Understanding the Precision Turbo SC6064 Turbocharger

The Precision Turbo SC6064 is a compact, high-efficiency turbocharger designed for compound setups where a smaller, quick-spooling unit pairs with a larger primary turbo. Its 64mm compressor wheel and 60mm turbine wheel are engineered for rapid response and excellent mid-range power. In a compound arrangement, the SC6064 acts as the secondary (high-pressure) turbo, compressing air that has already been boosted by the primary unit. This allows overall boost levels well beyond what a single turbo could achieve without sacrificing drivability. The SC6064’s journal bearing or optional ball-bearing cartridge ensures durability under sustained high-load operation.

It is common to see these turbos used on high-horsepower diesel and gasoline engines where air density and flow are critical for power targets above 800 horsepower.

Key specifications for the SC6064:

  • Compressor inducer: 64mm with a billet wheel design for efficient flow up to about 80 lb/min
  • Turbine exducer: 60mm with a divided housing option for improved pulse separation
  • Housing options: T4 or T6 turbine inlets, 1.00 A/R to 1.25 A/R for varied backpressure and spool characteristics
  • Maximum boost pressure: While originally rated around 30 psi, in a compound setup the SC6064 can safely produce 40-50 psi when paired with proper fuel and engine internals
  • Oil feed and drain: Requires 1/8" NPT feed and -10 AN or larger drain to prevent oil buildup at high RPM

Understanding these specifications helps you choose the right supporting components and tune correctly. The SC6064 is most effective when the primary turbo provides a pressure ratio of 2.5-3.0 before the secondary takes over. This division of labor prevents either turbo from operating outside its efficiency island.

Part 1: Selecting the Primary Turbocharger for a Compound Setup

The success of any compound system hinges on matching the primary (low-pressure) turbocharger to the SC6064. The primary must supply enough flow and pressure to the secondary while maintaining reasonable drive pressure. A poor match leads to either excessive backpressure (killing power) or lack of airflow (starving the secondary).

Ideal primary turbo characteristics for an SC6064 compound setup:

  • Compressor flow: 100-150 lb/min to feed the SC6064 at high RPM
  • A/R ratio: 0.90-1.10 for the turbine, depending on the exhaust volume and desired spool
  • Turbine size: Often a 80-88mm inducer turbine wheel to handle the combined exhaust from a large engine
  • Housing: T6 or larger divided housing to reduce backpressure and improve transient response

Popular primary turbos for this application include the Precision Turbo 6870, 7288, or a Garrett G42-1450. The primary turbo should be sized such that it reaches its efficiency zone by 4000-4500 RPM on a typical engine. If the primary spools too late, the SC6064 will have to work alone, causing excessive heat and slow response. Conversely, if the primary is too small, it will choke at high RPM and limit maximum power.

For a gasoline engine, aim for a primary turbo that produces about 15-20 psi of boost to the intake of the SC6064 by redline. On a diesel, the primary may need to provide 25-35 psi to the secondary. These numbers depend on your target total boost (e.g., 70 psi total on diesel). Use a boost pressure ratio map to calculate the combined pressure ratio across both turbos. The compound formula is: total pressure ratio = (primary pressure ratio) × (secondary pressure ratio).

For example, if the primary produces a ratio of 2.5 (about 22 psi gauge at sea level) and the secondary produces a ratio of 2.0, total ratio = 5.0 (about 58 psi gauge).

Part 2: Setting Up the Compound System – Piping, Mounting, and Intercooling

Turbo Placement and Piping

Physical layout matters for flow efficiency and heat management. The primary turbo is usually mounted close to the exhaust manifold, and the SC6064 is positioned downstream in the charge air path. It is common to see the primary turbo’s compressor outlet plumbed directly into the SC6064’s compressor inlet via a large-diameter pipe (3-4 inches). Avoid sharp bends and reduce transitions to minimize pressure drop.

The exhaust system must also be carefully routed. The primary turbine sees the exhaust from the engine, then its outlet is plumbed to the secondary turbine inlet. This secondary inlet should be as large as possible (typically 4-inch diameter). Use a flex joint and V-band clamps to simplify assembly and disassembly. Pay attention to heat shielding – wrap or coat hot side pipes to reduce underhood temperatures and improve spool by retaining exhaust gas energy.

Intercooler Sizing and Placement

Compound setups generate intense charge air temperatures. Even with efficient turbos, compressed air can exceed 250°F at high boost. An intercooler between the secondary turbo and the engine is mandatory. For performance up to 1500 horsepower, an intercooler core of at least 4" thick, 12" tall, and 24-30" wide with 3" inlet/outlet is typical. Bar-and-plate cores offer better heat rejection than tube-and-fin.

Consider a water-to-air intercooler if packaging is tight – it provides consistent intake temperatures and reduces lag.

For severe duty (e.g., drag racing or heavy towing), a secondary intercooler can be placed between the primary and secondary turbos. This lowers the temperature of the air entering the SC6064, allowing it to compress more dense air and reducing the secondary turbo’s outlet temperature. This "intercooled compound" configuration can gain 5-10% more power.

