One of the most important decisions you’ll
make when building your next engine is what rods to use.
Whether it’s a slightly warmed-over stock rebuild
or an all-out strip-stormer, any time you increase output,
the first thing that’s tested is the strength of
the connecting rods. Ignoring weight issues, most connecting
rod upgrades do not add significantly to power output.
What they do is far more important: They allow the ported
heads, hotter cam, extra carburetion and other hop-up tactics
to complete their mission.
As a piston reciprocates between top dead center (TDC) and bottom dead
center (BDC), the rod it’s attached to experiences power loads
and inertia loads. Power loads result from the expansion of burning gases
during combustion that push down on the head of the piston and cause
the crank to turn. Thus, power loads are always compressive in nature.
This compressive force is equal to the area of the bore multiplied by
the chamber pressure. A cylinder with a bore area of 10 square-inches
(3.569-bore diameter) with 800 psi of pressure is subjected to a compressive
load of 8,000 pounds. That’s 4 tons that the connecting rod must
transmit from the piston to the crank pin, and do it hundreds of times
per second at racing speeds.
Inertia loads are both compressive (crush) and tensile
(stretch). To better understand them, let’s pull
the heads off the engine and forget about the combustion
process for a moment. When the rod is pulling the piston
down the bore from TDC, the mass of the piston plus any
friction caused by ring and skirt drag imparts a tensile
load on the rod. Once the piston reaches BDC, the dynamics
shift. Suddenly the rod is pushing the mass of the piston
as well as the friction load back up the cylinder bore,
and a compressive load on the rod results. Then the piston
stops and reverses direction to head back down the bore,
so the inertia of the piston, once again, tries to pull
the rod apart as it changes direction. The size of the
load is proportional to the rpm of the engine squared.
So if crankshaft speed increases by a factor of three,
the inertia load is nine times as great. At 7,000 rpm,
a typical production V-8 with standard-weight (read “heavy”)
reciprocating parts can generate inertia loads in excess
of 2 tons, alternately trying to crash and stretch the
OK, now we’ll reinstall the heads, turn the fuel
pump and ignition system back on, and restore valve operation.
The principles of inertia loading are the same, but conditions
become even more severe now that the plugs are firing.
Even more tensile loading on the rod comes from the work
required to suck air and fuel through the intake tract
and into the combustion chamber during the intake stroke.
Once the piston reaches BDC, both valves close and the
rod must push the piston back up to TDC on the compression
stroke. But near the end of the trip toward TDC, the spark
plug fires and the compressed fuel mixture begins to expand
with opposing force before the piston reaches TDC. This
causes a sudden surge of compressive energy that must be
resisted until the orientation of the crank pin makes it
mechanically possible for the piston and rod to change
direction and be pushed back down to BDC during the power
stroke. Remember, the size of the loads is proportional
to the rpm of the engine squared. But that’s not
By far, the greatest test of a rod’s integrity is experienced near
the end of the exhaust stroke when the cam is in its overlap phase. In
overlap, both valves are open as the piston pushes the last remnants
of spent combustion gas out the exhaust port. The intake valve is held
open so that fresh intake charge is available the very instant the piston
begins generating suction on the downward intake stroke. What makes the
overlap period so hazardous is the fact that there is no opposing force
applied to the head of the piston (in the form of compressed gas) to
cushion the change in direction. This is the load that stretches the
rod, ovals the big end, and yanks hardest on the fasteners. If you don’t
want your engine to scatter, you’ve got to make sure the connecting
rods are always one step ahead of any performance upgrades. But which
ones are right for you? Read on for a complete rundown on the different
types of rods that are available today.