Forged Rods vs. Stock?
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On The Tree
Joined: Dec 2003
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Originally Posted by DavidNJ
I don't think there is a 'cast' connecting rod. I don't think I would call powdered metal 'cast'.
There are three issues in the rods (and bolts): power, duty cycle, and risk. Stock is designed for some margin (20%? 50%) above stock power/rpm levels. However is also designed to live close to the limits of the stock power levels for an indefinite duty cycle with zero risk of failure within manufacturing tolerances. Clearly, a bad one may get out now and then, but it is rare.
For higher power levels, what duty cycle and reliablity do you want? And at what weight. Before the fuel mileage mandates and performance wars, You could make the part fairly heavy At fixed engine speed on a dyno, a heavy piece is as good as a light one. However, for rapid acceleration, weight counts. And light weight reduces power/rpm limits and duty cycle.
If your spending money on rods, weight (500-600 for 6", racing 6" rods (steel 4130M) are under 500gm. with Honda or smaller journals) is as important as power.
There are three issues in the rods (and bolts): power, duty cycle, and risk. Stock is designed for some margin (20%? 50%) above stock power/rpm levels. However is also designed to live close to the limits of the stock power levels for an indefinite duty cycle with zero risk of failure within manufacturing tolerances. Clearly, a bad one may get out now and then, but it is rare.
For higher power levels, what duty cycle and reliablity do you want? And at what weight. Before the fuel mileage mandates and performance wars, You could make the part fairly heavy At fixed engine speed on a dyno, a heavy piece is as good as a light one. However, for rapid acceleration, weight counts. And light weight reduces power/rpm limits and duty cycle.
If your spending money on rods, weight (500-600 for 6", racing 6" rods (steel 4130M) are under 500gm. with Honda or smaller journals) is as important as power.
Isn't this a significant factor in how strong the rods and the crankshaft need to be? Most of us keep our redline below 7K in cars using factory parts. This is in part dictated by the limits of hydraulic lifters. This self imposed RPM limit also has much to do with the capacity of the factory rods.
One engine builder I know has told me that he believes the large end of LS1 rods begins to stretch around 7K. Is he correct? I don't know, but his theory is this. That the stretch causes the big end of the rod to go into an oval shape shrinking the diameter of the rod at the point where the cap meets the rod. The result is that the oil film between the bearing and the crank is wiped away resulting in a spun bearing. He believes this happens because the rod cap of a stock LS1 is fairly thin. It’s not nearly as thick as the forged units found in older SB Chevy engines. Weight no doubt is why GM made the change.
If he is right something else to consider are forged pistons. What happens to the piston weight using them? Are the forged pistons heavier compared to stock and does this necessitate forged rods?
Someone asked about specific brands of forged rods; does anyone know anything about the Manley rods?
