Subaru is conservative these days. It sticks with technologies it knows well, evolving its cars gradually over the years. Even when it does something new, it’s often long after the rest of the industry. But in the 1990s, Subaru was gloriously weird and experimental. Witness Subaru’s largely forgotten sequentially twin-turbocharged flat-four.
To the best of my knowledge, this is the only twin-turbo gas-powered four-cylinder automotive engine. Some automakers have done twin-turbo four-cylinder diesels, and before you jump down my throat about it, BMW’s "Twin-Power Turbo" engines use a single twin-scroll turbocharger. Not two turbochargers. So, Subaru stands alone here.
Welcome to The Rabbit Hole, a bi-weekly column where Senior Editor Chris Perkins explores his latest obsession with automotive technology. He speaks to the best in the business to understand how cars work and what the future of the automobile looks like.
Subaru introduced this twin-turbo boxer with the second-generation Legacy in 1993, when sequential turbocharging was all the rage. Porsche did sequential turbos first with the 959, and soon, other Japanese automakers embraced it. In the height of the asset-price bubble, the Japanese auto industry’s R&D budgets ballooned, ushering in all sorts of new technologies, like sequential turbocharging. Toyota was first among Japanese automakers, with the Supra Turbo’s 2JZ-GTE straight-six, then came Mazda with the 13B-REW twin-rotor in the RX-7.
The idea behind sequential twin turbocharging is simple. A small turbo spins up quickly, providing good low-RPM power and low lag, but runs out of puff at higher engine speeds; a large turbo takes time to spin up, manifesting in lag and a higher boost threshold, but with the benefit of better high-RPM performance.
Sequential turbocharging attempts to provide the best of both worlds, with a smaller turbo optimized for lower engine speeds and a larger turbo optimized for high engine speeds. The first turbo covers everything from idle to somewhere around 3,500-4,500 rpm, at which point some exhaust air gets diverted to the second turbo. Eventually, both turbos are going at full steam, working together to provide high-rpm power.
Subaru did things a bit differently. As the brochure for the 1993 Legacy notes, Subaru used two turbos of the same size to accomplish the same basic goal as other sequential turbo systems. To pre-spool the second turbo, the engine management system closes the first turbo’s wastegate, later diverting excess exhaust gas to the second turbocharger. Then, as engine speeds climb, the system opens up a switching valve to the second turbocharger, now allowing both turbos to operate together.
A great post on the Subaru Legacy International forum details the EJ20 twin-turbo variants further. The turbos in all versions weren’t identical, though they were always about the same size, and except for one later variant, the second turbocharger didn’t have a wastegate. The initial twin-turbo boxer in the second-generation Legacy GT made 246 horsepower, and from 1996 on, Subaru offered the engine with two outputs, 256 hp and 276 hp.
As with all other twin sequential turbo systems, Subaru’s was simple in concept, wildly complicated in execution. Under the hood is a mess of vacuum lines, plumbing, and solenoids to make the whole thing work. And controlling all those are some computers from the 1990s. It’s a lot of complexity and a lot of things that can go wrong for what turned out to be questionable benefits.
It seems Subaru wanted to address the relatively meager low-end torque of its single-turbo flat-fours, most notably used in the Impreza WRX, for the larger, heavier Legacy. And where in Japan, car owners are taxed based on engine displacement, further boosting a 2.0-liter rather than just making a bigger engine was advantageous.
But as a 2001 review of the Liberty (Australian-market Legacy) B4 from Australian magazine AutoSpeed notes, there’s a noticeable torque dip when the second turbo starts spooling up. It recorded up to 4.4 psi of boost-pressure dip, between 4,000 and 4,500 rpm, which is huge. A lot of enthusiasts call this zone the “Valley of Death.” And this was the best-developed version of that engine, the one that our friend from the forums considered the most reliable and the one that best delivered on the sequential-turbo promise.
These problems weren’t unique to Subaru. Sequential turbocharging always leads to uneven power delivery, and often, unreliability. It’s why when tuners get their hands on a 2JZ-GTE or a 13B-REW, they almost always delete the twin-turbo setup for a simpler, single turbo.
Still, Subaru’s system was popular. A 2003 Car and Driver article noted that, at the time, the Legacy B4—the last Legacy to get the twin-turbo boxer—was Japan’s most popular sports sedan. But, that same article noted that a lack of low-end torque was a “weakness” of the B4, and why when Subaru decided to bring it to the US for 2004, it went with a 2.5-liter single-turbo flat-four. The system was never designed to work with left-hand drive cars, so it remained mostly a Japan-only affair, with some sales in Australia and New Zealand later in its life.
Subaru discontinued the twin-turbo flat-four with the arrival of the fourth-generation Legacy in 2004. At the same time, it switched to a single twin-scroll turbocharger with its EJ20 flat-four, which helped address the traditional concerns around turbocharged engines without needing all sorts of complicated vacuum and hydraulic systems to make it work.
I can’t help but wonder why Subaru didn’t try a more conventional parallel turbo setup, with one turbo for each side. Yes, at the time there was a sort of sequential-turbo fever, but that layout makes more sense for an inline engine, like the 2JZ, or a rotary, like the 13B. Porsche abandoned sequential turbocharging after the 959, switching to parallel turbos with the 993 Turbo, something it stuck with until bringing the single-turbo flat-six back with the new, hybrid Carrera GTS.
No one has ever tried another twin-turbo gas four-cylinder of any sort since, and I highly doubt anyone will. It seems you only need so much airflow with four-cylinders, even if they are in a boxer arrangement. Even when Porsche made a turbo flat-four itself for the 718, it went single turbo. Today’s WRX is a good example of how far turbocharger technology has come in addressing response—its 258 pound-foot torque peak is a plateau from 2,000 to 5,200 rpm.
Still, Subaru’s sequential twin-turbo setup was a more interesting answer, if not a better one. It’s reflective of a totally different era, where Japanese automakers went crazy pushing all sorts of new technology with little to no regard to cost. What’s better? The rational solution, or the fun one?
The fun one, obviously! Unless you’re the one trying to make it work.
