Cast vs Forged
Eric Davis, via email: I have a question about pistons. I understand that forged pistons are stronger than cast pistons and should generally be used whenever nitrous or boost is involved, but I’m curious just as to why they are stronger.
Car Craft: Most of us are familiar with the terms “casting” and “forging” in relation to the foundation of pistons and engine components, without thinking about what separates the two metal processes. Forging is the controlled deformation of metal into a specific shape by comprehensive force, a process evolved from blacksmithing. Casting is pouring liquid metal into a mold. The major differences between the two include strength, structural integrity, and resistance to impact and fatigue. A significant distinction of the forging process is the production of directional alignment, more commonly known as grain flow. Correct grain flow allows for the near absence of structural defects or voids common in the casting process. When metal is forged, the molecular structure of the alloy is forced to directionally align, giving the part more consistent strength qualities. In the casting process, the alloy molecules are free to settle where they please, creating a random grain structure, and opening up the potential for weak spots. This is why forged pistons are fundamentally stronger than cast.
Specifically looking at the manufacturing process, forging pistons forces the material to flow and shape around the forging die, creating the base shape and structure of the pistons. This is generally done in two different processes: an isothermal hydraulic press or a mechanical press. The main benefit of the mechanical press is a high production rate. The force and speed are determined by the ram position, with full rated force only available when the ram is about bottom dead center. When pistons are forged via a mechanical press, the temperature of both the die and the puck must remain consistent and controlled throughout the process. A major difference with the isothermal hydraulic press is the use of computer numerical controls (CNC). Computer-controlled features allow the press to have full rated force available throughout the full travel of the ram. Additionally, the force and speed of the ram can be adjusted and controlled during the entire forging process, while also monitoring temperatures for easier consistency control.
After forging, the pistons do generally require more machining than cast pistons do. These processes include cutting the valve reliefs and ring lands, and cleaning up the pin bores, and other undercrown features. This extensive machining is where the higher cost of forged vs. cast pistons comes from.
Pushrod Design
Jim Heath, via email: Thank you for all the race coverage in last few years. I especially like the pictures from the pits that show engines taken apart. I’ve wondered about the shape and sizes of the pushrods in many of the engines. They’re much different than what’s in stock engines and even most street / strip engines.
Car Craft: We’re happy you’ve enjoyed the racing articles, especially the radial races we’ve been going to for a few years now. You’re right to notice how race engines have pushrods that look far different from most “performance” engines. Pushrods do far more than just connect the lifter to the rocker arm, they are a vital part of the engine’s valvetrain. Pushrod strength and resistance to bending are especially critical in aiding valve control in performance engine builds, especially with higher valve spring pressures required by solid tappet camshaft grinds.
Often a greater wall thickness is the only option when tight real estate concerns prevent upgrading the pushrod diameter. For a typical big-block Chevy for example, increasing the pushrod diameter from 3/8- to 7/16-inch may not present a clearance problem and offers a simple solution to improve performance. But larger intake ports quickly begin to impose space limitations that require pushrod designers to become more creative. Single and double tapered pushrods offer a way to increase overall compressive strength while offering the clearance needed either at or near the rocker arm or down at the lifter – oftentimes in both places.
There are actually four different Trend taper styles for the larger diameter pushrods. These include a full, half, double or neck-down taper designs. Some are dictated by clearance concerns but tapering can also be used to enhance strength. According to Trend Performance Design Specialist, Andy Anderson, “Our double taper 7/16-inch provides the customer a stiffer pushrod that will still work in most 3/8-inch pushrod assemblies.”
Wall thickness is another way to improve strength without adding diameter. Trend offers two options on wall thickness on most of its pushrod diameters, which creates the opportunity to weigh the relative advantages of strength versus weight. In the past, concerns about weight seemed to override strength, but evidence generated by Bob Fox’s SPINTRON proves that, in most cases, strength trumps weight with regard to pushrods.
One item to keep in mind here is that, while weight is always a concern, it is less of an issue on the lifter side of the rocker arm, since this side of the system’s total mass is only subjected to the acceleration dictated by the camshaft lobe. Consider, however, the increase in force the valve spring creates when multiplied by rocker arm ratios of 1.7:1 or higher. For example, a typical Pro Mod engine might need around 300 pounds of closed valve spring pressure and 1,400 pounds of open-valve load. While that number alone is impressive, it’s somewhat conservative when you consider that, if the rocker ratio is 1.7:1, then the pushrod must be able to easily withstand a load of 2,380 pounds (1,400 x 1.7) – or roughly equivalent to supporting the weight of the entire race car on that small column.
But these are just static loads that do not take into account the physics of the force = mass times acceleration formula. If we take what is commonly assumed to be conservative valvetrain acceleration rates of 100 g’s at an equally conservative 6,000 rpm, this jacks the load to nearly 240,000 pounds. These are instantaneous loads, but what’s really scary is when deflection does occur, loads can spike to well over 1,000 g’s. That’s when the numbers become very immense.
This is the reason for such large pushrod diameters and wide wall thicknesses when dealing with large displacement engines with ridiculous spring loads at equally outrageous engine speeds. The fact that these components survive this abuse is testament to the strength and durability of the Trend components.
One place where that load is concentrated is right at the pushrod tip. Extreme camshaft lift numbers push the rocker arm through a wide arc of travel, meaning the pushrod body cannot interfere with the rocker arm throughout this travel. It requires special attention in terms of a 180- or 210-degree radius around the end. This additional room just makes it easier for the components to work together.
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