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.

Lightening Things Up

In case you are just now joining us, lightweight is the name of the game in the racing piston game. One manufacturer has taken it a step further by producing its own software program that can “lighten” a shelf or custom piston for a customer who may be looking for an extra edge. And who isn’t?

Internal lightening, according to this manufacturer, is one of the more effective ways to reduce weight without changing the performance of the piston. In some forgings there may be areas that are thicker than an engine builder’s application may need so the lightening program is able to remove any unnecessary material. This is like getting free horsepower. It lightens the load on the other components in the engine as well.

Excess material along the inside of a piston is cut away to match the contour of the external features. Experts use their knowledge of piston design, and computer modeling software such as FEA and 3D, and after evaluating the needs of each individual customer they decide precisely how much inner material can be removed without decreasing reliability. Each internal profile design is saved to a computer then machined on a CNC mill.

Rings and Things

The accumulator groove is the groove between the first and second compression ring. It does make the piston lighter, but the real purpose is more abstract, explains one manufacturer. Pressure spikes that get trapped between the first and second compression rings tend to unseat to top ring. This action encourages ring flutter and loss of piston ring seal.

The void created by this groove between the rings tends to average the spike pressure of combustion, keeping the pressure low enough to prevent lifting the top ring while maintaining some pre-load on the second (oil scraping) ring.

The top ring end gap is often a significant part of the problem whenever there are piston issues. Most top land damage appears to lift the land into the combustion chamber. The reason is that the top ring ends butt and lock the piston at TDC. Crank rotation pulls the piston down the cylinder while leaving at least part of the ring and top land at TDC.

Actual running end gap will vary depending on the engine heat load. Piston alloy, fuel mixture, spark advance, compression, cooling system capacity, duty cycle and horsepower per cubic inch all combine to determine an engine’s heat load.

Most new generation pistons incorporate the top compression ring high on the piston. The high ring location cools the piston top more effectively, reduces detonation, smog, and increases horsepower. If detonation or other excess heat situations develop, a top ring end gap set toward the tight side will butt, with piston and cylinder damage soon to follow.

High location rings require extra end gap because they stop at a higher temperature portion of the cylinder at TDC and they have less shielding from the heat of combustion. At TDC the ring is usually above the cylinder water jacket. However, many of today’s current designs do a better job of keeping the rings cool, according to one manufacturer.

High rings locations work well in stock and street performance applications, and it helps reduce the crevice area for lower emissions and better fuel economy.  But in really high power applications (blown or turbocharged engines, or nitrous), many piston manufacturers recommend a piston design that has the top ring somewhat lower on the piston to keep the ring away from the heat. They say this improves the durability of the piston and top ring.

If a ring end gap is measured on the high side, you improve detonation tolerance in two ways: One, the engine will run longer under detonation before ring butt. Two, some leak down appears to benefit oil control by clearing the rings from oil loading. A small amount of chamber oil will cause detonation and significant horsepower loss. The correct top ring end gap with some pistons can be 50% to 100% more than the manufacturer’s specs, says an expert.

Ring options of 1/16? or stock 5/64? are offered in many applications. The 1/16? option reduces friction slightly and seals better at high rpm but the drawback is that it’s considerably more expensive. Stock (usually 5/64? compression rings) work well and won’t break your customer’s budget. Metric ring options are also becoming more common.

Piston to bore clearances of .0015?, .0020?, .0035? and .0045? were wide-open throttle dyno tested by one manufacturer. After 8 hours of maximum torque and 7 hours at maximum horsepower, the pistons were examined and all looked new, except the tops had normal deposit color. Even with 320° F oil temperature, the inside of the piston remained shiny and completely clean.

Excess clearance has been shown to be safe with many of today’s race pistons. The added skirt stiffness reduces piston rock, even if it is set up loose. Less rock allows you to run a tighter quench.  Some hypereutectic pistons with over .002? clearance may make noise.

As they get up to temperature they may still make noise because they have a very low expansion rate. Some manufacturers may use different hypereutectic alloy that expands at different rates than some others. When the skirts stay cool they don’t grow. Running additional piston clearance because friction is reduced can sometimes have a short-term horsepower improvement.

Pin oiling should be done at pin installation. Either pressed or full-floating, pre-lube the piston pinhole with oil or liquid pre-lube – never use grease. (If you are using a pressed pin rod, be sure to discard the spiral pin retainers.) A smooth honed pin bore surface with a reliable oil supply is necessary to control piston expansion.

A dry pin bore will add heat to the piston rather than remove heat. All pistons are designed to run with a hot top surface, cool skirts and pin bores. High temperature at the pin bore will quickly cause a piston to grow to the point of seizure in the cylinder.

Sources:

JE Pistons (www.jepistons.com), KB Pistons (www.uempistons.com), Wiseco Pistons (www.wiseco.com), Diamond Pistons (www.diamondracing.net), Speed Pro (www.federal-mogul.com), Mahle Motor-sports (www.mahlemotorsports.com) and Ross Pistons (www.rosspistons.com). Other suppliers of pistons and piston rings can be found in our exclusive Engine Builders Buyers Guides.