have a 0.06" beam offset in relation to the big end journal shoulders, Mod rods are centered. There is one company currently selling there rotating kits with offset chevy rods. I've approximated the wristpin force balance changes between centered and offset rods performing beam calculations but for those of you with offset rods in your engines, do you see any unusual wear around the wristpins, piston & bore or ring seal issues?

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  3. #2

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    Chris,

    For most of the last century the ICE engineers in Detroit chose to design all the new engines with a slight offset of the connecting rod beam to help accommodate the bore center offsets bank to bank used in the engine design. That trend continued until the advent of the LS and Modmotor engine families from GM and Ford. I have not checked the Gen III Hemi's so I don't have authoritative information on them but suspect it could be similar.

    With the advent of the LS series and Modmotor series both GM and Ford went to on center rods and have maintained that design criteria through current production. Depending on the offset you wanted to use you might be able to "get away" with it. It would likely not provide you any power advantage and most likely would decrease the operating life of several components.

    The offset beam would apply cylinder pressure (force) to the rod bearing but instead of evenly distributing it the force would be greater on the bearing shell side the beam was offset towards. This is probably not a big deal because, don't forget, all Detroit iron did this before the advent of the modern engine designs.

    The bigger bumps in the road would occur up the bore at the wrist pin and wrist pin / piston juncture. What you would do would be to offset the pin end of the rod from the center of the wrist pin. This has several bad effects. the most obvious is that one side of the piston would have a leverage advantage in terms of bending the rod beam. As cylinder pressures would increase the rod would exhibit a bending moment failure unless additional material was added to the beam section of the rod. It is always easy to add weight to rotating assemblies but it is not smart. Weight can more often than not be the protagonist in massive engine failures.

    A second significant bump in the road would be a flexure, that the rod would exhibit each ignition cycle. As you expend power budget units to oscillate the connecting rod back and forth from straight to slightly sprung that is horsepower that never sees the rear tires of the car. In time all metal that is flexed work hardens and then becomes much more susceptible to a mechanical failure (broken rod).

    As if the internal parasitic power loss to rod flexure were not enough don't forget the piston would no longer be square in the bore. The rocking back an forth from square to tilted would cause the rings to exhibit a reduced sealing quality. Combustion gases would increasingly escape to the crankcase exhibiting increased blowby and further reduced power output.

    There is more to the unhappy use of an offset rod in an on center engine but this should be enough to dissuade most. In the simplest of terms there is no upside and a lot of downside.

    Sticking with on center rods in an engine specifically built for them is a real good idea.


    Ed

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