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HPBG: Racing Piston Technology

In the movie "World’s Fastest Indian," Burt Munro (played by Anthony Hopkins) spent years building a 1920 Indian motorcycle, all the while trying to improve the pistons that he made in his garage.

By Brendan Baker

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Munro had a problem with melting his cast pistons so he kept trying to develop his own out of melted down GM pistons. Today’s piston makers use a little more modern techniques, but the principle is the same: try, test and improve.

On the outside, pistons tend to look the same. They are round slugs of metal with grooves and bores for the rings and wrist pin.

That is about where the similarities end, however. Today, every manufacturer has its own recipe for making a better piston, and the days of pouring a molten liquid into a mold is largely a thing of the past, at least for the performance side of things. Cast pistons may work fine in your vintage, mild street rod but pushing them much further than 400 horsepower will leave you with new additions to your collection of ash trays and planter art.

Today, there are numerous manufacturers in the aftermarket from which to choose a racing or high performance piston, so it can be a bit overwhelming for engine builders who are shopping for pistons. But all pistons are the same, right? That statement is, of course, completely false.

There’s a lot that goes into the design of a piston, whether it’s an off-the-shelf part or a specifically designed piece for a high horsepower racing application. Pistons come in all shapes and sizes from flat top to dish, domed, cast aluminum, forged, hypereutectic, short skirt, full round, and even asymmetrical.

What you choose for your next race engine build depends on what type of racing your customer does and if he or she has a budget or if there are rule restrictions that limit your choices, because piston manufacturers can make just about anything you want.

The trend in piston design today has been evolving for many years with pistons being made lighter and lighter with each passing year. But today there are a number of technologies and materials at a piston designer’s disposal to make designing a lightweight piston that has enough strength to last the distance and anti-friction properties to keep it from robbing any of the precious horsepower you so carefully assembled.

The high performance race engine by definition means that the limits are going to be pushed to the edge for most of its life. For piston designers, the limit is peak cylinder pressure. Maximizing cylinder pressure will give the most gains in horsepower, and can be reached by increasing the compression ratio through piston design with either a domed or flat top piston or a dished piston in the case of a power adder such as a supercharger, which dramatically increases cylinder pressures.

Finite Element Analysis
Finite element analysis (FEA) is one of the mostly widely used engineering analysis techniques in the world today. Engineers employ FEA to simulate how a physical system (usually an engineered product or manufacturing process, i.e., pistons) will respond to expected loading conditions.

Like all analysis models, a finite element model is an abstraction of a more complicated physical system. However, the physical world is much too complex to model at every level of detail.

One piston manufacturer said that to design a piston, it takes an engineer who not only understands how to make a piston through computer modeling and analysis but who can also understand what the computer doesn’t, who has experience to go beyond what the glowing screen tells him.

All Shapes and Sizes
Older style cylinder heads have larger combustion chambers and therefore may require a piston that incorporates a domed area on top of the piston for racing or performance applications in order to increase the compression ratio. While this has worked to great effect, adding a dome is extra weight and some engine builders choose to mill the head down to increase compression and use a smaller dome or to be able to use a flat top piston instead.

Weight is generally the name of the game for pistons and the rotating assembly, and it’s much easier on the components to move a lighter piece up and down, which in turn allows for quicker acceleration off the line or out of the corner.

Another trend today is shorter piston skirts, or “slipper” skirts. The main reason for shorter skirts is to reduce weight but it also reduces friction, and many manufacturers add an anti-friction coating to minimize scuffing effects on the cylinder walls.

Following “slipper” skirts are less than full round pistons. These pistons have reduced skirt area around the circumference of the piston and are generally braced or strutted underneath for added strength.  Some of these designs would have been impossible a few years ago, but due to better tooling and CNC machining, a lot more areas that used to be out of the realm of possibility are now possible. All pistons are slightly out-of-round when viewed from the top to allow for overall thermal expansion. 

The barrel shaped design refers to the side profile of the piston.  With a barrel shaped design, the upper ring land area of the piston is slightly smaller in diameter than the lower ring land and skirt area of the piston as viewed from the side.  Making the upper area of the piston slightly smaller allows more room for thermal expansion when the top of the piston gets hot and swells up.