Everybody’s trying to lose a bit of weight these days, and your pistons are no exception. Why is piston weight important and how can grams be shed without sacrificing strength? We explore the process of making pistons lighter.
At 6,500 RPM, a piston changes direction more than 200 times per second. In an engine with a four-inch stroke, that piston’s average speed is almost 50 miles per hour, and that’s just the average - it spends a considerable amount of time (in relative terms) sitting almost motionless at the top and bottom of every stroke before being shoved or dragged back in the other direction.
The bottom line is that the g-force a piston in even a mild performance application experiences is enormous, and we take it for granted that they’ll handle that stress over and over without failing and spontaneously disassembling the engine. While the acceleration a piston endures is a mathematical function of engine speed, crank throw, and rod dimensions, how much force that translates into is something we have some control over, in the form of piston weight. While it might seem counterintuitive, a lighter piston can be more durable than a heavier one if there’s excess material in the wrong places, and to get some insight into the how and why of making pistons lighter, we reached out to two very knowledgeable individuals - Wiseco’s Vic Ellinger and Nick DiBlasi.
It’s easy to look at components like pistons, rods, the crank, and the block as being utterly rigid when they’re laid out in the assembly room, but the forces that act on them in operation are so far beyond our ability to intuitively understand that it’s hard to get a feel for how they’re flexing and moving in operation.
Ellinger eloquently describes the controlled chaos going on inside a running engine; “There are constant pushing and pulling forces distorting every component of the rotating assembly - Think of a bucket of rocks on a string versus a pebble on the end of a string and spinning them round and round. The rings are fighting combustion cycles; the piston, rod, and pin are all trying to literally be thrown out of the block, while trying to seal and in turn push the vehicle on down the road. Cylinder walls are going barrel shaped, and main and rod caps are moving around fighting to stay clamped in place and provide support for the bearings.”
Interestingly, the pistons aren’t under the highest stress when you might think they are - during the power stroke. DiBlasi explains, “It’s actually the bearings, journals, and caps that see the most forces on the power strokes of engines, where the inertial loads of the exhaust stroke and deceleration are where many failures occur with heavy parts.” Pistons and rods are actually under some of the most extreme stress at TDC during changeover between exhaust and intake strokes, where there’s little resistance to the upward motion of the piston from pressure in the chamber, and the piston is suddenly changing direction as the motion of the crank starts to yank it back down the bore.
“Under compression, when parts are designed correctly, everything is happy and holds itself together,” DiBlasi explains. “Heavy weight really can make an impact when the mass becomes further and further from the centerline of the crankshaft.”
Simplicate and add Lightness
In order to lower those stresses, reducing the mass of the rotating assembly is key, and the biggest bang for the buck in terms of weight reduction happens with the pistons.
Per DiBlasi, “The more weight you can pull out of the rod and piston assembly, the less stress you put on the bearings and journals. When revving engines “high,” the load and speeds can do damage at an exponential rate. Every gram makes a huge difference at that point. An extremely heavy piston also pushes the limits of the rod bolts. The piston on its way back up to TDC will be doing everything it can to keep moving in that direction, and the rod bolts are the only thing in place to stop that.”
In some circumstances, like engines running high boost or a lot of nitrous, there’s no alternative to having extra mass in the piston in order to provide enough of a heat sink to prevent detonation and supply a broad enough path for that heat energy to make its way out of the piston and into the cylinder walls, and eventually the cooling system. But without those special conditions, lighter is going to be better.
“Lighter pistons are good for naturally aspirated engines looking for every possible advantage within the rules, or where lower horsepower levels are all that are going to be achieved and the lighter rotating mass can be used without risk of parts failure,” Ellinger explains. “Beyond that is how the engine responds,” DiBlasi adds. “Lighter rotating mass allows the engine to respond quicker when spinning up.”
