Types of Pistons Explained | Dished vs Domed | Forged vs Cast

Forged vs Cast Pistons

What are forged pistons? Forged pistons start life as a forged aluminum ingot, or blank, that is heated and then formed into the piston’s shape via a huge press and forging dies under extreme pressure. The cast piston begins as molten aluminum that is then poured into a piston mold and allowed to cool. Casting is cheaper, while forging takes specialized equipment that increases the price, resulting in quite different degrees of strength, longevity, impact resistance, fatigue, and cost. Forging eliminates voids in the metal, creating a stronger, although heavier, part through the alignment of the grain structure. The cast pistons grain structure is random due to the molten aluminum pour, which makes for a weaker, yet lighter, part.

In general, if your engine develops 450 horsepower or more with occasional hits of nitrous, then a forged piston is best. With that much expansion, your engine will require more ring gap, making piston ring installation critical. Newer forged pistons are coming onto the market with less expansion than just a few years ago. Still, there is another cast alternative with hypereutectic pistons.

Disadvantages of Forged Pistons

There is a downside, however. The 2618 aluminum used for piston forging has a higher coefficient of thermal expansion than the 4032 alloy used in cast pistons. To allow for that expansion in the cylinders, the forged pistons need higher piston-to-wall clearance. That means more piston rock while the engine warms up, which often can be heard on a cold running engine, causing more wear. Another disadvantage of forged pistons is that they also tend to be heavier and have a lower hardness rating. The forged piston’s price also comes at a premium over other piston material choices.

Once the forged piston is removed from the die, it requires a lot more machining than a cast piston to achieve a finished product. That means more time in the machining center, which translates to a more expensive part. So, if the engine you’re building will see far more street time than high rpm time, a cast piston is a better choice.

Forged vs Hypereutectic Pistons

Hypereutectic pistons have a higher silicon content than a traditional cast piston. Silicone is introduced for wear resistance. In this case, that means a silicon content of above 11 percent. This makes it stronger and more resistant to melting, taking on some characteristics of a forged piston, however it won’t expand as much as a forging. This makes a hypereutectic piston a sort of “best of both worlds” option, but only up to a certain point power wise, and your tune needs to be spot on.

Billet vs Forged Pistons

Another alternative is a billet aluminum piston. While a forged piston still has better characteristics than billet, we’ve seen a wave of billet pistons entering the market. But if they’re not as good as a forging, then why is there a need?

Today, manufacturers need shorter turn-around times for the unique piston designs racers demand. Each unique forged design needs a unique die. Make a change to a forged piston, a new die must be created. Billet blanks can be machined quicker and with more variables through precise CNC programming, without creating all the separate dies. It is at the point where some racers need unique piston designs for each cylinder due to the different degrees of heat in each cylinder. Both types of pistons start as a blank or ingot versus a molten pour. Where the forged piston uses a blank forging, the billet piston uses a cut length from billet bar stock. The main advantage of billet designs is that they can be machined immediately. Keep in mind that both the forging and the billet blank are heated to above 800-degrees, so their grain structures are optimized.

Can Manufacturers 3D-Print Pistons?

On the horizon is the ability to 3D-print pistons. Porsche and other automakers are already experimenting with the process, which requires around 1,200 layers of fused alloy per piston. The downside is that many layers means that many passes. It can take hours to create one piston. So, as more is learned and printing time falls, we may see more printed pistons available in the future.

What are the Parts of a Piston Called?

Each piston is made up of several specific features. Starting at the top is the top or crown, where combustion happens. It can feature several different designs including a dished, flat, or domed top. More on that in a moment.

Moving down the sides of a piston are the grooved ring lands where the compression rings are seated. There are usually two, followed by the oil control ring. Below that are recessed sides 180-degrees apart where the wrist pin is inserted. This holds the connecting rod to the piston. At the bottom of the piston is the skirt. Like the several types of pistons, piston rings are also produced in a range of materials, coatings, and designs making it critical to evaluate your engine specs to ensure you select the best piston rings for your application.

Dished vs Flat Top Pistons

What are flat top pistons? The simple answer is a piston with a flat combustion face or crown. You will find flat top piston with valve relief options and without depending upon the engine application. A flat top piston will usually provide the most even ignition for the best combustion efficiency. With the smallest surface area of all piston types, a flat top piston creates more force and the most even flame distribution, as there are little to no obstructions on the face of the piston. On small combustion chambers, however, the flat top piston can create too much compression. The dished piston, on the other hand, has a lowered piston surface (looking like a pie plate if you will) and provides for a lower compression value with all other engine aspects being the same. Usually used in a boosted application, such as a turbo or blower setup, dished pistons vs flat top pistons are preferred where a high-lift camshaft or a high compression ratio is not required.

