Parallel Twin Turbo Mode

Running the Subaru Legacy B4 Twin Turbo in Parallel Mode

Like some other JDM twin-turbo cars of the 90's, the Legacy B4 twin-turbo configuration operates in sequential mode, where the first turbo kicks in at lower RPMs, followed by the second turbo at higher revs to provide a smooth power delivery. However, a less common but fascinating modification enthusiasts explore is running the twin-turbo system in parallel mode.

In this article, we'll dive into what it means to run the Legacy B4 Twin Turbo EJ206 and EJ208 engines in parallel mode and what to expect from this configuration.

Sequential Turbos Explained

Before we explore parallel mode, let's break down how the stock twin-turbo setup works on the Legacy B4. In the sequential system, the engine utilizes two turbos, but they don't always operate simultaneously:

  • Primary Turbo: This turbo has a smaller hot-side and works at low to mid-range RPMs, providing quicker spool times and minimizing lag. It's ideal for daily driving, where low-end torque is necessary.

  • Secondary Turbo: The secondary, larger turbo kicks in at higher RPMs, supplementing the primary turbo with more boost to provide power at the top end of the rev range. This gives the driver a progressive boost and a smooth acceleration curve.

This system is intended to balance drivability with performance, especially in situations where daily use and fuel efficiency are considerations.

What Is Parallel Turbo Mode?

In contrast, a parallel turbo setup operates both turbos simultaneously across the entire rev range. In this mode, the two turbos share the load from the moment you step on the accelerator, eliminating the transition period or “Valley of Death” that occurs between the two turbos in a sequential system. However, switching to parallel mode comes with its own set of challenges and benefits.

How to Enable Parallel Mode

Previously, converting the Legacy B4 to parallel mode required mechanical modifications to the exhaust manifold and secondary turbo plumbing. However, with our ECU tuning software, this transformation can be performed without any physical alterations to the turbo or exhaust system.

The tune will need the following modifications to enable full-time parallel mode:

  1. Secondary Spool Settings: Set the following Boost Control maps to 0.0:

    • Secondary Turbo – Pressure Relief Valve Load
    • Secondary Turbo – Exhaust Control Valve Load
    • Secondary Turbo – Enable Delay, Minimum
    • Secondary Turbo – Enable Delay, Maximum
    • Secondary Turbo – Disable Load
    • Secondary Turbo – Spool Enable, Engine Speed
    • Secondary Turbo – Spool Enable, Manifold Pressure
    • Secondary Turbo – Spool Enable, Throttle

  2. Wastegate Duty Adjustments: For most cases, the secondary wastegate duty can be left alone.

  3. Fuel and Timing Adjustments: Since both turbos will now be generating boost simultaneously below 4000 rpm, fuel and ignition timing can be slightly more aggressive to take advantage of the lower intake and exhaust restriction.

Benefits

Switching the Legacy B4's twin-turbo setup to parallel mode offers several potential performance benefits:

  • Consistent Power Delivery: Parallel mode eliminates the "VOD" (Valley of Death) — a notorious lag period during the transition from the primary to the secondary turbo at 4000 rpm. Drivers experience smoother and more predictable power.
  • Fuel Consumption: The laggier nature of the parallel turbo setup can lead to decreased fuel consumption, especially when below 4000 rpm when normally only the primary turbo would be operating in sequential mode.
  • Primary Turbo Longevity: Since the load is shared in parallel mode, the primary turbo will see lower operating speeds during normal driving. This reduces heat and wear on the turbo internals.

Drawbacks

While parallel turbo mode has its perks, it also comes with one major drawback that drivers should consider:

  • Increased Turbo Lag: Running the turbos in parallel will lead to much greater turbo lag at lower RPMs compared to a well-tuned sequential system. This is because the smaller primary turbo in the stock configuration spools up quicker on its own, while in parallel mode, the both turbos together will take longer to come online.

Real-World Performance Results

Road Test Boost Comparison

Real-world testing of the Legacy B4 in parallel turbo mode has revealed mixed results across the RPM range. At lower RPMs, performance is notably reduced compared to the stock sequential setup. In sequential mode, the smaller primary turbo spools up rapidly, providing strong low-end torque and quicker acceleration. However, in parallel mode, the load is shared between both turbos, and the larger secondary turbo’s slower spool time results in diminished boost response at lower engine speeds. This leads to slower acceleration and less punch in the low-to-mid RPM range, where the primary turbo in sequential mode excels.

At higher RPMs, however, the difference in performance between parallel and sequential modes is minimal. This is because at higher engine speeds in sequential mode both turbos are already operating together, effectively running in parallel at that point. As a result, boost response and performance in parallel mode at higher RPMs is roughly equivalent to that of the stock configuration, offering no real gain in this range.

Therefore, while parallel mode might provide a different feel, it does not offer significant performance advantages over the stock setup, especially in the low-end, where sequential mode shines. This makes the sequential configuration more efficient and practical for most drivers, particularly for everyday driving or situations where low-end torque is needed.

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