The Precision 5857 turbocharger has earned a dedicated following among Mazda Miata enthusiasts who want serious power without sacrificing daily drivability. This compact, ball-bearing turbo is often chosen for its fast spool, high efficiency, and ability to deliver impressive horsepower gains on both stock and built engines. But what exact numbers can you expect? And how do real-world builds compare? In this article, we break down the specifications, installation requirements, tuning considerations, and actual dyno-proven horsepower results from the Precision 5857 on the Miata. Whether you are running a 1.6L, 1.8L, or even a built BP engine, you’ll get the data you need to decide if this turbo is right for your build.

Why the Precision 5857 for the Miata?

The Precision 5857 is a 57mm compressor, 64mm turbine unit designed for engines between 1.6L and 2.5L. Its compact frame and journal bearing (ball bearing available in Pro-Stock version) make it ideal for tight engine bays like the Miata’s. The turbo’s A/R choices (.60 compressor, .63 turbine) match well with the engine’s flow demands, allowing boost to come on strong as early as 3000–3200 RPM while still pulling hard to redline. Owners report that the 5857 feels responsive on the street yet supports 300+ wheel horsepower when paired with proper fuel and engine management. This balance is why many Miata builders choose it over larger frames that lag or smaller turbos that run out of breath.

Specifications and Design Features

Compressor and Turbine Details

  • Compressor wheel: 57mm inducer, 76mm exducer – cast aluminum, extended tip technology for improved flow
  • Turbine wheel: 64mm inducer, 56mm exducer – Inconel 713C for high-temperature durability
  • Compressor housing: .60 A/R, 3-inch inlet, 2-inch outlet standard (optional 4-inch inlet available)
  • Turbine housing: .63 A/R, T25 flange, 5-bolt outlet – suitable for both 1.6L and 1.8L Miatas with adapter plates or custom manifolds
  • Bearing system: Journal bearing standard; ball-bearing upgrade available in Precision Pro-Stroke versions
  • Maximum boost: 30 PSI (but safe limits on stock internals are lower)

The 57mm compressor is known for its wide efficiency island, meaning it delivers good airflow across a range of boost pressures and RPMs. This characteristic helps maintain drivability while supporting significant power gains.

Real-World Horsepower Gains: Dyno Results from Miata Owners

Stock Internal 1.8L (BP4W or BP-Z3) – Low Boost Setup

Many enthusiasts start with a stock engine and a conservative tune. On a 1.8L Miata with stock fuel injectors (upgraded pump), a basic intercooler, and a tune using a Megasquirt or Adaptronic ECU, the Precision 5857 at 8–10 PSI typically yields:

  • Wheel horsepower: 190–210 HP
  • Torque: 200–220 lb-ft
  • Boost threshold: ~3200 RPM

Example build: A 1999 NB1 with 130k miles, stock internals, 550cc injectors, Walbro 255 pump, CX Racing intercooler, and a Flyin’ Miata turbo manifold produced 202 WHP and 218 lb-ft at 9 PSI on 93 octane.

Built Bottom End – High Boost / E85 Tune

For owners who install forged pistons, rods, and stronger head studs, the Precision 5857 can be pushed beyond 20 PSI. With E85 fuel, proper engine management, and supporting mods, numbers climb:

  • Wheel horsepower: 280–340 HP
  • Torque: 280–320 lb-ft
  • Boost level: 18–25 PSI

Example build: A 2001 NB2 with built 1.8L (Wiseco pistons, Manley rods), Supertech valves, FIC 1000cc injectors, Aeromotive fuel pump, Garrett intercooler, and a Haltech Elite 1500 tuned on E85 produced 327 WHP and 298 lb-ft at 22 PSI. The car retained full boost by 3700 RPM and pulled hard to 7500 RPM.

1.6L Miata – Typical Results

The 1.6L Miatas (1990–1993) have smaller displacement and lower flow cylinder heads. However, with proper tuning and support, the 5857 still makes excellent mid-range power:

  • Stock 1.6L at 10 PSI: 170–185 WHP
  • Built 1.6L at 18 PSI: 250–275 WHP

The 1.6L’s lower redline (7200 RPM) means the turbo’s top-end flow is less critical, but the 57mm compressor spools quickly enough to deliver punchy torque from 3000 RPM.

