Friction is bad, strength is critical. Designing a piston skirt for power is a delicate balance of both. We explore skirt deisgn as a means to the perfect piston for the job.It's all about friction. That testimony quickly sums up the advantages of the box-strutted piston design engineered by Wiseco Pistons.
“You have a smaller skirt, so there’s less gliding on the cylinder wall,” says Nick D’Agostino, automotive sales manager at Wiseco. “You’re going to make more power.”
Box strutted is an adaption of the “slipper skirt” design that engineers brought to race engines a couple decades ago. The objectives were to lighten the piston, reduce the contact patch with the cylinder wall and provide clearance between the bottoms of the piston skirts and the crankshaft counterweights in stroker applications.
In addition to shortening the distance from the bottom of the skirt to the ring lands, the new strategy called for narrowing the space between the wrist-pin bosses. In other words, move the pin supports inward and away from the cylinder wall. By repositioning the bosses, the piston requires a shorter wrist pin; which, in turn, helps reduce reciprocating mass. The move also allows engineers to eliminate skirt surface area that previously was essential for helping support the wider wrist-pin boss locations.
“It’s really a symphony of design elements,” explains D’Agostino. “The shape of the skirt and how it relates to the cylinder wall. Then it comes back to the actual strut itself and how it relates to the wrist pin tower. The more splayed the angle of the strut, the wider the skirt. In higher levels of racing where there’s going to be more service, you’ll see a straighter strut and much smaller contact patch from the skirt area.”
Traditional full-round pistons offer stability and durability, along with simplicity, at a competitive price. The box strutted piston is designed for certain racing applications, particularly for naturally aspirated engines where the power increases from reduced friction and lighter mass can be validated on the dyno.
“Budget-wise, it’s more pricey to go down this road,” advises D’Agostino. “It’s usually custom work, but a significant measure of Wiseco’s shelf inventory is manufactured with a strutted forging.”
The piston crown and ring-land area of a box strutted piston are no different than a traditional full-round piston. But flip a box strutted piston upside down and design contrasts are quite evident.
“The traditional strut design will have two struts coming off the pin towers at an angle that goes to the skirt,” explains D’Agostino. “Then you can add struts between the towers and struts on the outboard side of the towers that go to the underside of the ring lands to create a box strutted design.”
Depending on the length and angularity of the support struts, piston engineers can achieve the similar effect of a longer wrist pin. The additional stability of outboard struts is especially valuable as the bore diameter increases.
“For example, if you have a 4.185in or 4.155in bore and a 2-inch wrist pin, we need to increase stability in the pin axis,” says D’Agostino. “Ideally for structure you’re going to want the longest pin possible. The reason you get away from them is weight savings—trying to get the lightest reciprocating assembly possible.”
Even with additional milling under the crown and machining out pockets around the struts, the actual box strutted piston structure can be heavier that its full-round counterpart.
“But it’s all strength. It’s definitely more rigid because of the strut locations,” explains D’Agostino, noting that when combined with a shorter wrist pin the overall weight is comparable if not less to a full-round version. “Also, due to the added structure you can mill it significantly more than you could a full-round and still maintain structural integrity around the crown."
Box strutted pistons work well in certain high-stress situations, but not so well in others.
“It could be a high-revving application where you don’t want to carry a 2.750 pin. But you could carry a 2.250 pin. Again, you can design the piston to simulate a longer wrist pin and give you more integrity on the pin axis,” says D’Agostino. “Where you don’t see them used very often is marine, street or in power-adder applications. At that point we’re talking durability.”
In drag racing and circle track applications where engine builders are chasing every last bit of weight and friction, box strutted designs fit well into a powertrain strategy. And there’s another possible benefit to squeezing the pin bosses closer together.
“You’re able to do piston-guided rods,” says D’Agostino. “You can’t do something like that in a traditional full round.”
Connecting rods are guided by either the crankshaft or the piston. For most automobile engines, the connecting rods are designed and sized so that the directional forces are controlled at the big end by the crankshaft. That’s one reason why rod side clearance is a critical measurement when assembling an engine.
Other factors associated with rod side clearance, of course, are friction and lubrication. If there’s not enough clearance, the rods won’t spin freely on the rod journal or oil flow across the bearings could be restricted. If there’s too much clearance, excessive oil will start flying around the crankcase and the rods could start slapping the crankshaft throws and fillet areas. Such movement will also relay stress to the little end of the connecting rod, possible causing it to bind on the pin.
By moving to a piston-guided rod, engineers can design thinner connecting rods to reduce weight. They don’t have to size the big end to ensure proper side clearance. Instead, the critical clearance is between the wrist-pin towers and the sides of the little end of the connecting rod.
“Generally speaking, you want .005- to .010-inch clearance from outer dimension of the rod width to the insides of pin bores. That’s pretty close clearance, but that’s what guides the rod.”
Piston-guided rods are used in numerous applications, including motorcycle and industrial, but only recently became a real option for US performance automobile engines when NASCAR engineers adapted the strategy.
“Some live and swear by it,” says D’Agostino. “The trend mushroomed when hand-me-down engines came out of Busch and were tore down by sportsman racers. Now our phones are ringing with guys who have SB2s as drag race engines and notice they have piston-guided rods. So, box strutted is the only way we can go because we have to capture a narrow space of one inch or less.”
Connecting rods are made specific to piston-guided applications, so both the engine builder and Wiseco have to know this when ordering pistons.
“In some cases, 1.2- or 1.3-inch spacing on a conventional piston, even some box strutted is wide, like 1.100-inch,” says D’Agostino. “So, if I have a 1-inch small end, it’s a completely different discussion. I may need to go to a billet or different boxed-strut forging. There are a lot of things to consider when talking about piston-guided rods.”
For either option, engineers still have to design the pin towers for proper engagement of the wrist pin.
“We need to capture 50 percent or more of the wrist-pin length between the two sides,” notes D’Agostino.
Box strutted designs are available for a variety of applications and in both 4032 and 2618 alloys. Some dirt-track engines will prefer the 4032 due to weight advantages while other high-stress applications require the stronger 2618 alloy. Also, some extreme horsepower applications may require oil squirters to cool down the piston. After all, there is less piston material to transfer heat to the cylinder wall. But that’s also why you go faster!