You're at the point in your project where it's time to start building the engine for your car. You're looking online to see what pistons and rebuild kits are available, but the first thing you find out is that there are a lot of choices available when it comes to pistons. The decision can be a daunting one, and that's probably why we get a lot of readers emailing us, asking what pistons they should use in their engine. Since you guys asked, we decided to put together this article about piston selection, and how to choose what's right for your individual application. There is no way for us to determine what piston you think will be best, but one thing to keep in mind, is that the piston you choose is subjected to violent explosions, extremely high temperatures, and if your engine isn't running correctly, situations that are akin to your piston being hit by a sledge hammer.
Automotive pistons are an extraordinary piece of design. Within a matter of seconds, they are subjected to temperatures reaching upwards of 1,000 degrees during combustion, only to be immediately subjected to a blast of cold air with every intake stroke. They reach speeds of around 7,000 rpm, and have to withstand side loads that try to push it through the cylinder wall. To say that a piston is abused, is an understatement. So, how do they survive? If used in an improper application, they don't. Take for example, if you install a stock, cast piston in an application that will see higher cylinder pressures than stock because of a turbo, supercharger, or nitrous. You can rest assured, that you will be seeing your engine's internals become external. But, why is that? To fully understand, we need to explain what the different pistons are, and when is the right time to use which one.
Determine Power Goals
One important factor to consider when choosing a piston is balancing on the fine line between strength and price. Before you can decide what type of piston your engine needs, you need to know how much power you are planning to make before you purchase pistons. Obviously, the more power you build into your engine, the more expensive the required piston will cost. The way you decide to make that power is also a huge factor. Will you be using nitrous oxide? Maybe a blower is in your future. These are all aspects that need to be considered.
Cast vs. Hypereutectic vs. Forged
The two most popular ways that a piston is manufactured, either results in a cast or forged piston. We've all heard these nomenclatures applied to pistons, but what does it mean—what's the difference? The difference is found in the way that the piston is actually made. A cast piston is built just like it's named. During the casting process, the melted aluminum alloy is poured into a mold that when cooled after filling, results in a piston-like "blank" being made. Casting a piston has a few advantages over forging when manufacturing. Cast tooling is generally designed to produce a near net casting. This near net shape minimizes the overall finish-machining that is required, reducing its cost. Cast pistons also offer excellent wear and thermal characteristics. This enables long ring-land and skirt life, as well as the ability to retain tighter side wall-clearances for quiet operation. The major drawback to running cast pistons is that the cast aluminum is limited in its ductility. Ductility is a solid material's ability to deform under tensile stress. What this means is that an over-stressed cast piston can fail more quickly.
When it comes to choices in aftermarket cast pistons, the choices begin with what we'll refer to as low-cost replacement pistons. These pistons are considered to be direct replacements, and are usually balanced closely to the weight of stock pistons. This is done so that "in theory," no balancing is needed when rebuilding. If you're building a performance-based engine, these pistons are not for you. With OE replacement style pistons, you can forget about features like valve notches for clearance to run higher-than-stock cams. It will also be difficult to get an adequate compression height for building any reasonable compression ratio. If building with any hopes of getting a high-performance engine, these pistons are almost always a risky purchase. But, if you're building a low rpm, daily-driver type of engine, these might be just what you're looking for.