The Cummins 6.7L inline-six diesel engine has long been a benchmark for reliability and power potential in the heavy-duty pickup world. When you pair this robust platform with a BorgWarner S366 turbocharger, you unlock a proven combination that can reliably push past the 550-horsepower mark. Whether you're building a daily driver, a tow rig, or a dedicated pulling truck, the S366 offers an excellent balance of spool response, top-end flow, and affordability. This guide covers everything you need to know about selecting, installing, and tuning for that power target – along with the supporting modifications required to keep your engine happy at these elevated output levels.

Understanding the BorgWarner S366 Turbocharger

The BorgWarner S366 is a member of the company's popular S-Series family, engineered for mid-to-high horsepower diesel applications. Its 66mm compressor wheel and 73mm turbine wheel place it squarely in the sweet spot for a 6.7L Cummins aiming at 550–650 horsepower. Unlike smaller turbos that choke on top end or larger units that suffer from excessive lag, the S366 delivers a broad power band with responsive spool characteristics – especially when paired with the factory variable-geometry turbo's (VGT) housing or an aftermarket divided T4 manifold.

Key Specs and Design Features

The turbo features a billet compressor wheel option, a 10-blade turbine wheel, and a choice of turbine housings – typically a 1.00 A/R or 0.91 A/R T4 divided. The 66mm inducer and 73mm exducer on the turbine side allow for efficient exhaust gas extraction while maintaining drive pressure levels that the 6.7L's bottom end can handle. For reference, the stock Cummins 6.7L turbo is around 58mm compressor wheel, so the S366 represents a 14% increase in airflow capacity right off the bat. This translates directly to oxygen volume, which your tuning software will use to add more fuel safely.

Unlike the factory VGT, the S366 is a fixed-geometry turbo. This removes the problematic vanes that can carbon up or fail, but it also means you must pay close attention to tuning and wastegate setup to control boost spikes. A quality wastegate (like a Turbosmart or Tial 44mm) is mandatory for reliable power production at 40–50 psi of boost.

Why the S366 for 550+ HP?

The S366 has become a staple in the 5.9L and 6.7L Cummins community because it hits a "Goldilocks" zone. It spools quickly enough for street driving (full boost by 2500–2800 RPM with a well-matched exhaust manifold) yet can flow enough air to support 600+ horsepower with proper fuel and tuning. For a 550 HP goal, the turbo is not at its absolute limit, leaving some overhead for safety and future upgrades. Compare this to a precision 62/68, which may struggle on the top end, or a S369, which requires more exhaust energy and may lag noticeably on a stock cammed 6.7L. The S366 is the proven performer that numerous shops use in 550–600 HP packages.

Installation Tips for the S366 on a 6.7L Cummins

Installing a BorgWarner S366 on a 6.7L Cummins requires planning. The 6.7L's engine bay is tight, especially with the factory intake horn and the bulky intercooler piping. Below are critical considerations for a clean, reliable installation that sets you up for successful tuning.

Exhaust Manifold and Up-pipes

You cannot simply bolt an S366 to the factory exhaust manifold on a 6.7L. The factory VGT is integrated into a unique manifold. You need an aftermarket T4 divided manifold – options include products from BD Diesel, Fleece Performance, Industrial Injection, or high-quality Chinese copies that have been port-matched. Choose one with a standard T4 flange. If you are replacing the manifold, consider upgrading to 304 stainless steel for durability and heat resistance. Also, replace the up-pipe to the turbine with a 2.5" or 3" diameter for minimal restriction.

Oil and Coolant Lines

The S366 uses -4 AN oil feed and -10 AN oil drain lines. On a 6.7L, you can often reuse the factory oil feed location but may need an adapter fitting to go from 1/8" NPT to -4 AN. The drain line is critical – it must be routed downhill with no low spots to prevent oil backup into the turbo. Use a -10 AN push-on line with a 45° or 90° fitting at the turbo. For coolant, the S366 does not require coolant lines (unlike the factory water-cooled VGT), but if your kit includes coolant circulation (some aftermarket housings have provisions), you can integrate into the heater hose circuit.

Intercooler and Charge Piping

The additional flow from the S366 will overwhelm the stock intercooler – at 550 HP, intake air temperatures can skyrocket, causing EGT issues and power loss. Upgrade to a 4" thick intercooler (e.g., Mishimoto or Spearco), at least 24" wide to fit behind the grille. Charge piping should be increased from the factory 2.5" to 3" or 3.5" aluminum tubing with bead-lock ends to prevent blow-offs. Use silicone couplers and T-bolt clamps rated for 60+ psi.

