Subaru is conservative lately. It sticks with applied sciences it is aware of effectively, evolving its automobiles progressively over time. Even when it does one thing new, it’s usually lengthy after the remainder of the trade. However within the Nineties, Subaru was gloriously bizarre and experimental. Witness Subaru’s largely forgotten sequentially twin-turbocharged flat-four.

To the most effective of my information, that is the one twin-turbo gas-powered four-cylinder automotive engine. Some automakers have achieved twin-turbo four-cylinder diesels, and earlier than you soar down my throat about it, BMW’s “Twin-Energy Turbo” engines use a single twin-scroll turbocharger. Not two turbochargers. So, Subaru stands alone right here.

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Subaru launched this twin-turbo boxer with the second-generation Legacy in 1993, when sequential turbocharging was all the trend. Porsche did sequential turbos first with the 959, and shortly, different Japanese automakers embraced it. Within the peak of the asset-price bubble, the Japanese auto trade’s R&D budgets ballooned, ushering in all types of recent applied sciences, like sequential turbocharging. Toyota was first amongst Japanese automakers, with the Supra Turbo’s 2JZ-GTE straight-six, then got here Mazda with the 13B-REW twin-rotor within the RX-7. 

Picture by: Subaru

The concept behind sequential twin turbocharging is straightforward. A small turbo spins up shortly, offering good low-RPM energy and low lag, however runs out of puff at larger engine speeds; a big turbo takes time to spin up, manifesting in lag and the next enhance threshold, however with the advantage of higher high-RPM efficiency. 

Sequential turbocharging makes an attempt to supply the most effective of each worlds, with a smaller turbo optimized for decrease engine speeds and a bigger turbo optimized for top engine speeds. The primary turbo covers every part from idle to someplace round 3,500-4,500 rpm, at which level some exhaust air will get diverted to the second turbo. Ultimately, each turbos are going at full steam, working collectively to supply high-rpm energy.

Subaru did issues a bit in another way. Because the brochure for the 1993 Legacy notes, Subaru used two turbos of the identical dimension to perform the identical fundamental objective as different sequential turbo programs. To pre-spool the second turbo, the engine administration system closes the primary turbo’s wastegate, later diverting extra exhaust gasoline to the second turbocharger. Then, as engine speeds climb, the system opens up a switching valve to the second turbocharger, now permitting each turbos to function collectively.

Picture by: Subaru

An awesome publish on the Subaru Legacy Worldwide discussion board particulars the EJ20 twin-turbo variants additional. The turbos in all variations weren’t an identical, although they had been all the time about the identical dimension, and apart from one later variant, the second turbocharger didn’t have a wastegate. The preliminary twin-turbo boxer within the second-generation Legacy GT made 246 horsepower, and from 1996 on, Subaru supplied the engine with two outputs, 256 hp and 276 hp. 

As with all different twin sequential turbo programs, Subaru’s was easy in idea, wildly difficult in execution. Beneath the hood is a multitude of vacuum strains, plumbing, and solenoids to make the entire thing work. And controlling all these are some computer systems from the Nineties. It’s a variety of complexity and a variety of issues that may go mistaken for what turned out to be questionable advantages.

It appears Subaru needed to handle the comparatively meager low-end torque of its single-turbo flat-fours, most notably used within the Impreza WRX, for the bigger, heavier Legacy. And the place in Japan, automobile homeowners are taxed based mostly on engine displacement, additional boosting a 2.0-liter relatively than simply making an even bigger engine was advantageous.

However as a 2001 assessment of the Liberty (Australian-market Legacy) B4 from Australian journal AutoSpeed notes, there’s a noticeable torque dip when the second turbo begins spooling up. It recorded as much as 4.4 psi of boost-pressure dip, between 4,000 and 4,500 rpm, which is big. Loads of fans name this zone the “Valley of Loss of life.” And this was the best-developed model of that engine, the one which our good friend from the boards thought-about essentially the most dependable and the one which greatest delivered on the sequential-turbo promise.

Picture by: Subaru

These issues weren’t distinctive to Subaru. Sequential turbocharging all the time results in uneven energy supply, and sometimes, unreliability. It’s why when tuners get their fingers on a 2JZ-GTE or a 13B-REW, they nearly all the time delete the twin-turbo setup for an easier, single turbo. 

Nonetheless, Subaru’s system was well-liked. A 2003 Automobile and Driver article famous that, on the time, the Legacy B4—the final Legacy to get the twin-turbo boxer—was Japan’s hottest sports activities sedan. However, that very same article famous {that a} lack of low-end torque was a “weak spot” of the B4, and why when Subaru determined to carry it to the US for 2004, it went with a 2.5-liter single-turbo flat-four. The system was by no means designed to work with left-hand drive automobiles, so it remained principally a Japan-only affair, with some gross 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. On the similar time, it switched to a single twin-scroll turbocharger with its EJ20 flat-four, which helped deal with the standard considerations round turbocharged engines while not having all types of difficult vacuum and hydraulic programs to make it work. 

Picture by: Subaru

I can’t assist however marvel why Subaru didn’t strive a extra typical parallel turbo setup, with one turbo for both sides. Sure, on the time there was a form of sequential-turbo fever, however that format makes extra sense for an inline engine, just like the 2JZ, or a rotary, just like the 13B. Porsche deserted sequential turbocharging after the 959, switching to parallel turbos with the 993 Turbo, one thing it caught with till bringing the single-turbo flat-six again with the brand new, hybrid Carrera GTS. 

Nobody has ever tried one other twin-turbo gasoline four-cylinder of any type since, and I extremely doubt anybody will. It appears you solely want a lot airflow with four-cylinders, even when they’re in a boxer association. Even when Porsche made a turbo flat-four itself for the 718, it went single turbo. At present’s WRX is an effective instance of how far turbocharger expertise has are available addressing response—its 258 pound-foot torque peak is a plateau from 2,000 to five,200 rpm.

Nonetheless, Subaru’s sequential twin-turbo setup was a extra attention-grabbing reply, if not a greater one. It’s reflective of a completely completely different period, the place Japanese automakers went loopy pushing all types of recent expertise with little to no regard to value. What’s higher? The rational answer, or the enjoyable one? 

The enjoyable one, clearly! Until you’re the one attempting to make it work.