19/01/2021
The Subaru Impreza, renowned for its legendary all-wheel-drive (AWD) system, offers a thrilling driving experience. However, for those seeking to push the boundaries of performance, upgrading the differential can be a game-changer. An aftermarket Subaru Impreza differential is not merely a replacement part; it's a sophisticated piece of engineering designed to optimise power delivery to the wheels, ensuring you have the right amount of grip when you need it most. Whether you're navigating twisty B-roads, tackling track days, or simply aiming for a more engaging drive, understanding the role and benefits of an aftermarket differential is crucial.
The Science of Traction: Why Differentials Matter
At its core, a differential is a mechanical device that allows the drive wheels on the same axle to rotate at different speeds. This is essential because, when cornering, the outer wheel travels a greater distance than the inner wheel. Without a differential, this difference in speed would cause tyre scrub, strain on the drivetrain, and unpredictable handling. Subaru's symmetrical AWD system, with its centre differential, is already a sophisticated setup. However, the factory differentials, particularly in standard road-going models, are often designed with a bias towards durability and quiet operation over outright performance. Aftermarket differentials aim to rectify this by providing more control over how power is distributed between the wheels.
Types of Aftermarket Differentials for the Subaru Impreza
When you venture into the world of aftermarket differentials, you'll encounter several key types, each offering distinct advantages:
1. Limited-Slip Differentials (LSDs)
LSDs are perhaps the most common and popular upgrade for performance-oriented Impreza owners. Their primary function is to limit the speed difference between the wheels on a given axle. This means that if one wheel starts to lose traction (e.g., on a slippery surface or during hard acceleration), the LSD will transfer some of the torque to the wheel that still has grip. This significantly improves acceleration and handling, especially out of corners.
There are several sub-types of LSDs:
- Clutch-Type LSDs: These use a series of clutch plates that are compressed by springs or hydraulic pressure. The more the clutches are engaged, the more torque is transferred to the wheel with more grip. They offer a highly tunable performance but can sometimes be a bit grabby or chatter during low-speed manoeuvres.
- Viscous-Coupled LSDs: These utilise a silicone fluid that thickens when subjected to shear stress (i.e., when wheels spin at different speeds). While simpler and smoother than clutch-types, they can overheat under sustained aggressive use and may not offer the same level of lock-up.
- Torsen (Torque-Sensing) Differentials: These are gear-based differentials that automatically bias torque towards the wheel with more traction. They are smooth in operation and require no special additives but are generally more expensive and can be less aggressive in their lock-up compared to some clutch-types.
2. Spool Differentials
A spool differential, also known as a locked differential, permanently locks the two axle shafts together. This means both wheels on that axle *always* rotate at the same speed. While this provides maximum traction in off-road or drag racing scenarios where both wheels are expected to maintain grip, it's generally unsuitable for a road car like the Impreza. On a tarmac surface, a spool will cause severe tyre scrub, drivetrain stress, and extremely poor handling during cornering, making it impractical for most Impreza applications.
3. Helical Gear Differentials
Similar in principle to Torsen differentials, helical gear differentials use a series of interlocking gears to transfer torque. They are known for their smooth operation and durability. Brands like Quaife offer popular helical LSDs that provide excellent traction bias without the drawbacks of clutch-type units.
Benefits of an Aftermarket Impreza Differential
Upgrading your Subaru Impreza's differential can yield significant improvements:
- Enhanced Traction: This is the primary benefit. By ensuring power is sent to the wheels with grip, you'll experience vastly improved acceleration and stability, especially when exiting corners or driving in adverse weather conditions.
- Improved Handling Dynamics: A well-chosen LSD can make your Impreza feel more planted and predictable. It helps to reduce understeer and oversteer, allowing for a more confident and engaging driving experience.
- Reduced Drivetrain Stress: While it might seem counterintuitive, a performance differential, when properly implemented, can actually reduce stress on the drivetrain compared to an open differential that allows excessive wheel spin.
- Cornering Speed: The ability to put power down effectively through a corner means you can carry more speed, leading to faster lap times on a track or more enjoyable driving on winding roads.
- Customisation: Aftermarket options allow you to tailor the differential's behaviour to your specific driving style and intended use, whether it's for spirited road driving or competitive motorsport.
Considering a Differential Upgrade: Key Factors
Before you decide to upgrade, consider these important points:
Front vs. Rear vs. Centre Differential
Subarus have differentials in the front, rear, and a centre differential that manages power distribution between the front and rear axles. Upgrades are available for all three locations. The most common and impactful upgrades for road performance are often to the front and rear differentials, particularly the rear.
Which Differential to Choose?
Your choice will depend on your budget, driving style, and intended use:
| Differential Type | Pros | Cons | Best For |
|---|---|---|---|
| Clutch-Type LSD | Highly tunable, aggressive lock-up, excellent performance | Can be grabby/chattery, requires specific fluid, potential wear | Track days, spirited driving, rally |
| Torsen/Helical LSD | Smooth operation, durable, no special fluids, progressive lock-up | Less aggressive than some clutch-types, can be more expensive | Daily driving, fast road, touring |
| Viscous LSD | Smooth, simple, cost-effective | Can overheat, less effective lock-up than others, limited lifespan | Mild performance enhancement, budget builds |
| Spool | Maximum traction (in specific conditions) | Unsuitable for road use, severe tyre wear, drivetrain stress | Drag racing, off-roading (not recommended for Impreza road use) |
Installation and Maintenance
Differential installation is a complex mechanical task that requires specialised knowledge and tools. It is highly recommended to have the work carried out by a reputable workshop experienced with Subaru vehicles. Furthermore, depending on the type of differential chosen, you may need to use specific gear oils and adhere to a stricter maintenance schedule. Changing gear oil is a vital part of this maintenance. While the archived topic doesn't provide details, generally, you'll need to locate the fill and drain plugs on the differential housing, drain the old oil using a suitable pump, and refill with the correct grade and quantity of new gear oil. Always consult your workshop or the differential manufacturer's recommendations.
Frequently Asked Questions
Q1: Will an aftermarket differential affect my car's fuel economy?
While a more aggressive differential might slightly increase fuel consumption due to increased mechanical drag, the effect is typically minimal for most road users. Performance gains usually outweigh this minor drawback.
Q2: Can I fit an LSD from a different Impreza model to mine?
Gear ratios and physical fitment can vary significantly between Impreza models and generations. It's crucial to ensure compatibility or opt for aftermarket units specifically designed for your model year and variant.
Q3: Do I need to change my gearbox oil when upgrading the differential?
No, the gearbox oil and differential oil are separate. However, it's always good practice to ensure all drivetrain fluids are in good condition.
Q4: How much does an aftermarket differential cost?
Costs vary widely depending on the type, brand, and location of the differential. Expect to pay anywhere from £300 for a basic LSD to over £1000 for high-performance units, plus installation costs.
Q5: Is a front or rear differential upgrade more beneficial for an Impreza?
For most road-focused Imprezas, upgrading the rear differential often provides the most noticeable improvement in handling and traction out of corners, complementing the factory's front bias.
Conclusion
An aftermarket Subaru Impreza differential is a potent modification that can transform your car's performance. By understanding the different types available and their respective benefits, you can make an informed decision that enhances your driving pleasure and capability. Whether you choose a smooth Torsen unit for daily driving or a more aggressive clutch-type LSD for track use, the result is a more connected and dynamic driving experience, truly unlocking the potential of Subaru's famed AWD system. Always prioritise quality components and expert installation to ensure you maximise the benefits and longevity of your upgrade.
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