Factors That Influence Horsepower Output

Engine Condition and Compression Ratio

Higher compression ratio engines (9.5:1 vs. 8.8:1) will make more power at the same boost level, but require careful tuning to avoid knock. A fresh engine with good ring seal and no vacuum leaks will always outperform a worn one.

Fuel Quality and Fuel System

Pump gas (91–93 octane) limits boost to about 12–15 PSI before detonation becomes a risk. E85 allows significantly higher boost (18–25 PSI) and reduces exhaust gas temperatures, enabling the 5857 to deliver 300+ WHP reliably. Upgraded fuel pumps (Walbro 255 or 450) and injectors (550cc minimum for 200 WHP; 1000cc+ for 300 WHP) are mandatory.

Tuning and Engine Management

A stand-alone ECU (e.g., Megasquirt, Haltech, Adaptronic, Link) is strongly recommended. Even a piggyback system like a Powercard may work at low boost, but fine-tuning the fuel and ignition maps unlocks the true potential of the 5857. Professional dyno tuning is worth every dollar – a poor tune can leave 20–40 HP on the table and risk engine damage.

Intercooler and Intake Temperature

Reducing intake air temperature (IAT) is critical. An intercooler that flows well and has efficient heat rejection can drop IATs by 50–70°F, allowing more aggressive timing and higher boost. Many 5857 builds use a CX Racing or Flyin’ Miata intercooler with 2.5-inch piping. On warmer days, a water-methanol injection system can further stabilize IATs and add a safety margin.

Exhaust System Backpressure

The turbine housing .63 A/R requires a free-flowing exhaust to avoid choking the turbo. A 3-inch downpipe and exhaust (or at least a 2.5-inch system) significantly reduce backpressure and improve spool and top-end power. Don’t use a restrictive catalytic converter – a high-flow cat (or test pipe) is recommended for builds over 250 WHP.

Supporting Modifications: The Complete Checklist

Installing the Precision 5857 is not just about bolting on the turbo. To achieve reliable, high horsepower, plan for these supporting parts:

  • Manifold: Cast iron or tubular stainless steel with T25 flange – Flyin’ Miata, Kraken, or aftermarket custom
  • Downpipe: 3-inch to 2.5-inch merge (if using stock exhaust path) – ceramic or coated to reduce heat
  • Intercooler: 600+ HP rated bar-and-plate core with 2.5-inch inlet/outlet
  • Bypass valve: A strong recirculating or blow-off valve (Tial Q50 or similar) to prevent compressor surge
  • Fuel system: Walbro 255 lph or Aeromotive 340; injectors sized for target power (e.g., 550cc for 200 WHP, 1000cc for 300 WHP); fuel pressure regulator if returning fuel system
  • Engine management: Standalone ECU with wideband O2 sensor and electronic boost control
  • Cooling: Larger aluminum radiator (e.g., Koyo or Mishimoto) with dual fans to manage increased heat load
  • Clutch: Upgraded clutch (e.g., ACT or Exedy Stage 1 or 2) – stock clutch slips after 200 WHP
  • Oil and coolant lines: -4 AN feed, -10 AN return; water lines to turbo if using water-cooled center section (the 5857 is oil-cooled only; you may add a water-cooled adapter)

Garrett GT2560R (Mitsubishi TD05H 16G)

The Garrett GT2560R is a classic choice for Miatas. It spools slightly faster than the Precision 5857 (boost by 2800–3000 RPM) but maxes out around 240–260 WHP. The 5857 offers a broader power band and can support over 300 WHP with built internals. If your goal is 200–240 WHP and immediate throttle response, the GT2560R may be better. For a build aiming at 280–340 WHP with a few more RPM of top-end pull, the 5857 wins.

