The limiting factor here is not the Duramax long block. It’s the stock L5P turbocharger. At the power level being tested, the turbo is in choke on the turbine side, overspeeding, running past the observed EGT limit, and building far more turbine inlet pressure than boost. That imbalance is hard on everything. It holds exhaust in the cylinder, raises piston and exhaust valve heat, and adds stress without making clean power. That’s why pushing the stock turbo deeper into the tune is the wrong move. The fix is more airflow from a properly sized turbo, not just more fuel. The plan is a 7576 Precision turbo with external wastegate control so the engine can keep moving air without the stock unit becoming a restriction. The takeaway is simple: once drive pressure gets upside down, the turbo stops being a power adder and starts becoming the problem.
In this episode, Gale Banks explains to Diesel World editor Adam Lautenberg that the current Duramax program is intentionally pushing an L5P toward its mechanical limits. The objective is not simply to make more power, but to identify the engine's physical boundaries: when head-gasket sealing fails, when head-bolt clamping is overcome, and ultimately what level of cylinder pressure begins to threaten rods, pistons, the crankshaft, or the block itself.
Banks says the team is already beyond what the stock turbocharger can support, so the next phase is to understand exactly why it is done and what hardware changes are required. The testing is being conducted with explicit limits in mind, including turbo speed, exhaust gas temperature, and firing pressure, but those limits may be raised later if necessary to determine where the engine's sealing and structural margins actually end.
Banks describes the stock L5P turbo as a well-engineered unit for a stock pickup-truck application, but not something that can be pushed indefinitely. The problem is choke, primarily on the turbine side. The turbo uses variable geometry to increase exhaust-gas velocity at the turbine wheel and improve spool-up, but at full power that same geometry and housing arrangement begin to impede flow.
He identifies the stock unit as a 61-62 turbocharger, meaning a 61 mm inducer and a 62 mm exducer. In stock form, that sizing is nicely matched to the engine's intended use. However, once power is pushed far beyond factory levels, the turbo becomes a restriction rather than an asset. Banks is blunt about claims of stock-turbo L5P combinations making more than 600 horsepower: in his view, trying to take the stock turbo "to the moon" is a fool's errand, and anyone doing so should be prepared to damage the engine.
To frame the testing, Banks references several operating limits he has developed through work with the engine and with BorgWarner. For turbo speed, he cites 130,000 rpm as the maximum, and only for short periods. For exhaust gas temperature, he uses 1,508 degrees as the short-duration limit. He also notes a factory maximum firing pressure of 183 bar, but for this testing he set the limit at 200 bar.
He clarifies that this is cylinder pressure occurring after top dead center, when combustion force is pushing the piston down the bore. At 200 bar, he estimates the load on the piston crown at roughly 36,000 pounds of total force. In a running engine that force is applied through motion rather than as a static load, but increasing cylinder pressure still raises the risk of bending rods or fracturing hard parts.
Banks also explains what that pressure does to the head gasket and fasteners. Each firing event tries to lift the cylinder head away from the block, stretching the head bolts and cycling the deck surface around the threaded areas. That repeated distortion is why multi-layer steel shim head gaskets are effective: they provide spring-back after compression and help maintain sealing load even as the head and block flex under combustion pressure.
At the test point Banks discusses, the engine is operating at 3,000 rpm. He says the stock turbo setup is moving 59 pounds of air per minute, while the engine itself is pumping about 350 cfm at that speed based on piston displacement. Boost pressure is 31 psi, and the calibration is being held at 18:1 air-fuel ratio.
That 18:1 target is deliberate. Banks says the OE calibration is around 18:1 at peak power, and he considers that roughly the smoke threshold. Richer mixtures such as 17:1 or 16:1 are common in aggressive tuning, but Banks prefers to stay at 18:1 or leaner. He notes that with the Derringer they go to 19:1, which improves fuel economy and reduces soot loading into the DPF while still making more power. Throughout this development work, the goal is to remain at 18:1 or leaner, meaning more air relative to fuel and cleaner combustion.
Banks then breaks down where the engine's power is coming from in terms of air density. He says the engine is inducting 837 cfm of ambient air, and that the density of the ambient air itself contributes 180 horsepower to the total 567-horsepower output. The turbocharger's density increase is worth another 177 horsepower.
The intercooler, however, contributes even more. Banks says the intercooler is worth 211 horsepower, outperforming the turbo in terms of density contribution. He describes it as a water-cooled intercooler originally built for a marine engine. In this test, charge air enters the intercooler at 431 degrees and exits at 82 degrees.
He distinguishes between effectiveness and efficiency. The intercooler's effectiveness, based on temperature reduction alone, is 97 percent. When pressure drop through the intercooler is included, the measured efficiency is 87 percent. Banks notes that intercooler efficiency is rarely discussed because few people have a way to measure it directly, but by his measurements this intercooler is exceptionally good.
