The real problem here is not boost on the cold side. At 510 horsepower, the stock L5P turbo and the larger Precision 7675 delivered about the same compressor pressure, intercooler performance, and manifold air density. The big change showed up on the hot side. With the stock turbo, turbine inlet pressure was 43 psi. With the Precision, it was 23 psi. That 20 psi drop, along with 130 degrees less turbine inlet temperature, changed how the engine breathed. Excessive drive pressure leaves hot inert exhaust trapped in the cylinder, displacing fresh air and hurting volumetric efficiency. Cut that back, and the engine fills the cylinder better, burns cleaner, and makes the same power with less fuel. That is exactly what showed up in the data. At the same 510 horsepower, fuel use dropped 4.8%, heat rejected to the oil dropped 9.1%, and heat rejected to the coolant dropped 10.8%. Volumetric efficiency improved from about 80% to 87%, and air-fuel ratio moved from 21.6:1 to 23.8:1. The tradeoff is response. The larger fixed-geometry turbo took longer to reach the target power, so if you run a setup like this on the street, converter and calibration strategy become part of the package.
Gale Banks introduces a new turbocharger installed on the dyno-cell L5P Duramax: a Precision Turbo 7675. It replaces the stock turbocharger, but the swap required several supporting changes because the Precision unit is not a variable-geometry turbo. The team modified the mounting foot, added a Turbosmart wastegate, and repiped the compressor discharge into the charge-air cooler because the new compressor outlet is larger. Although the 7675 is much larger than the stock turbo, its peak speed rating is similar at about 132,000 rpm, implying stronger materials and a more robust design for the larger rotating assembly.
The intake side was also upgraded to support the expected airflow. The stock mass airflow sensors were limited to about 60 lb/min, and the original-equipment L5P turbo was already operating in that range, so Banks doubled the MAF capacity to 120 lb/min by using two sensors. The filtration was doubled as well with two Banks Big Ass Filters, chosen for extremely low restriction. On the flow bench, Banks says these filters show virtually no pressure drop at 1,000 cfm. The exhaust retained the same 4-inch outlet, and the turbo was wrapped with heat blanketing. With the hardware in place, the goal was not maximum power yet, but a controlled comparison: stock BorgWarner turbo versus Precision 7675 at the same operating point of roughly 510 horsepower and 3,000 rpm.
The immediate question was whether the larger non-VGT turbo would suffer unacceptable lag or poor throttle response compared with the stock variable-geometry unit. Banks notes that turbo discussions often focus on peak power while ignoring response, so this test was designed to examine both steady-state efficiency and transient behavior.
After warming the engine and confirming coolant flow and operating temperature, the team loaded the engine to match the previous stock-turbo run. The target was 510 horsepower at 3,000 rpm, duplicating the earlier BorgWarner test point. The engine settled at about 510 horsepower with roughly 891 lb-ft of torque at that horsepower peak. All channels were logged during the run using the in-house instrumentation package. Banks emphasizes the density of the recorded data: 100 channels sampled 20 times per second, yielding 2,000 data points every second. That level of logging allowed him to reduce the run into direct stock-versus-Precision comparisons.
At the matched 510-horsepower point, the Precision turbo produced several measurable improvements. Accelerator pedal position dropped by 5.3 percent for the same output, indicating the engine needed less commanded effort to make the same power. Maximum cylinder pressure fell by about 1 percent. Rail pressure remained the same at the stock engine limit of 2,000 bar, but main injection duration was 4.8 percent shorter with the Precision setup.
Heat rejection also improved substantially. Coolant flow stayed the same at 125 gallons per minute, but the heat rejected into the engine oil dropped 9.1 percent. Coolant temperature rise through the engine fell from 12 degrees with the stock turbo to 9 degrees with the Precision. That translated to more than 1,000 fewer BTU per minute rejected into the coolant, or about 10.8 to 11 percent less heat, while still making the same horsepower. Summarizing the steady-state result, Banks reports that compared with the stock turbo at the same 510 horsepower, the Precision setup used about 5 percent less throttle, 4.8 percent less fuel, and rejected significantly less heat to both the oil and coolant.
Banks explains these results by framing the crankshaft as the engine's power broker. Only the power stroke makes a deposit; the intake, compression, and exhaust strokes all consume power. The intake stroke acts like a vacuum pump. The compression stroke consumes work, and in a diesel it becomes even more complicated because combustion begins before top dead center, adding negative work during the end of compression. The exhaust stroke must push hot gases out through the valves, ports, manifolds, piping, aftertreatment, and, in a turbocharged engine, the up-pipes, turbine housing, and turbine wheel.
