Why the 70mm BorgWarner S366 Turbo Is the Go‑To Upgrade for 6.7L Cummins Engines

The 6.7L Cummins diesel engine has earned a reputation for bulletproof reliability and class‑leading torque. From heavy‑duty work trucks to competition builds, this inline‑six platform powers everything from construction site haulers to drag‑strip contenders. Many owners eventually push beyond the stock turbocharger’s capabilities, especially after adding fuel or exhaust modifications. The factory turbo, though durable, becomes a restriction once power levels climb past roughly 450–500 horsepower at the wheels. Enter the 70mm BorgWarner S366 turbocharger – a proven upgrade that unlocks the 6.7L’s true potential without sacrificing the drivability that makes Cummins trucks so popular.

Unlike some aftermarket turbos that require extensive fabrication, the S366 is a direct‑fit option for many 6.7L applications (2007.5–present) when paired with the appropriate adapter plate or up‑pipe kit. Its 70mm compressor wheel strikes an ideal balance between airflow capacity and spool characteristics, making it suitable for daily drivers, tow rigs, and high‑horsepower race trucks alike. This article dives deep into the engineering, installation, supporting modifications, real‑world results, and potential trade‑offs of running a BorgWarner S366 on a 6.7L Cummins.

Understanding the BorgWarner S366 Turbocharger

Compressor and Turbine Specifications

The BorgWarner S366 is a journal‑bearing turbo that features a 70mm inducer compressor wheel with a 67.5mm exducer, designed to flow approximately 66–70 lb/min at peak pressure ratios. The compressor housing is typically a T4 or T6 flange with a 2.00″ outlet for a 4″ intake tube. The turbine section uses a 68mm wheel with a 76mm exducer and a 0.91 or 1.00 A/R divided T4 housing, though different A/R options are available to tailor spool and top‑end power. A 0.91 A/R turbine housing strikes a popular compromise: it spools quickly for a 70mm turbo while still supporting 600+ horsepower with proper fueling.

The S366 is designed for diesel and high‑performance gasoline applications, but its robust billet or cast compressor wheel options make it especially durable in the EGT‑heavy diesel environment. The journal bearing construction keeps cost manageable while still delivering long service life when using quality synthetic oil and adhering to proper cool‑down procedures.

How the S366 Compares to the Stock Turbo

The factory 6.7L Cummins turbo (typically a variable geometry unit from Holset or Garrett) is tuned for spool and emissions compliance. It might flow enough for 450–500 flywheel horsepower, but beyond that, drive pressure climbs rapidly, and exhaust gas temperatures spike. The S366, being a fixed‑geometry unit, eliminates VGT failure points (sticking vanes, actuator issues) and supplies a much larger volume of cooler, denser air. This reduces exhaust backpressure and allows the engine to breathe freely at higher rpm, which is critical for sustained high‑load operation like towing uphill or racing.

Key Benefits of Upgrading to a BorgWarner S366

Substantial Horsepower and Torque Gains

The most immediate benefit of the S366 is the raw power increase. On a mildly modified 6.7L (turbo‑back exhaust, intake, and a tune), the S366 can add 100–150 wheel horsepower over the stock turbo. With supporting fuel system upgrades – such as larger injectors, a CP3 pump, or a dual‑fueler – power can exceed 600 wheel horsepower at the rear wheels. Dyno results from well‑known diesel shops show many 6.7L trucks running the S366 in the 550–650 whp range on custom tuning, with peaks touching 700+ whp on race fuel or water‑methanol injection. Torque similarly improves; owners report 950–1,100 ft‑lb at the wheels, making the truck feel dramatically stronger during passing and merging.

Improved Throttle Response and Lower Drive Pressure

Despite its larger size, a correctly sized S366 with a 0.91 A/R turbine housing spools only a few hundred rpm later than the stock VGT. The fixed geometry delivers a predictable, linear power curve that is much easier to control than a badly mapped VGT. Drive pressures (exhaust backpressure measured upstream of the turbine) are significantly reduced compared to a stock or smaller VGT when running at high boost levels. Lower drive pressure means the engine doesn’t have to fight to push exhaust out, reducing pumping losses and improving overall efficiency. The result is a turbo that feels responsive and sharp from the mid‑range onward, with clean, smoke‑free power when properly tuned.

Enhanced Towing and Hauling Capability

Truck owners often worry that a larger turbo will ruin low‑end torque for towing. In practice, the S366 with a 0.91 A/R housing still makes usable boost by 2,000 rpm on a 6.7L, and the higher airflow volume keeps exhaust gas temperatures in check when climbing grades with a heavy trailer. The ability to maintain lower EGTs while pulling 10,000+ pounds is a huge safety margin. Many owners report that their trucks run cooler while towing after the S366 upgrade compared to the stock turbo, especially if they also upgrade the intercooler and exhaust. The increased torque also makes merging easier and reduces the need to downshift on highway grades.

