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         One of the most important decisions you'll make when building an engine
is what rod to use. Whether it's a slightly warmed over stock rebuild or an all out
strip stormer, anytime you increase output, the first thing that's tested is the
connecting rods. Ignoring weight issues, most connecting rods 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 compressive load of 8,000 pounds. That's 4 tons that the connecting rod
must transmit from the piston to the crankpin, and do it hunderds of times per
second at racing speeds.                                                                                             

Inertia loads are both compressive (crush) and tensile (stretch). 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 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 poor rods.                  

  Remember, the size of the loads is proportional to the rpm of the engine
squared. But that's not all. 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.