Advancements in Piston Technology

You hear it in the pits from drivers, crew chiefs, and race teams. Announcers talk about it throughout the race. You see the results on the track.

“Our race leader is out-muscling the competition on the straights. He has a stronger engine than the rest of the field today.”

In all types of racing, from Winston Cup to a Saturday-night dirt track, most drivers who end up in the winner’s circle do so because their race cars had a little more horsepower than the competition. While driving skills, good equipment, proper preparation, and a little luck are ingredients for winning, there isn’t a driver around who wouldn’t trade a day’s pay for a small horsepower advantage.

The complex array of parts within an engine all contributes to horsepower, torque, and reliability of a race engine. Each component plays an important part in deriving maximum performance and reliability. The technological finesse of each component can result in a horsepower difference between similar engines. At the very core of this horsepower is the piston, which sees every facet of an engine’s operation: combustion, spark, lubrication, heat, rapid motion, mechanical stress, and energy transference. That makes pistons one of the key components of engine horsepower and reliability.

Development of Racing Pistons

About 25 years ago, racers didn’t have many options when it came to choosing a piston. First there were the stock pistons—big, heavy, cast pieces seen on some of the more potent factory engines. Then a few companies, like TRW, offered higher-performance forged pistons, modified for specific valve reliefs and dome shapes. Finally, one could manufacture a custom piston from specialty machine shops. This was especially prominent in Southern California, where the hot rod industry had its initial roots and, with a large aerospace-manufacturing base, specialty machine shops were plentiful. Names like Mickey Thompson and Art Sparks were among the first to develop specialized performance pistons for race cars.

In the late ’70s and early ’80s, specialized pistons became a necessity as racing cylinder heads produced by companies like Brodix and Brownfield (now AirFlow Research) required racing pistons designed for their use. Today, the tremendous growth of racing and racing components has led many racing-piston companies to compete for the most horsepower. Names such as JE Pistons, KB Pistons, CP Pistons, Ross, Wiseco, and others push the boundaries of piston technology.

As with so many parts of a race car, technological advancement generally means lighter weight, greater strength, and greater durability. The criteria for piston design is much more complex; besides the issues of piston weight and strength, air/fuel flow, compression, combustion, detonation, and temperature are greatly determined by the piston design. Because the piston is at the very point where horsepower is generated, its design is of critical importance.

The advancement of race engines necessitated advancements in piston technology. For example, in the ’80s, Winston Cup cars generally ran in the mid-7,000–rpm range. With continued engine development, that number has steadily risen to the point at which a Cup car can see just above 9,000 rpm on superspeedways. Short-track cars won’t run at that high an rpm, but with higher compression and more horsepower, even Saturday-night racers should understand the importance of current piston technology. Today, specialty pistons are not only demanded by every class and type of racing engine, but often pistons are designed to the exact specifications of each individual customer. It is not unusual to find proprietary piston relief and dome shapes unique to engine builders. At the higher levels of engine building, these shapes can be closely guarded secrets. For most racers, even off-the-shelf piston selection will determine important engine-performance characteristics.

Piston Design

The need for higher-performance pistons coincided with the advancements of manufacturing technology in the ’80s. High-speed CNC machining allowed piston manufacturers to increase the prototyping and production cycles of pistons, and 3-D modeling software greatly increased design time. Complex cam and barrel shapes, before difficult to design and machine, were now easily contoured and, just as importantly, repeated with high-tolerance consistency. This, as we will discuss, is a key aspect of manufacturing high-performance pistons.

Piston material also has made great strides. Aluminum pistons (as all racing pistons are) are subject to enormous physical stresses, yet must maintain their structural integrity for thousands of cycles. Advancements in high-strength aluminum permit parts with greater strength, durability, and tolerance to high heat conditions. This, coupled with more sophisticated forging techniques, has allowed engineers much more flexibility in piston design.

The specific design advancements of racing pistons are too numerous to outline here—designers dissect and optimize every characteristic of a piston.

However, by focusing on a few aspects, we can realize how sophisticated pistons have developed and contributed to the ever-escalating horsepower increases of today’s racing engines.

Here is one example: A piston ring is the seal between the piston and the piston cylinder wall, and its ability to maintain its sealing capability at high rpm is critical to horsepower. Leakage here robs the piston chamber of the pressure produced by the combustion of air/fuel, compression, and spark. Between two identical engines, the one with better sealing capability will produce greater horsepower. The evolution of CNC-machining capabilities and improved accuracy and tolerances have led to lighter rings that react quicker to higher piston speeds. Ring flutter, a negative condition seen at high rpm, has been substantially reduced, which leads to greater and more consistent horsepower.

To further explain this point, note that the accuracy in which ring grooves are measured are units of microns; that is, 50 millionths of an inch. The ability to both machine and measure tolerances at that level starts to reveal the precise nature of these parts.

