Upgrading to 67mm Turbos: A Deep Dive into 6G72 Twin Turbo Performance

For enthusiasts of the Mitsubishi 6G72 V6, the twin-turbo configuration represents a proven platform for serious power. The stock turbos—often TD04 or TD05 variants depending on the application—are adequate for factory output but leave significant headroom on the table. Upgrading to a set of 67mm turbos is one of the most transformative steps you can take. However, this is not a simple bolt-on affair. It requires a thorough understanding of the engine’s capabilities, supporting modifications, and careful installation. This article examines the real-world power gains you can expect from 67mm turbos on the 6G72 twin-turbo, along with a comprehensive installation guide designed to help you avoid common mistakes.

Understanding the 6G72 Twin Turbo System

The 6G72 engine family has powered everything from the Mitsubishi 3000GT VR-4 and Dodge Stealth RT/TT to the GTO and even some export-market sports cars. The twin-turbo variant features a 3.0-liter (or later 3.0-liter DOHC) V6 with a 60-degree bank angle, dual overhead cams, and four valves per cylinder. In stock form with twin turbos, these engines produce in the range of 300-320 horsepower at the crank. The block is robust, with forged connecting rods and cracked-cap main bearing caps in many versions, making it a solid foundation for increased boost.

But the stock turbos are the bottleneck. Their compressor wheels are small, and at higher boost levels they struggle to flow enough air efficiently, generating excessive heat and high exhaust backpressure. Stepping up to 67mm turbos—where the inducer diameter is approximately 67mm—dramatically increases the airflow capacity. This allows the engine to produce power well above the 500-600 horsepower range, provided the fuel system, intercooling, and engine internals are upgraded to match.

Power Gains from 67mm Turbos

When you replace stock turbos with properly sized 67mm units, the power increase is substantial. Exact numbers depend on boost pressure, cam timing, compression ratio, and the quality of the tune. On a healthy 6G72 with supporting modifications, you can expect the following gains:

  • Horsepower gains of 100–180 whp over a well-tuned stock turbo setup. Many builders report wheel horsepower figures in the 450–550 whp range on pump gas (91–93 octane) at moderate boost levels (18–22 psi). With E85 and higher boost (28–32 psi), 600+ whp is realistic.
  • Significantly increased torque output across the mid-range. The larger turbines allow the engine to sustain higher exhaust flow without choking, leading to a much flatter torque curve. Expect peak torque in the 450–500 lb-ft range on pump gas.
  • Improved spool characteristics despite larger wheel diameters. While 67mm turbos might spool slightly later than stock (full boost may come on around 3500–4000 RPM instead of 2800–3200 RPM), the power delivery is much more linear and the engine pulls hard all the way to redline. With proper exhaust manifold design and a well-matched turbine housing AR (0.63–0.81), spool can be surprisingly quick.

It is important to set realistic expectations. To achieve these gains, you must also address fuel delivery, ignition timing, and engine breathing. A 67mm turbo upgrade without injectors, fuel pump, intercooler, and tuning is dangerous and can quickly destroy the engine.

Selecting the Right 67mm Turbo Kit

Not all 67mm turbos are the same. For the 6G72 twin turbo, you need a symmetric pair—typically a dual-scroll or single-scroll design depending on your intake manifold and headers. Common choices include:

  • Precision 6766 or 6768 CEA: A popular billet wheel option that flows over 80 lb/min. Excellent for mid-600 hp goals.
  • Garrett GTX3076R Gen II: Actually a 67mm inducer (76mm exducer). Known for quick spool and high efficiency. Many 3000GT owners run these with great success.
  • BorgWarner S360: 67mm inducer with S300 frame. Excellent durability and cost-effectiveness. Pair with a 0.83 AR twin-scroll housing for best results.

You will also need a turbo installation kit that includes oil feed and drain lines, water lines, gaskets, and all necessary fittings. Many aftermarket suppliers offer complete upgrade kits specifically for the 6G72 twin turbo. Ensure the kit includes new exhaust flanges and downpipes that mate to your existing exhaust system. For more technical specs, check resources like 3SI.org (3/Si community) or the Mitsubishi Media Center for factory specifications.