BorgWarner EFR 6258

The EFR 6258 is a 58mm compressor, 65mm turbine turbo with integrated wastegate and recirculation valve. It spools similarly to the 5857 but offers dual ceramic ball bearings and a more compact package. However, it requires a T25 or T3 flange and different manifold. The EFR is about 30–50% more expensive, and replacement parts are harder to find. The 5857 is a more cost-effective option with comparable power potential.

Precision 5431 (Smaller Frame)

The 5431 (54mm compressor) is another Precision turbo that spools very early (boost by 2800 RPM) but runs out of steam at around 260 WHP. The 5857 is the upgrade path for those who want more top-end power without sacrificing too much spool. For canyon driving and autocross where low-end torque matters most, the 5431 might be preferred; for track days or drag racing, the 5857 is superior.

Tuning Depth: Timing and Boost Curves

Optimizing the Precision 5857 on a Miata requires a thoughtful tuning strategy. Because the turbo can flow upwards of 35 lb/min at 25 PSI, the engine’s volumetric efficiency plateau becomes a factor. A common tuning approach is to use a “tapered boost” curve – keep boost lower in the mid-RPM to avoid torque spikes that could break the stock drivetrain, then allow boost to climb as RPM rises. On a built engine, you can run 18 PSI by 3500 RPM and slowly increase to 22 PSI at 7000 RPM. This yields a torque curve that feels linear and reduces stress on rods and ring lands. Timing is also critical: start conservative (10–12° at peak torque) and advance toward redline (18–20°) as manifold pressure decreases. Use a knock sensor feedback strategy to protect the engine.

Reliability and Longevity

The Precision 5857 is known for robust construction. The journal bearing version is oil-cooled only, so proper oil supply and drainage are essential. Use a restrictor (0.030–0.050 inch) to limit oil pressure to 30–40 PSI at idle, and ensure the drain line has no kinks and slopes downward to the oil pan. The turbo can last tens of thousands of miles if oil changes are frequent (3,000–5,000 miles) and the engine is tuned safely. Many owners report 30,000+ miles with the same turbo on their Miata before needing a rebuild. The ball bearing Pro-Stroke version adds durability for high-boost continuous use, but it costs about 30% more. On a daily-driven Miata at 250 WHP, journal bearing is sufficient. For track cars or high-boost E85 builds, consider the ball bearing upgrade.

Cost of a Precision 5857 Miata Build

Below is a rough budget for a complete, well-executed build (prices are estimates in USD as of 2025):

  • Precision 5857 turbo (journal bearing): $800–$1,000
  • Turbo manifold (cast T25 flange): $400–$700
  • Downpipe and exhaust (3-inch stainless): $400–$800
  • Intercooler and piping kit: $300–$600
  • Fuel pump (Walbro 255): $100–$150
  • Fuel injectors (1000cc): $200–$400
  • Standalone ECU (Megasquirt or Haltech): $600–$1,200
  • Wideband O2 sensor and gauge: $150–$250
  • Clutch upgrade: $300–$600
  • Radiator and fans: $300–$500
  • Oil and coolant lines/fittings: $150–$300
  • Tuning (dyno time): $400–$800

Total estimate: $3,500–$5,500 for a low-boost (200–240 WHP) build; $5,500–$8,000 for a high-boost (300+ WHP) built motor build. The Precision 5857 itself is competitively priced and offers great value per horsepower compared to turbo kits that cost thousands more.

Final Verdict: Is the Precision 5857 Right for Your Miata?

If you want a turbo that spools quickly, supports a wide range of power levels (200–340 WHP), and is backed by a strong aftermarket support network, the Precision 5857 is an excellent choice. It fits the Miata’s character – responsive low-end for street driving, yet capable of impressive top-end rush when you push it. The key is honest self-assessment: if your goal is under 250 WHP on a mostly stock engine, you might be fine with a smaller turbo and simpler setup. But if you have plans to build the engine, run E85, and chase 300+ WHP, the 5857 is a proven platform. With proper planning, quality parts, and a careful tune, your Miata can transform into a balanced, powerful sports car that surprises many heavier machines on the road or track.