Performance Auto Parts
High Performance Racing Parts and Engine Components!
Including Custom Engine Kits and Crate Motors!
Including Custom Engine Kits and Crate Motors!
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Carrillo Connecting Rods |
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Cat Power Performance Rods |
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Childs & Albert Connecting Rods |
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Crower Connecting Rods |
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Eagle Specialty Products Connecting Rods |
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MANLEY Connecting Rods |
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Oliver Connecting Rods |
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Pauter
Connecting Rods High Performance IMPORT Rods |
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SCAT Connecting Rods |
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 poor rods.
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 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. But which ones are right for you? Read on for a complete rundown
on the different types of rods that are available today.
Cast Steel and
Stock Forged Steel connecting rods
Aftermarket Forged,
True Billet Steel
and Fully Machine Forged steel
Aluminum and Titanium Rods
Facts of Interest
Cap Screw Vs. Bolts
&
H-Beam Vs. I-Beam
Call 1-877-354-3812 to order your Connecting Rods Today!!
We would like to thank the following sponsor sites for the information that has been compiled and written above!
Performance-Auto-Parts.com
Cedar Valley, UT 84013
Toll Free 1-877-354-3812
