Wiseco FA20 Subaru Pistons Bring Proven Performance and Strength!

September 5, 2019 / by Bradley Iger

In the realm of Subaru tuning, Wiseco has become a household name. Our newest support for the brand comes in the form of high-tech forged pistons for direct-injected FA20 engines. Read on for a comprehensive list of upgrades from the failure-prone OE parts. 

All FA20 pistons are forged from ultra-strong 2618 aluminum and feature complex injection bowls compatible with factory fuel injection.
“Our Subaru EJ257 piston is actually the best-selling part that we offer,” says Wiseco’s Mike Skeen. “We’ve been providing an off-the-shelf option for STI guys since 2003, and even before that as custom parts.”

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It didn’t take long for tuners to discover the limitations of the factory slugs, Skeen notes. “The main limitation in the OE stuff was the materials they used. The EJ257 uses a cast piston from the factory, and the fact that these engines are rather high strung means that it’s fairly easy to exceed the limitations of the part when you start looking for more power. The primary concern is ringland failure. A lot of people want to crank up the boost and get a little more aggressive with the tunes because it’s a really capable platform. The problem is that some of the stock internals just aren’t ready for it.”

Whether it’s a road racing, rally, or street build, stepping up a more stout piston not only allows builders to turn up the wick a bit more, but also provides a reliable foundation for modification down the road. “If you look at the purchase trends, the fact that so many of these are standard-bore pistons tells us that many people are taking that extra step beforehand to make sure they can safely start making the engine really perform.”

And that tradition continues with Wiseco’s new Subaru FA20 series pistons. Originally developed for the Subaru BRZ, the FA series engine was designed from the get-go with performance in mind. Today, the naturally aspirated FA20D variant can be found under the hood of the 2012-2017 Toyota GT86, Scion FR-S and Subaru BRZ, while the turbocharged FA20F engine provides motivation for the 2015 and newer WRX for the US markets, the 2014-2018 Subaru Forester XT, and the 2012-and-up Legacy 2.0GT in Japan. 

Shaping the direct injection bowl of the piston is absolutely critical to performance, efficiency, and durability. The goal is to maximize atomization, and avoid fuel spray landing on the cylinder wall, which can wash down the precious oil film that lubricates the piston. 

While the FA20F represents one of Subaru’s most sophisticated production engine designs to date, like the EJ257 there are still a few bottlenecks that must be addressed in order to effectively introduce more performance into these power plants. Here we’ll take a closer look at how the FA20F series differs from both its predecessors (as well as other variants in the FA series), and check out the features and benefits of Wiseco’s latest Subaru offering.

FA20 Architecture Considerations

“One of the things that has come along in the last five to ten years – and is really here to stay – is direct injection,” Skeen notes. “With the FA20F’s original equipment, they’re utilizing a very high pressure fuel system and they’re timing the injection of fuel with that high pressure, almost like a diesel engine. So with that, the stresses on the parts have only increased over time, and depending on the specific architecture of an engine, the piston often has to be designed to make sure that high pressure fuel is going where it’s supposed to go for proper atomization.”

That direct injection system in turn dictated some of the design features of Wiseco’s FA20F piston. “We have a fuel bowl on our FA20F piston,” says Skeen. “The idea there is to reroute the directly-injected fuel – that’s a big design concern because it affects the function and efficiency of the engine, which we obviously want to preserve whenever possible.”

Thicker ringlands, thicker-wall wrist pins and a strutted forging all contribute to greater piston strength. 

This differs significantly from a more traditional port injection system, Skeen explains. “With port injection you’re spraying in at the port and getting some air to tumble to get some swirl and generate atomization. It’s almost a “swamp cooling” effect of the vapor and the fuel – it increases the density of the charge a little bit, and cools down what would otherwise be some pretty warm air going into that engine, especially in a turbocharged application. With a direct injection system, that air is entering the cylinder pretty lean at that point, so you’re really not getting that same benefit. That means the parts are under greater stress – you’re looking at higher temperatures, and the timing of the event with the fuel being sprayed in at high pressure at the last moment means that it basically ignites as soon as it gets into the cylinder because the compression of it is already happening.”

And the piston, in turn, must be designed to accommodate those changes. “Typically what we find is that the injection is occurring on an angle, and you definitely don’t want that fuel to spray on the cylinder wall because that can cause some wear issues,” Skeen continues. “If it’s spraying down, we want to ramp that back up into the chamber so we can ignite the mixture instead of washing the cylinder wall with it and potentially removing the oil film. So that’s where the fuel bowls come in – capturing and redirecting that fuel spray.” 

Above And Beyond OE

Wiseco FA series pistons are available in both low and high compression variants. The piston on the left features a deeper dish whereas the right piston has less dish. Both maintain the necessary fuel pocket to re-direct fuel spray from the direct injector. 

With the design concerns of the direct injection system addressed, Wiseco’s mission was to take the piston’s durability well beyond the factory hardware. “The prime advantage here is that we use forged 2618 aluminum,” Skeen tells us. “That’s going to be a lot stronger than a cast OE part. You’re going to find that the 2618 material is also much more resistant to detonation than pretty much any other aluminum alloy that’s used in engines.”

The strength of forged pistons versus their cast counterparts offers other benefits that might not be obvious at first glance, too. “A forged part doesn’t come apart the same way a cast part would,” Skeen says. “What you’ll see with both cast and hypereutectic pistons is that when a part of one fails, it’ll actually fracture or basically explode.”

Cast material changes over time and becomes brittle, and that’s what leads to potential fractures. Forged components, on the other hand, typically do not suffer the same fate. “While a forged part is subject to wear like everything else in an engine, when you come to the end of the service life of the product, you’re able to tear the engine down and replace the component,” he adds. “You’re not facing the aftermath of catastrophic engine failure. You’re often looking at a new engine when you have a failure with a factory cast part.”

The forged side relief design of the FA series reduces the length of the wrist pin. This minimizes weight without any reduction in strength. 

And while modern engines typically deliver more performance than their predecessors, most manufacturers’ primary goal is still maximizing efficiency. “Everybody’s chasing the CAFE standards,” Skeen says. “But our customers are using these products to really push beyond that artificial barrier. It’s tough to ballpark because it’s really subject to a particular car’s tune, but the piston should easily be able to handle the limits of the connecting rod. On a good tune, these pistons should be happy in a 600-800 horsepower build.”

As with all other pistons in Wiseco’s lineup, the FA20F receives the ArmorGlide skirt coating. “This is particularly beneficial in street cars,” Skeen says. “It really helps reduce the engine noise that makes some folks nervous about going to a forged piston. That, in combination with the way we profile the skirt, enables us to have an extremely quiet part. We get a lot of customer feedback with our current Subaru offerings – everyone is always commenting on how much quieter the Wiseco part is versus the competitors in the segment.”

Wiseco also offers piston options for the FA20F for folks who want to take things a step further. “We have a DLC (Diamond-Like Carbon) pin option for the FA20F that drops right in, and we keep those on the shelf,” Skeen says. “DLC has huge benefits in terms of increased lubricity in the pin bore, so that’s going to help hold on the oil and keep it there on the bearing surface where’s most critical.”

Beyond that, Skeen points out that Wiseco can also fulfill the requirements of just about any build under the sun through their custom line. “We can design a custom piston based on the demands of an application – customizing compression ratios, bore sizes, compression heights, and other features as needed.”

He also hints that other new Wiseco-developed rotating assembly components for the FA20F might just around the corner. “It’s something we’re actively pursuing. So keep your eyes peeled for that, too.”

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Written by Bradley Iger

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