Supporting Modifications Required

Fuel System

Stock fuel injectors (typically 360–390 cc/min depending on year) are completely inadequate for 67mm turbos. You must upgrade to at least 1000 cc/min injectors—preferably 1200 cc/min or larger if you plan on E85. A high-flow fuel pump (Walbro 450 or equivalent) in the tank, along with a fuel pressure regulator and -6 AN feed line, is mandatory. Do not overlook the fuel rail; the stock plastic rail can flex under high pressure. Many builders switch to a billet aluminum rail for safety.

Intercooling

Larger turbos produce more heat. The stock side-mount intercoolers will quickly become heat-soaked. Upgrade to a front-mount intercooler (FMIC) with a core size of at least 550x250x65 mm. Alternatively, you can run a single massive intercooler with custom piping. Ensure the charge pipes are at least 2.5 inches to avoid restriction. A quality air-to-air intercooler with an efficient bar-and-plate core reduces intake temperatures by 60–80°F, directly increasing power output.

Engine Management and Tuning

The stock ECU cannot handle the increased airflow and injector size. You have two primary options:

  • Standalone ECU: A Haltech Elite 2500, FuelTech FT550, or AEM Infinity is the best choice for full control over fuel, spark, and boost. Expect to pay $1,500–$3,000 for the unit plus calibration costs.
  • Piggyback or Flash: For those on a budget, a chip tuner like a ChipTorque or a reflash of the stock ECU (if supported) can work, but you lose fine control over individual cylinder timing and boost-by-gear. For serious power, standalone is the way to go.

You will need a professional tune on a dynamometer. Do not attempt to tune with an off-the-shelf map. Each engine has unique fuel trims and timing tolerances. A poorly tuned 6G72 with 67mm turbos can detonate within minutes, cracking pistons.

Installation Tips for 67mm Turbos on the 6G72

Installing 67mm turbos on the 6G72 twin-turbo is a major job involving removing the engine or at least the intake manifold and front components. The following step-by-step guide assumes you have the car on jack stands, the battery disconnected, and the cooling system drained.

Preparation and Parts Gathering

Before touching a wrench, ensure you have all the following items on hand:

  • New turbos (pair of 67mm units with appropriate AR housings)
  • Exhaust manifold gaskets (both turbo-to-manifold and manifold-to-head)
  • Oil drain gaskets and new oil feed restrictors if required
  • Water line o-rings or copper washers
  • Upgraded fuel injectors (recommended: 1200 cc/min high-Z or low-Z depending on ECU)
  • High-flow fuel pump and fuel pressure regulator
  • Boost controller (electronic manual or MAC valve)
  • Vacuum lines, hose clamps, and zip ties
  • New spark plugs (one range colder than stock, e.g., NGK BKR7EIX)
  • Engine oil and coolant for refill

Work in a clean area. Label every connector and hose with tape and marker—you will thank yourself during reassembly.

Removing the Old Turbos

Begin by removing the intake pipe, air filter housing, and any under-hood braces. You will need to access the turbos from above and below.

  1. Disconnect the battery negative terminal.
  2. Drain the cooling system by removing the lower radiator hose and opening the radiator cap. Also, drain the engine oil.
  3. Remove the front exhaust pipe (downpipe) from each turbo. Use penetrating oil on the bolts—they are often seized.
  4. Unbolt the intake manifolds: remove the upper intake plenum and then lower intake manifold. This gives access to the rear turbo area.
  5. Remove the heat shields and any brackets holding the turbo assemblies.
  6. Disconnect the oil feed lines from the top of the turbos. Expect some oil spill—keep rags handy.
  7. Unbolt the exhaust manifolds from the cylinder heads (8 bolts per side). The manifold and turbo may come off together.
  8. On the front bank, you can usually remove the turbo from the manifold once the assembly is out. On the rear bank, you may need to lift the engine slightly or remove the starter motor for clearance.

Take photos as you go. The 6G72 is tight, and reinstallation is much easier with reference images.