Boost Reference and Wastegate Setup

Because the S366 is fixed geometry, you must install a wastegate. The wastegate should be mounted on the exhaust manifold downpipe before the turbo inlet. The boost reference line should come from a port in the charge pipe after the intercooler (not the compressor housing) to ensure accurate boost control. Set wastegate spring pressure according to your tuning strategy – typically 18–20 psi for a starting point, then controlled electronically or via an aftermarket boost controller later.

Other Mechanical Considerations

Don't forget to upgrade the exhaust system – a 4" turbo-back (or 5" if you plan for more power later) reduces backpressure significantly. Also, check valve clearances and consider a valve spring upgrade if your tuning will push over 45 psi. The factory rocker arms are generally fine at this power level, but ARP head studs (625+ material) are mandatory to clamp the cylinder head properly.

Tuning for Maximum Performance with the S366

Tuning is where the rubber meets the road. No matter how well you install the turbo, without a refined tune you will have high EGTs, smoke, or a lack of power. The factory ECM on the 6.7L Cummins can be tuned via flash tools like EFI Live, S&S Performance's custom calibration, or standalone systems like Haltech or Motec. For most builds, EFI Live is the standard.

Fuel Maps and Air-Fuel Ratio

With the S366 flowing more air, you must command more fuel delivery – but only to the point where your air/fuel ratio stays rich enough to avoid cylinder damage. Target an air/fuel ratio around 17:1 under full load (measured on a wideband O2 sensor installed in the downpipe). Use your tuning software to increase the fueling command tables (often called "duration" and "desired fuel mass") in both the high boost and transient areas. Start conservative – add 15–20% more fuel than stock, then monitor EGTs and boost. Work your way up.

A good rule of thumb: for 550 HP on a 6.7L, you'll need roughly 200–220 cubic millimeters of fuel per injection cycle (depending on injector size). Factory injectors are around 100 mm³. That means you almost certainly need aftermarket injectors – 75% over stock or full flow 200% over injectors from Exergy or Industrial Injection. The tune must be recalibrated for the larger flow rate.

Exhaust Gas Temperature Management

Watching EGTs is the single most important task during tuning. Pre-turbine EGTs should not exceed 1,450°F sustained. Peak readings of 1,500°F for short bursts are acceptable but not for daily driving. Your tune should cut fuel back if EGTs exceed a safe threshold. Use a good quality thermocouple pyrometer installed about 4–6 inches before the turbo inlet. Keep in mind that running too lean (air/fuel ratio > 20:1) can cause EGT spikes, while too rich can cause excessive smoke and soot buildup.

Boost Control and Timing

Set your boost target between 38–45 psi for 550 HP. Your wastegate spring should be set to crack open at around 25 psi, then use an electronic boost controller (e.g., manual or solenoid-driven) to hold boost to your target. Timing maps must be retarded slightly compared to stock to prevent detonation – around 10–12° BTDC at peak torque under high boost is common. Consult a professional tuner if you are unsure; a bad timing map can crack pistons instantly.

Data Logging

You cannot tune blind. Invest in an Edge Insight CTS3, a standalone logger (like the EFI Live DSP5 with data capture), or a dedicated tablet running software to log RPM, boost, fuel pressure, EGT, rail pressure, and throttle position. Key data points to monitor for the S366 setup: boost pressure rising cleanly without surging, steady fuel rail pressure (target 26,000–30,000 psi), and low smoke output. If you see rail pressure dropping below 22,000 psi, your fuel system is maxed out.

Supporting Modifications for a Reliable 550+ HP Build

The S366 turbocharger and a good tune are only part of the equation. To deliver 550 horsepower consistently, your supporting systems must be upgraded. Below are the most critical modifications.

Fuel System Upgrades

The factory CP3 fuel pump on a 6.7L can support roughly 500–550 HP, but it is already near its limit with a single pump. For safety, consider a dual CP3 setup or a single high-flow CP3 unit (like from Exergy or S&S). Along with the pump, you need 100–200% over injectors (e.g., Exergy 155% or 175%), a lift pump delivering at least 10–15 psi of inlet fuel pressure to the CP3, and upgraded fuel lines (3/8" or 1/2" from tank to pump). Keep the factory fuel rail; it is adequate for this power level.