Domed Piston vs Dished Piston

In most instances, dished crowns pose the least design issues for several reasons. Because dished pistons are popular with boosted engines, they require less compression than more radical high-lift cams. They usually come as an inverted dome, which is the mirror image of the combustion chamber, or as a symmetrical dish for two- three- or four-valve applications. Piston design methodology dictates soft edges and gradual transitions for the best burns.

A dome creates an obstruction for the flame front. If you need a dome for that extra compression, the lower and broader it is the better. The design becomes more complicated when the dome needs space for a valve pocket, known as a valve relief. With limited real estate, the domed pistons end up with more of a peak, which interferes with the atomized fuel entering the cylinder. In general, builders are moving away from highly domed crowns because of improvements in cylinder head design and fuel injection advances. Now, higher combustion ratios, usually for racing engines, are achieved with flat top pistons vs dome pistons for an overall smaller combustion chamber.

What is "Valve Relief" on a Piston

The more cam lift you introduce, the deeper and larger valve relief you need. More lift means the valve needs to penetrate deeper into the combustion chamber. Since the piston dome is the floor of the combustion chamber, it needs a relief to give the valve a place to go when the piston is at top dead center. You wouldn’t want to lower the piston’s crown (use a dished piston) because you would lose compression. In the end, you are juggling piston designs for the best spark control and flame travel, without the valves banging the piston crowns, but also giving the desired compression ratio. Always check piston to valve clearance when assembling an engine. You may find you’ll need to change your camshaft specs or even your pistons, but better to find out during mockup/measuring/assembly than after you twist the key and hear bad noises!

CR is the distribution of the atomized fuel mixture inside of the cylinder at bottom dead center (BDC), divided by the volume at top dead center (TDC). The TDC is the measurement of the distance between the piston crown and the top of the cylinder head deck. Generally, picking the lower end of the CR you want is better than going above. Variances in gasoline quality and fuel delivery means the potential for problems with too much CR. If your engine is boosted, you can make up for a slightly lower CR by kicking in an additional pound or two of boost to make up for it.

What Pistons Are Best for Boost?

A boosted or forced induction engine is one that uses a turbocharger or supercharger. Either one pushes more air into the cylinders, which means more atomized fuel, giving more power. Nitrous oxide gives an immediate boost based on the driver’s determination of when to introduce it. With turbo or supercharged applications, the higher the rpm the more air is introduced, meaning there is always a certain amount of boost. Nitrous is boost on demand if you will, while a turbo or supercharger is always providing boost when the engine rpm is above a certain level. The best pistons for nitrous may not always be the same piston choice for a turbo or supercharged application either

What do compression ratios mean for boosted applications? These applications will always prefer a lower compression ratio for best performance. Generally, a boosted application will run best with compression ratios in the 8:1-10:1 range. What about naturally aspirated engines? What is a good compression ratio for these you ask? Your typical naturally aspirated engine will run best with a higher compression ratio of 10.5:1-12:1 or even higher if the proper fuel and tuning are implemented. In conjunction with the lower compression ratio and dished piston, boosted applications will be much safer with a forged piston. Cast pistons and boost are simply a recipe for destruction, even at moderate boost levels.

Keith Black Pistons

The name Keith Black Pistons is synonymous with drag racing and general high-performance components. Speedway offers a wide range of Keith Black hypereutectic pistons for Chevy small- and big-block applications, as well as Ford small-block engines. There are also rotating assembly kits that include pistons, crank, and connecting rods. As discussed earlier, the hypereutectic piston is of a cast design, offering a better than cast piston performance at a lower entry point than a true forged piston. If budget is a concern and you have the choice for your application of a cast vs hypereutectic pistons, go with the hypereutectic option.

Icon Forged Pistons

Forged pistons don’t get any better than Icon Pistons. If your engine build warrants a forged piston for their known strength and benefits of forged pistons, the Icon brand is certainly up to the task to help your engine build perform at its best. Speedway Motors has a wide variety of Chevy, Cadillac, and Ford forged pistons as well as complete rotating assemblies that include the pistons, crank, and connecting rods.

Mahle Pistons

If an LS engine build is in your future, then look no further than Mahle Pistons. Mahle’s PowerPak piston packages are available for LS V8s and small block Chevy applications and feature matching rings and piston wrist pins in the kit for a complete package ready to hang on your rods. If you’re looking to build a stout LS1, 5.3, 6.0, or LSX combo and need LS pistons, Mahle is a wise choice.

• The diameter of the cylinder bore
• Crankshaft stroke
• Head gasket bore diameter
• Head gasket compressed thickness
• Combustion chamber bore
• Piston dome volume
• Piston deck clearance volume

Bore gauges and dial indicators help you determine some of the information to determine the compression ratio value. Speedway Motors has several tools available for those wanting to take their own measurements. From here, you can go to several online calculators to come up with the compression ratio. There are still other factors dealing with both the static and dynamic compression ratio, affecting camshaft timing.