Installing the 67mm Turbos

  1. Clean the exhaust manifold faces thoroughly. Use a wire brush and brake cleaner. Install new manifold-to-head gaskets. Apply a thin coat of copper anti-seize to the manifold bolts.
  2. Mount the new 67mm turbos to the manifolds. Use new turbo-to-manifold gaskets. For the water-cooled center sections, apply new o-rings to the water line ports.
  3. Install the oil drain flanges. Ensure the drain line has a downhill slope to the oil pan. If you are using a oil restrictor in the feed line (most 67mm turbos require one to prevent seal failure), install it now.
  4. Bolt the manifold/turbo assemblies to the heads. Torque the manifold bolts in a crisscross pattern to 30–35 ft-lbs.
  5. Connect the oil feed lines. Use -4 AN braided lines with appropriate fittings. Connect the water lines as per the turbo manufacturer’s instructions. Note: 67mm turbos often have 1/8 NPT or 1/4 BSP ports; adapt accordingly.
  6. Install the intake piping and boost controller lines. If using an electronic boost controller, route the vacuum line from a compressor housing pressure source to the controller, then to the wastegate actuator(s).
  7. Reinstall the intake manifold and plenum with new gaskets. Torque the lower intake manifold to 15–20 ft-lbs in a spiral pattern.
  8. Connect all sensors and electrical connectors. Double-check that the Mass Air Flow (MAF) sensor or MAP sensor (if using aftermarket ECU) is correctly wired.
  9. Refill the cooling system with distilled water and proper coolant. Refill with fresh engine oil (5W-40 synthetic recommended). Prime the oil system by cranking the engine with the fuel pump relay removed until oil pressure registers.

Tuning: The Make-or-Break Step

Do not start the engine without a base tune loaded into your ECU. Even a brief run on stock calibration can cause lean conditions. Here is the tuning process:

  • Break-in: First, start the engine and let it idle for 20 minutes to seat the piston rings and turbo seals. Check for oil and coolant leaks.
  • Base fuel map: Set fuel pressure to 43.5 psi (3 bar) with the vacuum line disconnected. Enter injector dead times and flow rates. Create a rough fuel table using VE (volumetric efficiency) calculations.
  • Timing map: Use conservative timing values (starting with 10 degrees at idle, ramping to 15–18 degrees at full boost) until you can get on a dyno.
  • Dyno tuning: A professional tuner should dial in the air-fuel ratio to 11.5–12.0 for pump gas, 11.2–11.8 for E85 at full boost. Watch for knock retard. Adjust boost control to target 18–22 psi initially.
  • Wastegate setup: Set the wastegate base pressure to around 10 psi. Use the boost controller to raise boost gradually. Monitor exhaust gas temperatures (EGT)—keep them below 1600°F during tuning.

For detailed tuning resources, consult Haltech’s tuning guides or the EFI University for advanced strategies.

Common Mistakes and Pitfalls

  • Neglecting oil drain routing: A too-small or too-restrictive drain line will cause turbo seal failure. Use -10 AN minimum drain line with a gravity-fed slope.
  • Using stock fuel system: Even with 67mm turbos at low boost, stock injectors will hit 100% duty cycle and cause a lean condition. Always upgrade fuel first.
  • Inadequate intercooling: A stock sidemount will lead to high intake air temperatures (IAT) above 140°F, causing knock and power loss. Invest in a proper FMIC.
  • Over-boosting without control: 67mm turbos can easily produce 25–30 psi at redline. Without a boost controller and wastegate spring, you risk engine damage. Always install a quality boost controller.
  • Skipping break-in procedure: New turbo bearings need a break-in period. Let the engine idle for 10 minutes before any load. Changing oil after the first 500 miles is essential.

Upgrading to 67mm turbos on the 6G72 twin-turbo is a rewarding project that transforms the car into a true high-performance machine. With careful planning, proper parts selection, and meticulous installation, you can enjoy reliable power gains that far exceed stock. The key is to approach the upgrade methodically: fuel, air, tune, and then boost. Take your time, validate every connection, and always prioritize engine safety. The result will be a 6G72 that pulls hard from mid-range to redline, offering an exhilarating experience that makes all the effort worthwhile.