Better Efficiency Under Load

When the engine is under heavy load, a restrictive turbo causes the fuel system to work harder to maintain power, leading to higher fuel consumption. The S366’s superior airflow allows the engine to build power with less boost pressure (and therefore less fuel enrichment) for the same demand. Under moderate throttle, some owners see a 0.5–2.0 mpg improvement compared to a stock turbo when driving empty, with even larger gains when towing. Efficiency gains are most noticeable when the turbo is not constantly being pushed against its surge limit – a common issue with stock turbos on tuned trucks.

Supporting Modifications for a Successful S366 Build

Dropping an S366 onto an otherwise stock 6.7L will yield some gains, but to realize the turbo’s full potential – and to keep the engine reliable – several supporting upgrades are strongly recommended.

Fuel System Upgrades

The stock 6.7L fuel system (CP3 injection pump and injectors) can handle roughly 500–550 wheel horsepower. Beyond that, the injection pump runs out of volume and pressure. To feed the S366 enough fuel for 600+ horsepower, you’ll need at least a fuel pump upgrade (e.g., a 10mm or 12mm stroker pump, or a dual CP3 setup). Larger injectors with a higher flow rate (e.g., 30–50% over stock) will also be necessary. Without adequate fuel, the S366 will simply push air that the engine can’t burn, leading to excessive smoke and high EGTs.

Exhaust and Intake Modifications

A 4″ or 5″ turbo‑back exhaust is essential. The stock exhaust is too restrictive for the increased flow. A straight‑through exhaust not only reduces backpressure but also helps the turbo spool more quickly. On the intake side, a high‑flow cold air intake or a large filter with a 4″ or 5″ diameter tube ensures the compressor can gulp enough air. Avoid restrictive mesh filters; a dry or oiled cotton element with a large surface area is ideal.

Intercooler and Charge Air System

As boost pressures climb above 40 psi (common with an S366 tune), the stock intercooler can become a bottleneck. High charge air temperatures reduce oxygen density and increase the risk of detonation. A larger air‑to‑air intercooler (or a water‑to‑air setup for extreme builds) will lower intake temperatures by 20–50°F, directly translating to more power and lower EGTs. Also consider upgrading charge pipes and boots to handle higher pressures without blowing off.

Head Studs and Gaskets

The 6.7L Cummins is known for its ability to handle high cylinder pressures, but above roughly 600 wheel horsepower, the stock head bolts can stretch, leading to head gasket failure. Installing ARP head studs is an inexpensive insurance policy that allows you to run 50+ psi of boost without worrying about lifting the head. Many builders also use a multi‑layer steel (MLS) head gasket for an extra seal.

Transmission Strengthening

Torque levels above 900 ft‑lb will overwhelm the 68RFE automatic transmission in the 2007.5–2018 trucks. A built 68RFE or a swap to the stronger 48RE/Aisin is often necessary for trucks used for racing or heavy towing. For daily driving with occasional loads, a revised valve body, upgraded torque converter, and a larger transmission cooler can suffice up to the mid‑600 whp range.

Tuning – The Foundation of Everything

No amount of hardware makes a difference without proper calibration. The S366 requires custom tuning to set appropriate fuel timing, boost target, and turbo control (for the non‑VGT mapping). A reputable diesel tuner will program the ECM to deliver clean, smoke‑free power while keeping EGTs under 1,300°F during sustained heavy loads. Many tuners offer “tow tunes” that protect the engine by reducing fueling at high boost and high rpm. A poor tune can destroy the turbo or engine quickly – invest in quality tuning.

Installation Considerations and Challenges

Direct Fit vs. Adapter Kits

On 2007.5–2018 6.7L Cummins trucks, the S366 can be fitted using a T4 style up‑pipe adapter that replaces the factory VGT turbo’s exhaust manifold connection. Most aftermarket companies offer a kit that includes a new up‑pipe, downpipe (to connect the S366’s 3.5″ or 4″ outlet to the existing exhaust), oil supply and drain lines, and silicone hoses for the intake and intercooler. Some kits retain the factory location of the turbo, making installation straightforward for a skilled DIYer.

Clearance Issues

The S366 is physically larger than the stock VGT, so clearance with the firewall, inner fender, or valve covers can be tight. Some kits require slight modification to the turbo heat shield or a small dent in the firewall for the compressor cover. On 2013+ trucks with a larger cabin filter housing, an aftermarket intake tube or relocation kit may be needed. Always dry‑fit the turbo before final assembly to check for contact.

Oil Supply and Drain

The S366 uses a -4 AN (or #4 push‑lock) oil supply line from the engine oil galley or factory turbo feed location. The drain line is typically a -10 or -12 AN line that must run downhill to the oil pan. Adequate drain line slope and a large drain fitting (e.g., a 19mm or 5/8″ tube) are critical to prevent oil from backing up into the turbo seals. Install a restrictor in the supply line (common with journal‑bearing turbos) to keep oil pressure at 30–45 psi at idle.