Less is More
With these factors in mind, how can pistons go on a diet without sacrificing strength? Some of it comes down to “genetics” - how the piston is manufactured. The two common methods for mass-produced pistons are casting, where molten aluminum is poured into a mold, and forging, where an aluminum blank is heated, then shaped in a die under immense pressure. There are pros and cons to each method in terms of strength, the materials used, and of course price, and one isn’t necessarily better than the other for every application.
“Typically a forged piston is heavy in most cases,” Ellinger explains. This is because many forged piston manufacturers use a single base forging for a wide variety of different final piston designs, whether it’s going to be a high compression domed crown or a low compression dish, or a number of other design factors like a full skirt or reinforced pin bosses. You can always remove material from a forging, but the time it takes to machine away the excess that doesn’t need to be there for a particular application costs money, and as a result, forged pistons often end up heavier than they need to be.
Ellinger continues, “Wiseco is very good at designing near-net forgings for a specific application with the luxury of having our own in-house forge shop.” That ability to create forged pistons that closely match the final desired shape means less mass to remove in the first place. DiBlasi adds, “Depending on the application, forged pistons can be significantly lighter, the same weight, or heavier. Typically, in our naturally aspirated applications, pistons are lighter as our base material and design is superior to OEM. In extreme duty applications where we see demands of high boost, it is common to see weights like cast and heavier. Many of our drop-in replacement parts are designed to match OEM weights so little to no balancing is required.”
Reducing piston mass is an opportunity for some creative engineering, as every part is a potential place to add lightness, starting with the design of the base forging and continuing through the machining operations that turn it into a ready-to-run component. DiBlasi says, “The as-forged design can reduce weight in virtually every area of the piston. Since our base material is stronger, we can utilize better piston geometry and thinner cross sections of the piston to reduce overall weight. The crown thickness, skirt thickness, and pin boss towers are the largest areas that material can be removed. A cast piston may have a crown thickness of .220” whereas a forged variant can be superior in strength at .180” thickness. Also, by utilizing a design with a shorter pin, we can reduce pin weight by 25% and move the pin boss towers inward to reduce the amount of aluminum on the forging.”
Ellinger continues, “Typically under the dome and valve reliefs are your biggest areas to alleviate extra weight. Finite Element Analysis and examining run pistons can give you signs of where to pull weight out without having issues of cracking and heavy wear start to show up.” DiBlasi adds, “Our FEA software allows us to see the high stress areas in the piston. We can remove weight in our modeling software and see a direct impact to the finished product. Many iterations are designed when forgings are being created to understand where each forging can utilize additional weight reduction. From that point, we develop specific design standards for the forging.”
What’s it Worth?
Overall, what does that add up to? According to Ellinger, it can be more than ten percent of the total weight of the original piston. “Usually a full undercrown milling operation can be as much as 45-60 grams depending on the design of the dome. At a starting point of 480 grams, that is a substantial reduction in weight.”
DiBlasi adds more specifics, saying, “Each application is going to be different, but we can look at our naturally aspirated flat top LS3 in a standard 4.065” bore piston, for example. By reducing the pin boss width, crown and skirt thicknesses, we can pull 30 grams out of the piston in just the forging. Shorter pins equate to another 12 grams going from 2.500” to 2.250”. This equates to about 7-10% mass reduction.”
As part of a complete strategy, lightweight Wiseco pistons can significantly reduce the reciprocating mass in your engine without any compromise in strength or durability. “Piston components are another resource to reduce weight further on the rotating assembly,” DiBlasi notes. “Using high grade, stronger pin material allows us to run a thinner wall wrist pin in conjunction with the shorter pin. Also, running a smaller ring package such as 1.0mm, 1.0mm, 2.0mm can reduce up to 30 grams in comparison to a 1/16, 1/16, 3/16 ring set.” While there’s inevitably a cost increase involved with extensively lightened pistons, lightweight rods, upgraded pins, and other components, if you’re in search of reliable high RPM power in a naturally-aspirated application, it’s money well spent.