Air Intake System

The S366 draws a lot of air. A pod filter (e.g., S&B or AFE Stage 2) with at least 5" inlet is recommended, but ensure it is properly shielded from hot engine bay air. A forced cold air intake with a duct from the grille is ideal. The stock air box can be modified but may still become restrictive above 45 psi of boost. Many builders use a 4" aluminum intake tube with a Donaldson or K&N conical filter.

Exhaust System

Backpressure is the enemy of the S366's turbine efficiency. Run a full 4" downpipe (preferably bellows-type to avoid cracking) into a 4" exhaust system with a free-flowing muffler or a performance-oriented resonator. A straight 5" system is also common but may be louder. Ensure the downpipe has a v-band clamp at the turbo for easy removal.

Transmission and Drivetrain

The stock 68RFE automatic transmission in the Cummins 6.7L will fail quickly at 550 HP unless fortified. A full rebuild with billet input/output shafts, heavy-duty clutches, improved valve body (from Revmax, Goerend, or SunCoast), and a higher stall torque converter is mandatory. For manual transmissions (G56), the stock clutch will slip; install a dual-disc clutch from South Bend or Valair rated for 600+ hp and 1000+ lb-ft. Do not forget driveshaft upgrades – especially the carrier bearing and u-joints.

Cooling and Engine Internals

The 6.7L's bottom end is strong enough for 550 HP with good stock internals – but only if you control heat. Add a heavy-duty radiator (e.g., Mishimoto or aluminum full-core) and an oil cooler if not already upgraded. Consider a larger engine oil pan for added capacity. For the cylinder head, install ARP 625 head studs and replace the head gasket with a fire-ring or multi-layer steel (MLS) gasket from companies like Mahle or Fel-Pro. Valve springs should be upgraded to handle higher valve lift and RPM (dual springs recommended).

Common Issues and Troubleshooting with the S366 Setup

Even with careful installation, you may encounter hiccups. Here are the most common problems and how to solve them.

Boost Leaks

If you are not hitting target boost or see a lack of power at higher RPM, inspect every charge pipe connection, intercooler end tank, and the intake manifold gasket. Use a boost leak tester (a simple PVC cap with air fitting) to pressurize the system to 30–40 psi and listen for hissing sounds. Fix leaks with T-bolt clamps and proper silicone couplers.

High EGTs Under Load

High exhaust gas temperatures can indicate insufficient air flow or too much fuel. Check that your wastegate is not opening prematurely (spring too weak) or that your intercooler is not heat-soaked. Also, verify your air filter is clean and intake air temperature sensor is reading correctly. Sometimes retarding timing 2–3° can drop EGTs 50–100°F.

Turbo Surge

If you hear a fluttering or "sneezing" sound when lifting off the throttle, you may be experiencing compressor surge. This happens when the throttle plate closes while the turbo is spinning fast, causing airflow reversal. Solution: add a blow-off valve (BOV) on the charge pipe near the throttle body, or adjust your tune to taper boost earlier. A recirculating BOV is best for street use.

Excessive Smoke

Black smoke (soot) is unburned fuel – you are too rich for the air available. Lean out the tune slightly or increase boost to 40+ psi. White smoke usually indicates raw fuel from poor atomization – check injectors and fuel pressure. Blue smoke is oil – inspect turbo seals or engine blow-by.

Engine Misfires or Rough Idle

After the turbo swap, some people experience a rough idle. This is often due to air in the fuel system after removing injectors. Bleed the system properly. If it persists, check injection timing and fuel rail pressure consistency. Also, verify all exhaust manifold bolts are tight to avoid exhaust leaks that confuse the oxygen sensors.

Conclusion

Reaching 550+ horsepower on a Cummins 6.7L with a BorgWarner S366 turbo is a well-established path for diesel performance enthusiasts. The turbo itself is an excellent match for this engine when paired with a proper T4 manifold, wastegate, and upgraded charge air system. However, the real key to success lies in the supporting modifications – fuel system, transmission, head studs, and especially tuning. A conservative, well-logged tune will keep your engine reliable and your EGTs manageable. Take the time to verify every connection, use quality components, and do not skip the data logging. If you're new to diesel performance, consider working with a reputable tuning shop for the initial calibration. Once sorted, the S366-equipped 6.7L will deliver strong, clean power that puts a smile on your face every time you hit the accelerator.