If you’re like many of us, you don’t spend a lot of time thinking about the pistons in your engine. Most of the time you don’t need to, but there are times when burnt, scuffed, or cracked pistons need to be replaced or upgraded. If you’re rebuilding your engine, chances are you’re probably upgrading to a set of forged aluminum pistons. But which alloy should you go with? And should you buy into new technologies? Here are some tips on choosing the proper pistons for your engine.
2618 vs. 4032 Aluminum Alloy Piston Comparisons:
No matter which type or brand of piston you decide to use, they are all made from a combination of both aluminum and silicon, and almost all forged aftermarket pistons are manufactured from either 4032 or 2618 aluminum alloy. It is the amount of silicon, though, which determines the piston’s overall strength versus wear resistance properties.
2618 Aluminum Alloy
A 2618 aluminum alloy has a higher overall strength and can ultimately take a bit more abuse than 4032 alloys. A 2618 piston is ideal for forced-induction engines that experience higher temperatures and/or any racing application. In comparison to the 4032 alloys, 2618 with its low-silicon alloy requires larger piston-to-wall clearances due to its higher coefficient of thermal expansion, which causes the piston to grow more when exposed to heat.
Pros
- Fatigue life
- High-temperature strength
- Conductivity (heat transfer)
- High strength
Cons
- High expansion rate (more clearance required)
- Poor wear index (lower silicon content)
- Piston rattle or slap due to larger piston-to-wall clearance
4032 Aluminum Alloy
A 4032 aluminum alloy is designed for high-performance applications where a strong and quiet piston is required. A 4032-alloy piston requires less initial piston-to-wall clearance than 2618 due to its higher silicon content, and is ideal for street/strip applications.
Pros
- Wear index (good at resisting wear due to higher silicon)
- Low expansion rate (less clearance required)
- Low density (lightweight)
Cons
- Limited temperature strength (lower strength at high temperatures)
- Notch sensitivity (brittle)
Gas Ports
Vertical Gas Port
Lateral Gas Port
Gas Ports are small holes drilled into the top of the piston crown or on the sides of the top ring land that lead to the back of the top ring groove. When gas ports are used, pressure is directed to build up behind the compression ring and seal it against the cylinder wall. As a result, less pressure can leak past the top ring and more horsepower is generated. Generally, lateral gas ports (right) are drilled through the bottom side of the top land and extend to the back wall of the ring groove and are used for endurance or road racing. Vertical gas ports (left) have the holes drilled from the deck of the piston into the top ring groove and behind the ring, and are mainly used for drag race applications, which accelerates ring wear from the increased pressure on the ring face. Gas ports work best for race engines that are torn down and rebuilt often, and are not recommended for daily street cars.
Wristpins
The piston’s wristpin is arguably the most highly stressed part in an engine, as shown by the red areas in the FEA (finite element analysis) image shown here. The expansion of burning gases in the combustion chambers applies tremendous force on the piston top. That force is transferred to the connecting rod via the wristpin. To give you an example of how much force is applied to a wristpin in race applications, consider the inside of an 850hp NASCAR Sprint Cup engine running at 9,000 to 9,500 rpm: A great deal of stress from both cylinder pressure and inertia loads applies crushing force equivalent to six tons, hammering each wristpin about 77 times each second. This punishing, cyclical load can last up to 600 miles in some races.