We’ve talked about it in detail before, but sometimes a quick refresher is needed. When it comes to piston shape, it was realized generations ago that full-circumference piston skirts weren’t required in performance applications, and were in fact, a detriment to performance.
As time has marched on, and our understanding of what is happening in an engine on a micro-level has improved, more refinement of piston shape has taken place — far beyond just a simple “slipper skirt.” Now, the shape of a modern piston’s skirt is a well-calculated oval shape, designed to minimize friction within its peak operating envelope.
Oval Peg Into A Round Hole
To explain the concept and its execution in an easy-to-understand manner, we bring you this video by Ben Strader of EFI University. Strader has tons of experience with piston design for custom applications and shares with us the salient points about why a piston’s skirt isn’t round.
“Depending on what you are planning using your engine for, whether it’s going into a boat, or a street car, or a road-racing car, or a drag-race application, you might find that you want a different piston shape,” says Strader. When he says “a different shape” he’s not talking about the shape of the crown.
“First and foremost, the piston has to seal the combustion chamber, so that the power we want to extract from the engine during combustion is sealed in pushes the piston down, transfers the energy to the connecting rod, and then the crankshaft,” he explains. The only way to seal the cylinder is for the piston to be the same shape as the bore — ideally, round.
However, when things change temperature, they change size, and even sometimes shape. The most drastic example of this in a piston has to be drag racing, due to the large temperature change in an extremely short amount of time. That makes it a perfect example of how this concept works.
“In a drag racing application, there isn’t a lot of time to get the engine warmed up. In fact, a lot of the professional teams will pump chilled water through the engine block, because it keeps the heads and intake manifold colder, which in turn helps increase air density and make more power,” says Strader, who has been a part of some of the top-level drag racing teams in addition to his own program.
“We need the piston to be round to seal well, but also lightweight, stiff, and strong. One of the ways modern manufacturers make the pistons lightweight and strong is through the piston skirt design. In a drag race engine, we want to eliminate weight and drag. So instead of having a full-round skirt like a traditional piston, we have a slipper skirt. Then the structure to hold the piston pin is brought inboard, resulting in an “inboard-box” design piston. It offers a lot of advantages because it’s lightweight and very stiff and strong.”
However, that skirt’s shape can have some unintended consequences, like acting as a heat-sink in certain areas of the piston, thus causing uneven thermal expansion. “Remember that all of the heat affecting the piston is being generated at the crown,” Strader explains. “The heat will move from the crown and radiate downward. The mass of aluminum in the skirt area draws the head further from the crown than the areas without the skirt’s mass. This causes the piston to grow at a different rate in the pin direction than the thrust direction.”
Since the problem is obviously understood, there is a solution to it, right? “Over time in development, we’ve been able to learn just about how much that piston will grow in the amount of time we need to use it. The only way the piston is going to seal properly is if it gets round in a hurry,” says Strader.
“If I started with a perfectly round skirt, and it grew in one direction and not the other, the piston would immediately go out of round and have less sealing as you go down the track. To combat that, we’ve developed a piston shape that is absolutely not round.”
X Marks The Spot
While this might seem like an extreme example, it is. But, it illustrates a concept that exists in all pistons, known as “ovality” and can be measured via a specialized fixture. However, that brings up another point: if the piston isn’t perfectly round at room temperature, how do you measure things like piston-to-wall clearance?
“When I buy pistons from any manufacturer — it doesn’t matter which one — you’ll get a card in the box that gives you the recommended clearance. Regardless of what that number is, the really important thing is that they tell you a gauge point on the piston, which is where you need to measure on the skirt,” Strader explains.
Piston-To-Wall Clearance Explained
Piston-to-wall clearance is a critical specification, and getting it right is crucial when assembling an engine. Expressed in thousandths of an inch, it’s simply the bore diameter minus the piston diameter. However, the key factor is measuring the piston diameter at the indicated datum point. With all the different shapes built into a modern piston, the location at which you measure can give you wildly varying readings. The piston manufacturer’s clearance specification is based on the measurement at precisely that point and nowhere else.
So, in addition to not being perfectly round, pistons aren’t perfect cylinders, either. Thermal expansion happens in three dimensions, and all three are important for the proper performance of a piston.
“The heat happens on the top of the piston, which means the top of the piston will swell a lot faster than the bottom will. It’s tapered so that when everything grows, it’s a perfect circle,” says Strader. “The trick is, how much? Too much taper and the piston never gets to the proper shape and doesn’t seal the combustion chamber. Not enough taper, and it grows too fast, gets stuck, and galled in the cylinder, and then you have a mess.”
So the salient points to take away from the article are, make sure you measure your piston’s size at the proper point, and know that an aftermarket piston isn’t a perfect cylinder shape, for good reason.