Oil and Coolant Lines

Both turbos require reliable oil supply and drainage. Use a dedicated oil feed line from the engine’s main oil gallery, ideally with a restrictor (0.035" – 0.045" orifice) to prevent over-pressurizing the turbos. The oil drain lines must angle downward with no dips or restrictions – a -10 or -12 AN drain line is common. If the turbos are mounted higher than the oil pan, consider a scavenge pump to return oil. Failure to drain oil properly leads to seal failure and smoke.

For water-cooled SC6064 (optional feature), run coolant lines from the engine’s cooling system to help reduce oil coking after shutdown. Use 5/8" heater hose and ensure proper flow direction.

Part 3: Tuning the Engine Management System for a Compound Setup

Tuning a compound turbo engine requires a standalone ECU or a flash-tunable factory ECU with custom logic. The fundamental challenge is that airflow changes dynamically between the two turbos, affecting fuel and ignition requirements across the RPM band.

Fuel Delivery and Air-Fuel Ratio

The increased air density demands proportional fuel. With total boost exceeding 50 psi, gasoline engines require high-octane fuel (E85, race gas, or methanol) to resist knock. Diesel engines need sufficient injection duration and high-pressure common rail programs. Set your target air-fuel ratio (AFR) based on fuel:

  • Gasoline (pump 93): 11.5-12.0:1 under boost, with rich tip-in to avoid lean spikes
  • E85: 7.5-8.5:1, but can tolerate leaner at low load for economy
  • Methanol: 4.5-5.5:1
  • Diesel: target 1.0-1.3 lambda (≈14.7-19.1:1 AFR), but smoke limited by injection timing

When tuning, start with rich mixtures and gradually lean out while monitoring exhaust gas temperature (EGT) and knock. EGT before the primary turbine should not exceed 1650°F on gasoline or 1300°F on diesel under sustained load. Use a wideband sensor on each bank if possible.

Ignition Timing Strategy

High boost and high cylinder pressure require retarded ignition timing compared to naturally aspirated. A typical starting point for gasoline at 30 psi is 12-15 degrees BTDC at peak torque, advancing to 20-22 degrees at redline if knock allows. With compound boost over 50 psi, timing may need to be as low as 8-10 degrees BTDC. Use a knock detection system and data log to dial in safely. For diesel, injection timing affects cylinder pressure and EGT – advance timing slightly for power, retard for lower EGT and less stress on head gaskets.

Data Logging and Sensor Requirements

You cannot tune a compound system blind. Essential sensors:

  • Manifold absolute pressure (MAP) – both post-primary and post-secondary
  • Intake air temperature (IAT) – post-intercooler and post-secondary to measure intercooler efficiency
  • Exhaust gas temperature (EGT) – pre-primary turbine and pre-secondary (if using a second probe)
  • Boost pressure ratio – to compute turbo efficiency and surge margin
  • Fuel pressure – especially important when using high-flow injectors and low-pressure systems
  • Oxygen sensor – wideband lambda

Log all parameters at 10 Hz or faster during pulls. Watch for sudden EGT rises, knock events, or boost oscillations. A compound system can exhibit "boost lag" during gear changes if the turbos are poorly matched – data logging reveals the transition zone.

Part 4: Boost Control Strategies for a Compound Turbo Setup

Controlling boost with two turbos requires a coordinated approach. The wastegate on the primary turbo controls the overall boost level, while the secondary turbo’s wastegate (if present) or a blow-off valve handles over-speed protection. Most compound systems run the primary wastegate as the primary boost controller and leave the secondary wastegate closed or set to open at a much higher pressure (e.g., 10-15 psi above target). This prevents the secondary from overspeeding if the primary fails.

Electronic boost control is highly recommended. A single solenoid can control both wastegates using a smart controller that learns the system dynamics. For example:

  • Set primary wastegate duty cycle to maintain target total boost (e.g., 50 psi)
  • Secondary wastegate set to crack open at 45 psi to protect the secondary turbo from surge if the primary falls out of its map
  • Use a boost-ramp function to gradually increase boost with RPM to prevent sudden torque spikes that can break drivetrain components

Another method is to use a “gate” style boost controller that references boost from the secondary compressor outlet. This ensures that as the secondary spools, the primary wastegate duty is adjusted to keep the secondary from overspeeding. Some tuners prefer to run the primary open (no wastegate) and rely on the secondary wastegate alone – but this is less precise and risks oversping the primary.

Part 5: Cooling System Enhancements for High Heat Load

Compound turbocharging dramatically increases heat rejection requirements. The engine cooling system must handle the additional intercooler heat load, radiated heat from the turbos, and higher cylinder head temperatures. Upgrade your radiator to a high-flow aluminum unit with at least two 1" thick cores. For extreme duty (track days, sled pulling), consider a custom radiator with integrated oil and transmission coolers.

