The faster and further we want to go, the harder diesels are pushed, creating greater demand on engine components to handle the abuse. As such, custom piston options continue to increase as we continue to innovate within the diesel industry to find out what works and what doesn’t. It’s often been the school of hard knocks, but these lessons allow piston manufacturers to design custom options based on what they’ve learned and what they already know about Original Equipment (OE) durability and function.
First, the common feeling among piston manufacturers is that they want to be contacted when selecting pistons. There are so many different options available that it is a huge advantage to take advantage of their knowledge.
“Because we’ve been through all the hard knocks of diesels, we know what works, what doesn’t, and what you need to do in each application,” says Rick Canning, technical specialist at CP Carrillo.
Most stock diesel pistons are cast iron, and cast iron is traditionally at the bottom of the performance food chain. Cast OE diesel pistons, however, have proven durability and can work, at least to some degree, with the performance crowd. One advantage is that they have a steel insert for the top ring. In addition, they use a top ring key type, creating a radial movement that cleans carbon build-up from the groove, a major advantage for diesels.
The cast pistons also feature oil cooling galleries behind the rings to help control heat, and it’s the combination of these design aspects that allows them to handle mild power increases. In the case of Mahle Motorsports, its cast pistons are among the company’s best-selling diesel pistons, according to Joe Maylish.
Cast pistons have many advantages, but when you’re pushing performance well beyond stock levels and looking for more than the occasional drag race, you may be pushing more than it can handle, which means it’s time to upgrade the forged pistons.
“Forging produces a tighter grain structure, crushing the molecules and making them strong,” says Canning. High-powered traction or drag engines are a common application for forged pistons, and while trucks are the obvious home for most diesel engines, they are increasingly appearing between car fenders, which has helped push the power limit even further. , demonstrating its place in the performance.
“People who are putting in really high amounts of boost and trying to make outrageous amounts of horsepower, choose to go with forged,” agrees Maylish.
However, forged is not without drawbacks, and one of them is the rate of expansion. “Heat from combustion is dissipated at the top of the piston, and since it’s a forged piston, the diameter of the pitch has to be reduced so that when it gets a lot of heat, it won’t grow and hit the cylinder wall” , Canning. he says
Many forged pistons use low-expansion alloys to keep the piston fit as tight as possible without creating clearance problems, and because of the extreme heat, clearance is critically important. The average cast aluminum piston will need a cylinder wall clearance of about .005″-.006″. A low-expansion forged piston requires .007″-.008″, and a high-expansion alloy may require .010″-.012″ or more. Because of these variable expansion rates, piston manufacturers always emphasize the importance of measurements during assembly.
“With a diesel, even the best components can be destroyed if they don’t go together properly,” Canning notes.
Forged pistons pose another problem. Although stronger than cast, they lack the long-term durability, because you can’t forge a piston around a steel insert for the top ring, nor can you create an oil cooling gallery behind the rings . The advantages of a key ring on a diesel are great for longevity, but the downside of this style of ring is that it will cause accelerated wear without a hard material to protect it, which is the purpose of the steel insert. This is the only real advantage of stock pistons and a reason many people stick with them as long as they can.
The movement of a key ring in its groove becomes more pronounced when additional cylinder wall space is required to compensate for piston expansion.
“Diesels are very hard on ring grooves,” says Canning. “Forged diesel pistons are hard anodized to provide extra protection. We also don’t use key ring grooves. We use a conventional rectangular ring groove because when the floor diameter is driven in from the bore, what you have for the ‘vertical clearance and recoil remain the same.
Another aspect of custom diesel pistons is the relocation of the top ring groove due to the extreme heat and pressure loads on top of the piston.
“To make these pistons live in high-power, high-heat applications, in certain applications, we drop the top ring .600,” he says.