Another area where pistons have seen dramatic improvement is in the their skirt profiles and designs. Due to improved forging and machining technology, the piston weights have been dramatically reduced without sacrificing structural integrity. Skirt designs have been made considerably smaller, eliminating significant weight. In fact, advanced software programs that utilize techniques like finite element analysis (FEA) allow designers to test parts for strength and performance under different physical conditions (stress, temperature, pressure) before a single part is machined. That means a designer can “chisel” a part on a computer screen to minimize the weight and maximize the strength before any real chips are cut. New, low-profile piston designs have lowered piston gram weights significantly, resulting in greater horsepower possibilities.

Advances in materials and design have made wristpins, the all-important connection between the connecting rod and piston, stronger and lighter with less deflection. The list of improvements goes on: lubrication methods, unique left- and righthand forgings, specialized dome shapes and valve reliefs, accumulator grooves—all contribute to allowing engines to run more reliably with greater horsepower output.

Accuracy and Performance

Perhaps the most important gain technology has brought pistons is repeatability on the production line with startling consistency and accuracy. Because many manufacturing operations occur to produce just one piston, maintaining close tolerances is essential to engine performance.

First, an engine with eight cylinders has eight assemblies (pistons, rings, and wristpins) that can either fail or limit performance. Any one of the pistons that exhibits excessive blow-by, loses lubrication, or fails structurally will most likely put a driver out of the race. There is no redundancy here—each part of a racing engine must perform flawlessly.

At the highest levels of engine building, the stakes are even more critical.

Take the case of restrictor plate racing on superspeedways. With limited carburetion, cars fight for every drop of horsepower gain. Cylinders that lose compression at any rpm will take them out of the race. The playing field is already made level (perhaps too level) with restrictor plates—no team can afford anything but maximum engine performance. Saturday-night racers, many of whom race under spec engine constraints, face the same fate if their engines are anything but 100 percent. An engine builder, therefore, must have confidence that each piston falls within the most demanding tolerance specifications as any other piston. In the earlier days of racing pistons, parts had to be made with significant redundancy engineered into the part.

Limitations in materials, manufacturing accuracy, tolerances, measuring, and so on made over-design a requirement for dependability. As design capabilities, manufacturing, and measuring accuracy have improved, pistons are produced within close tolerance guidelines, and less redundancy also is now required. This has led to both a positive and negative result.

As previously stated, the positive is a lighter and stronger part. In the competitive world of auto racing, the edges of weight and strength must be pushed to run up front. Engine builders must build their engines to the limit of the specification criteria. On the negative, because pistons (as well as other components) reach the limits of weight reduction and strength, pistons tolerate less abuse. Racing pistons cannot be expected, no matter how technologically advanced, to tolerate anything less than optimal running conditions. By the sheer competitiveness of the sport, their designs and running conditions have already been pushed to the edge.

Design Time

Not to be lost in the piston-design manufacturing process are the vastly reduced times required from initial concept to production. In the past, it often took months to design a piston, produce a forging, manufacture tooling, and machine the part. Because of high-speed CNC machining and computer-design programs, that period has been cut down to weeks, and for prototyping new designs, even days. It is not uncommon for manufacturers to receive requests for piston designs (by an engine builders) and cut and ship the new designs in a few days. This acceleration of design time is a major factor of the increased competitiveness for horsepower.

For most racers and engine builders, custom pistons are not affordable or practical; pistons stocked by manufactures still represent the latest technology and are more than adequate for most classes of racing.

Cost of Piston Technology

The cost factor is always a consideration of racing parts, and pistons are no exception. There are three costs here: first, the direct costs of piston design; second, its production costs of it; and third, the labor costs of engine building.

To begin with, the research and development of racing pistons is expensive. Design, prototyping, dyno testing, and track testing are time-consuming and expensive. Due to the sophisticated nature to which racing has evolved, even the most basic racing requires state-of-the-art design.

Second are the direct costs of manufacturing. In many cases, these costs have actually gone down as high-speed machining equipment has increased productivity. Additionally, as racing has grown, so has the need for volume production. Increased production runs always tend to lower prices. Also, the sheer competitiveness of the racing parts industry has forced manufacturers to compete on both the quality and performance of pistons while maintaining a viable price.

Third, and perhaps most important yet continually overlooked, is the cost of engine building. When a piston fails or is assembled incorrectly, an entire engine must be disassembled. That is a lot of time and expense. To that add the fact that there are eight pistons, and every one must perform perfectly for an engine to yield the maximum horsepower. Inconsistency from any piston will result in poor performance from that cylinder, hence the engine’s horsepower. The cost to manufacture, inspect, and properly assemble pistons is critical. The result of imperfections is felt both in the wallet and on the track.

Trickle-Down Technology

There is no question that the technology gained from piston manufacturers developing NASCAR-type pistons ends up in the engines of Saturday-night racers. Although design and specifications may differ, it is the expertise developed in machining and quality control that goes into every racing piston.

Over the past 10 years or so, piston manufacturers have been able to steadily reduce piston weight by about 20 percent. That represents real increases in horsepower output. On the track, where a few extra horsepower is usually the difference between passing or being passed, every component of horsepower and reliability must be examined and optimized. The piston is right in the thick of horsepower, and every engine builder and racer must remain on the technological edge of piston development. With so much riding on the consequences of their use, there can be no other option.