Water/methanol injection is a game-changer for compound setups. It reduces IAT by 100-150°F, suppresses knock, and helps keep exhaust valves cool. Install a progressive controller that starts spraying at 10-15 psi boost and ramps up to full flow near peak boost. Use a mix of 50/50 water and methanol for best results. Ensure the nozzle is placed at least 12 inches before the throttle body to allow atomization.

Also consider upgrading your engine oil cooler. Many high-horsepower builds use a large air-to-oil cooler or a water-to-oil heat exchanger. Oil temperatures should stay below 240°F under load; sustained temps above 260°F degrade oil viscosity and turbo bearing life.

Part 6: Exhaust System Optimization for Compound Turbos

Backpressure is the enemy of compound turbo performance. Each turbo adds restriction to the exhaust path. Use the largest diameter exhaust possible – 4-inch or 5-inch downpipe from the secondary turbine, and 3-4 inches from the primary turbine. Merge the two exhaust streams into a single pipe after the secondary turbine. Avoid mufflers with restrictive baffles; use a straight-through design or a cutout for competition use.

Exhaust gas temperature (EGT) management is critical. High EGT before the primary turbine can overheat the turbo and cause wheel damage. If your primary turbine is too small, EGT will spike. A rule of thumb: primary turbine pressure ratio should stay below 2.5:1 at peak boost. If you see pressure ratios above 3.0:1, consider a larger housing or a different primary turbo.

For vehicles running high boost on diesel, a free-flowing exhaust is essential to keep drive pressure low. Excessive drive pressure can lift heads and blow gaskets. Some tuners install a pressure sensor in the exhaust manifold to monitor drive pressure vs. boost pressure. Ideally, drive pressure should be equal to or lower than boost pressure. If it exceeds boost by 20% or more, the system is choking.

Part 7: Testing, Tuning for Reliability, and Common Pitfalls

After assembly and initial tuning, perform a series of controlled tests starting at low boost (10-15 psi) and gradually increasing. Always have a spotter watch for smoke, coolant loss, or unusual noises. On a dyno, perform a 30-minute low-load heat cycle to check for leaks and stabilize temperatures. Then do moderate dyno pulls (half throttle) before full throttle runs.

Common issues in compound setups and how to fix them:

  • Turbo surge during lift-off: Install a blow-off valve on the charge pipe between the SC6064 and the throttle body. Set the spring tension to open at about 10-15 inHg vacuum.
  • Oil leaks from the secondary turbo: Usually due to excessive crankcase pressure or inadequate drain. Check PCV system and ensure drain line slopes continuously downward.
  • Boost oscillation: This can happen when the wastegate is incorrectly sized or the controller gains are too high. Reduce wastegate duty cycle gain and increase the cycle time. Also ensure the wastegate springs are strong enough to hold the diaphragm closed.
  • High EGT on one cylinder bank: Indicates an air/fuel imbalance. Individual cylinder EGT monitoring is advised for engines over 1000 hp.

Reliability also depends on regular maintenance. After every track day or heavy towing session, inspect the turbos for shaft play and check oil condition. Change engine oil and filter after the first 500 miles of break-in, then every 3000 miles or annually. Use high-quality synthetic oil (5W-40 or 15W-50 depending on climate) to withstand the heat and shear. Clean air filters frequently – dusty conditions can erode compressor wheels.

Part 8: Maintenance Schedule for Longevity

A Precision Turbo SC6064 compound setup is a high-stress system. Following a strict maintenance schedule prevents expensive failures:

  • Every 1,000 miles or after each event: Check all boost pipes for leaks (soapy water test). Confirm wastegate operation by letting the engine idle and manually actuating the arm. Inspect oil drain lines for kinks or leaks.
  • Every 5,000 miles: Replace oil and filter. Flush intercooler and charge air cooler cores for oil residue. Inspect turbo compressor wheels for debris or contact marks. Check intake system for cracks or loose connections.
  • Every 15,000 miles: Remove and inspect the SC6064 and primary turbo for radial and axial play. Replace if wear exceeds 0.003 inches. Replace spark plugs (gasoline) or fuel injectors (diesel) as they wear under high boost.
  • Annually: Replace all coolant and consider a pressure test of the cooling system. Flush intercooler water (if water-to-air). Inspect exhaust system for cracks and corrosion.

Document all maintenance and tuning changes in a log. This helps diagnose issues quickly if performance degrades.

Conclusion

Tuning a Precision Turbo SC6064 compound setup is a rewarding endeavor that demands thorough engineering and attention to detail. By selecting the correct primary turbo, designing the plumbing for minimal restriction, tuning the ECU with robust data logging, and implementing smart boost and cooling strategies, you can achieve power levels and throttle response that a single turbo cannot match. Reliability comes from correct matchmaking, careful calibration, and disciplined maintenance. Whether you are building a street beast, a dedicated drag car, or an all-out diesel sled puller, the SC6064 compound system offers a proven path to four-figure horsepower without sacrificing drivability. Stay methodical, always prioritize safety, and enjoy the relentless power delivery of a properly tuned compound setup.