Expansion can increase as boost levels and performance increase, and it’s not the only heat-related factor to consider either. Most stock pistons have a bump or peak in the center of the bowl that helps disperse the fuel sprayed by the injector into the surrounding bowl. As the bowl curves upward, there is usually a lip or overhang that redirects the air/fuel mixture toward the center of the bowl. This is specific by design for a cleaner burn to reduce emissions, but heat tends to concentrate in the upper lip area of the bowl, and in an engine running at high boost, heat can build up on the lip or protrude and cause a detonation or cracks. to the piston
“Hot spots are a big problem in diesels,” says Canning. “Sharp edges cause problems and the heat will penetrate where it can burn holes in the pistons. We made non-nitrous piston plates with generous radii that go from the plate to the valve pockets and around the valve pockets. All it has a generous radius around it to avoid any kind of heatsink. The entire top is completely smooth, machined and radiused.”
Custom work is often done on OE-style pistons, machining the lip designed around the bowl, along with beveling or blending the top edge of the bowl to eliminate the possibility of hot spots. It’s proven to be effective, but it doesn’t have the same advantages as custom pistons, which are designed without hot spots from the start.
Many factors are considered when designing a diesel piston, including the type of injector and spray pattern, as well as the fuel and ignition strategies that will be used in the tuning. This can affect the design or inclusion of a peak, and a common question asked by consumers is related to the injection angle of the dish.
“We made a wide bowl plate that covers all the different angles,” Canning says, again reinforcing the idea that it’s critical that piston manufacturers know everything about the application to provide the correct piston. “Small plates make less power without nitrous, but with nitrous, a small plate is better.”
Another aspect of a custom piston that can improve strength is the pin head. The pin end of a diesel connecting rod is tapered toward the top to increase the bearing surface and spread the load over a wider area at the bottom of the wrist pin on the downstroke rod, and also at the top between the pin. and piston Unlike machining the taper in a forge, it is an advantage to duplicate the shape of the rod in the forge.
“Since we have Carrillo rods and CP pistons, we doubled the taper of the Carrillo rod in the CP piston forging,” notes Canning.
Another piston option is steel, which offers the performance of forged aluminum, but the added benefit of durability. Steel pistons are incredibly strong and able to withstand heat. Based on these factors alone, they may seem like the ideal choice, but they also have drawbacks. If any wear occurs between the cylinder wall and the piston skirt, steel is likely to damage the cylinder wall, while aluminum is easier on the cylinder walls and any damage will only go to the piston .
Coatings are a technology that adds multiple options and can vary between custom pistons. Superior thermal coatings aren’t meant to be a substitute for quality, but when you’re pushing the limits, they can be that little extra edge that gets you through without piston damage. They can also help prevent hot spots and give you a slightly larger window to adjust.
“We offer a thermal barrier coating on the piston crown,” Mahle’s Maylish told us. “It’s an extra layer of protection against thermal shock, to prevent excess heat from penetrating the piston crown.”
Hard anodizing is a common process to protect ring grooves, but some companies offer a nickel-based anti-wear coating. Although it adds thickness to the material, anodizing is a process that increases the thickness of the natural oxide layer on metal, unlike an applied coating, which requires a different type of machine finish.
Skirt coatings are designed to reduce friction, with the primary intention of reducing debris and reducing piston skirt fatigue.
“The material we use in the Mahle piston skirts is Grafal,” says Maylish. “It has a damping property, and when there is potential for contact with the cylinder wall, the Grafal will prevent metal-to-metal contact.”
Oil is another important aspect of diesel pistons. They are cooled by spraying the underside of the piston and directing some of the oil into the cavities behind the rings. Since the galleries themselves cannot be cast into a forged piston, many custom pistons are designed to direct the oil spray in order to maximize cooling.
Ultimately, one of the most important factors when ordering diesel pistons is to provide the piston manufacturer of your choice with all the information about their construction and intended use. Piston manufacturers encourage their involvement to ensure you get the pistons you need.
“If you have any questions of any kind, give us a call,” Maylish reiterates. “We have people who have been on our sales staff for over 20 years and people who have been on our engineering staff for over 30 years. We can help you.” EB
