How a twin-turbo boxer engine redefined mid-sized performance
In short: In 1999, the Subaru Legacy B4 arrived to challenge luxury sedans. By pairing a specialized twin-turbocharged boxer engine with standard all-wheel drive, it offered a unique blend of high-performance torque and tax-efficient engineering that targeted both enthusiasts and pragmatic Japanese buyers.
The third-generation Legacy B4 utilized a sophisticated 'phase 2' all-alloy, 2.0-liter DOHC boxer engine. Unlike traditional engines, the horizontally opposed cylinders allow for a lower center of gravity and reduced vibration, as the crankshaft requires less weighting and eliminates the need for balance shafts. In the RSK trim, a sequentially staged twin-turbocharger system was employed to smooth out the power delivery. A primary turbo handles low-end boost, while a secondary turbo engages between 4000 and 4500 rpm to provide mid-to-high range breathing, aiming to eliminate the noticeable power dips seen in previous models.
Performance varied significantly by market. The Japanese-market RSK model, running on 100-octane fuel, delivered 276 bhp (206 kW) and could accelerate from 0–60 mph in 5.2 seconds. In contrast, Australian B4 models were detuned to run on 98 RON fuel, producing 258 PS (254 hp) for manuals and 239 PS (236 hp) for automatics, with 0–60 mph times of 5.6 and 6.6 seconds respectively. This performance was particularly attractive in Japan, where the engine's size allowed drivers to bypass certain tax liabilities associated with larger-displacement engines.
To handle this power, Subaru implemented significant chassis and braking upgrades. The B4 featured a multi-link rear Chapman strut arrangement and a 'performance rod' that increased side-axis stiffness by 500 percent. Braking was managed by a Bosch 5.3i four-channel ABS system using a 'select low' method for the rear wheels, which calculates pressure based on the wheel with the lower coefficient of adhesion. This combination of engineering—from the low-slung boxer engine to the high-stiffness suspension—was designed with the ambitious goal of matching the performance of the BMW M3.
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- Summary by Intellect · source: Subaru Legacy (third generation)
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