Boost Control and Wastegate

Some S366 kits include an external wastegate (e.g., a 38mm or 44mm gate) that must be plumbed into the exhaust system. This is essential for controlling boost pressure; without it, the turbo can overboost, leading to dangerous cylinder pressures and potential engine damage. The wastegate reference line (boost signal) should be taken from a clean source like the intake manifold or boost pipe. The wastegate spring pressure typically set to 30–35 psi for a daily driver, with boost climbing to 40–50 psi under full load.

Real‑World Performance Results and Owner Experiences

Dyno Proven Power

Numerous independent dyno tests confirm that a 6.7L Cummins equipped with an S366, 5″ exhaust, cold air intake, and a custom tune can lay down 550–650 rear wheel horsepower and over 1,000 ft‑lb of torque. For instance, a 2016 Ram 3500 with the 68RFE transmission showed 587 whp and 1,045 tq on a Mustang dyno – a gain of 180+ whp over the stock tune. With upgraded injectors and CP3 pump, the same truck hit 712 whp at 47 psi of boost. The torque curve is broad, starting around 2,500 rpm and pulling hard to 3,800 rpm.

Driving Impressions

On the street, the S366 feels far more responsive than many owners expect. The lag is minimal (about 2,800–3,200 rpm to full spool, depending on tuning and load). The power delivery is smooth and progressive, making everyday driving enjoyable. Towing a 12,000 lb fifth‑wheel, the truck maintains speed better than stock, with lower EGTs (1,100–1,200°F on grades versus 1,350°F stock). Many owners report that the truck actually generates less soot during hard acceleration because the combustion is more complete with the improved air supply.

Potential Trade‑Offs

  • Low‑RPM Response: If you spend most of your time idling around town or crawling off‑road, the S366 will feel laggy compared to a VGT. A 0.91 A/R housing is a good compromise, but some owners prefer the smaller 0.83 A/R for faster spool, at the cost of top‑end power.
  • Noise: The fixed‑geometry turbo produces a distinct whistle and whoosh sound that some love and others find intrusive. It is louder than a VGT at idle and under light throttle.
  • Cost: A complete S366 kit with all necessary adapters, lines, and wastegate can cost $1,500–$2,500. Combined with supporting modifications, the total build can run $4,000–$8,000.
  • Emissions: The S366 is not compatible with the factory DPF/SCR system – you must delete the emissions equipment to fit the larger downpipe and tune. This is illegal for on‑road use in many regions.

Comparing the S366 to Other Turbo Options for the 6.7L

BorgWarner S369 SX‑E vs. S366

The S369 has a 69mm compressor wheel and flows slightly more (70–75 lb/min) than the S366, making it a popular choice for 700+ whp builds. However, the S366 spools faster and produces more usable mid‑range torque, making it a better all‑around performer for trucks that see a mix of towing, daily driving, and some racing. The S369 is often favored when the engine is built to handle 800+ hp and lower spool isn’t a priority.

BorgWarner S475 vs. S366

The S475 is a larger 75mm turbo that flows in excess of 100 lb/min – huge for a 6.7L. It is a monster on the top end (1,000+ whp capable) but has significant lag and poor low‑range response. The S475 is best reserved for all‑out race trucks that rarely see street driving. The S366 is far more practical for a street‑driven truck.

Garrett GTX3582R vs. S366

The GTX3582R is a smaller ball‑bearing turbo that spools very fast, sometimes matching a stock VGT. It’s excellent for trucks that want quick response and can produce around 500–600 whp. However, it runs out of steam compared to the S366 at higher rpm and struggles to keep EGTs low on sustained heavy towing. The S366 is the better choice for trucks that regularly haul heavy loads.

Factory VGT (Holset HE351VE / Garrett GT3256V)

Stock VGTs offer excellent low‑end response but are limited to about 450–500 whp before backpressure becomes problematic. They also suffer from reliability issues (actuator failure, sticking vanes). The S366 is a proven “fix” for those who want more power and have had enough of VGT problems.

Conclusion

The 70mm BorgWarner S366 turbocharger is one of the most versatile and widely respected upgrades for the 6.7L Cummins diesel. It delivers substantial horsepower and torque gains, improves throttle response and towing performance, and reduces drive pressure – all while staying reliable when properly supported. Whether you’re building a weekend toy, a daily driver that sees the occasional race, or a heavy‑duty tow rig, the S366 fits the bill. The key to success lies in pairing the turbo with the right supporting modifications (fuel, exhaust, intercooler, head studs) and, most importantly, custom tuning from an experienced diesel calibrator.

If you are currently running a mildly modified 6.7L and want more grunt without the cost and complexity of going to a massive 700+ hp setup, the S366 offers the best bang for the buck. It is a proven performer with a vast community of owners and a wealth of tuning support. For those who need a reference point for parts and tuning, reputable suppliers like BorgWarner’s official page and performance retailers such as Fleece Performance or Industrial Injection offer complete kits and expert advice.

Upgrading to an S366 transforms the 6.7L Cummins from a capable workhorse into a true performance powerplant. With proper planning, installation, and tuning, you can enjoy a truck that is both more fun and more capable – and that still gets you home reliably.