The stock turbo is not just near its limit; it is beyond it in several ways. On the 3,000 rpm pull, Banks says the turbo reached 134,000 rpm, exceeding the 130,000 rpm short-duration limit. Exhaust gas temperature also surpassed the accepted limit, reaching 1,549 degrees versus the 1,508-degree maximum.
The most serious issue is turbine inlet pressure. With 31 psi of boost, the engine is seeing 53.3 psi of turbine drive pressure. That means drive pressure exceeds boost by 72 percent, or by 22.5 psi in absolute difference. Banks describes this as completely upside down.
That excessive backpressure creates multiple problems. It is parasitic, because the engine must work harder to push exhaust out. It also traps exhaust in the cylinder, increasing heat load on the pistons and exhaust valves. Banks refers to this as a kind of poor man's exhaust gas recirculation, because the retained exhaust effectively contaminates the fresh charge. In his view, this is exactly what you do not want in a high-output diesel.
The compressor side is also out of its efficient operating range. Banks says a healthy compressor should be around 75 to 77 percent efficient in its sweet spot, with some racing combinations reaching into the 80 percent range. The stock L5P turbo, by contrast, is only at 63.5 percent compressor efficiency in this test.
That low efficiency means the compressor is generating excessive heat rather than delivering dense air efficiently, which helps explain why the intercooler is contributing more to power than the turbo itself. In Banks' assessment, both sides of the stock turbo are finished: the turbine is choked, the compressor is inefficient, and the unit is simply over the moon for this application.
The planned replacement is a 75/76 Precision turbocharger, repurposed from another project. Banks says they were supplied by Precision Turbo, and the setup will use a TurboSmart wastegate and controller. Rather than making a small incremental change, the team is moving to a much larger turbocharger immediately.
When Lautenberg asks whether high drive pressure mainly harms the turbo by increasing lateral load on the turbine wheel and stressing the bearings and shaft, Banks answers that thrust is the real issue. He uses an old-school mismatch example: a small T3 hot side trying to drive a larger T4 compressor. That kind of combination asks a tiny turbine wheel to accelerate a much larger compressor, and the two sides simply do not match.
Banks argues that this mismatch still appears in some modern small-displacement turbo applications. The desire for rapid spool can push engineers toward undersized turbine hardware, but there is a limit to how far that can go before top-end flow suffers badly. Variable geometry helps, but it can still choke at high power. His broader point is that turbocharger matching must account for both transient response and full-load airflow, or the result will be exactly what the L5P is showing here: overspeed, heat, and excessive drive pressure.
Banks says the long-term answer to this spool-versus-flow conflict may be electric supercharging. He dismisses the cheap electric blower devices often sold online as nonsense, but emphasizes that real electric superchargers have been under serious development for decades, largely for emissions and throttle-response reasons.
He recalls using one roughly 20 years ago on a Cummins-powered push truck at Bonneville. That truck was used to launch the Banks streamliner, a 432 mph piston-engined car that held the title of the world's fastest piston-engined car for about a decade and a half. Because the push truck had only a four-speed transmission from a rear-drive IndyCar and was producing smoke while taking too long to accelerate, Banks installed an electric supercharger supplied by Walt, a former Garrett president who had been involved with such systems since the 1970s.
According to Banks, the electric supercharger transformed the push truck. It eliminated smoke, allowed the engine to use all its fuel cleanly, and improved the launch enough to effectively lengthen the streamliner's acceleration distance from five miles to a full five miles rather than wasting part of that distance waiting for the push truck to get up to speed. He credits the system with adding roughly 6 to 7 mph to the streamliner's terminal speed during FIA record runs, where the car exceeded 400 mph. He says electric supercharging is coming in a serious way, and that the industry should expect much more discussion of it.
The conversation closes with Banks reflecting on his early diesel turbo work. He points to a turbocharger from 1981 and identifies it as Banks Diesel Turbo number one, developed for the GM 6.2 diesel. It used a T4 rotating group along with Banks-designed turbine housing, compressor cover, and wheel trims. That kit, introduced in late 1981, helped launch the company's diesel aftermarket business.
He then recounts how GMC approached him in the mid-1980s. John Rock, who was running GMC, recognized that Banks was successfully turbocharging GM diesel trucks in the aftermarket without creating warranty problems. GMC proposed a dealer-specified option that would allow customers to buy a turbocharged diesel pickup or Suburban through GMC dealers. Banks agreed, and the turbocharged GMC 6.2 became available through dealers in 1988.
Banks notes that the first turbo-diesel pickup truck sold in the United States through a dealer was therefore a Banks-turbocharged GMC 6.2, arriving a year before the 1989 Dodge Cummins. For him, that history underscores a larger theme running through the episode: turbocharging works only when the hardware is properly matched to the engine and the intended use, and once the stock hardware is out of range, the data will show it clearly.