That means turbocharger selection affects not only boost production but also how much parasitic work the engine must do to breathe. Banks argues that the turbo and engine exist in a symbiotic relationship, and some pairings are better than others. The Precision turbo did not improve output by simply making more boost at this test point; instead, it reduced the engine's breathing losses and heat burden while maintaining the same power.
Ambient conditions were not identical between the two runs. Ambient pressure was the same at 14.4 psi, but ambient temperature was 9 degrees higher during the Precision test. As a result, ambient air density was lower with the Precision setup: 71.7 lb per 1,000 cubic feet versus 73.3 lb per 1,000 cubic feet for the stock run, a little over a 2 percent disadvantage. Banks uses this to reinforce a broader point: the purpose of turbocharging, supercharging, intercooling, and Ram-Air systems is to increase the air density entering the intake manifold and ultimately the cylinders.
The revised intake system helped before the turbocharger even began compressing air. By doubling the filters and MAF sensors, the team reduced inlet pressure loss by 55 percent. Banks evaluates intake systems by how much horsepower they consume rather than how much they add. In this case, the stock intake arrangement represented an 11.5-horsepower parasitic loss, while the revised dyno-cell setup reduced that loss to 6.6 horsepower. He describes the goal as minimizing the robbery of horsepower by the intake system.
Across the compressor itself, the two turbos looked surprisingly similar at this power level. Compressor pressure rise was 30.3 psi for the stock turbo and 30.9 psi for the Precision. Compressor temperature rise was 289 degrees for the stock unit and 297 degrees for the Precision, suggesting similar compressor efficiency. Density gain across the compressor was also close, about 75 lb per 1,000 cubic feet for the stock turbo and about 74 for the Precision. The major difference was shaft speed: the stock turbo was spinning at 116,000 rpm, while the larger Precision made the same contribution at only 92,000 rpm. Banks characterizes the stock turbo as approaching its limit while the Precision was still loafing. Compressor horsepower contribution was nearly identical, about 183 horsepower for the stock turbo and 180 for the Precision.
The intercooler results were also nearly a wash. The dyno cell uses a marine-style water-cooled intercooler with a cupro-nickel heat exchanger core for strong thermal transfer. Intercooler inlet temperature was 354 degrees with the stock turbo and 375 degrees with the Precision. Pressure drop through the intercooler was about 1.25 psi in both cases, which Banks considers acceptable as long as the cooler removes substantial heat without sacrificing too much boost.
Density gain across the intercooler was essentially the same for both setups, right around 66 lb per 1,000 cubic feet. That yielded an intercooler horsepower contribution of about 161 horsepower for the stock turbo and 160 for the Precision. In other words, the cold side of the system-intake, compressor, and intercooler-did not explain the dramatic differences in fuel use, exhaust gas temperature, and heat rejection. The manifold air density with the Precision setup at 510 horsepower was 208.3 lb per 1,000 cubic feet, produced by multiplying the ambient density by a boost-system factor of 2.9. Banks notes that without the boosting system, the engine would inhale only ambient-density air and make roughly one-third the power.
The real difference appeared on the hot side. Turbine inlet pressure with the stock turbo was 43 psi, while the Precision needed only 23 psi to make the same power. Turbine inlet temperature also dropped sharply, from 1,385 degrees with the stock turbo to 1,255 degrees with the Precision, a reduction of 130 degrees. Those two numbers-20 psi less drive pressure and 130 degrees lower turbine inlet temperature at equal horsepower-are the core of the comparison.
Banks then compares turbine inlet pressure to boost pressure, calling the relationship turbo drive ratio. With the stock turbo, turbine inlet pressure exceeded boost by 13 psi. With the Precision, turbine inlet pressure was actually 8 psi lower than boost. He describes that as a swing of 21 psi between the two setups. Expressed as a percentage, the stock turbo showed a negative 31 percent turbo drive ratio, while the Precision showed a positive 34 percent ratio.
That exhaust-side improvement explains the lower fuel consumption and lower heat rejection. Excessive drive pressure corks up exhaust flow leaving the cylinder. When the exhaust stroke ends and the intake stroke begins, residual exhaust gas remains trapped in the cylinder. That inert gas contains no oxygen for new combustion, occupies cylinder volume that should be filled with fresh charge, and preheats the incoming air. The result is poorer cylinder filling and lower volumetric efficiency. Banks reports about 80 percent volumetric efficiency with the stock turbo under this high-drive-pressure condition, versus 87 percent with the Precision. Lower exhaust-port pressure relative to intake-port pressure helps fresh air enter the cylinder, improving air mass and combustion quality.
With the Precision turbo, the engine operated with more air and less fuel at the same power. Air-fuel ratio improved from 21.6:1 to 23.8:1. That means more complete combustion, lower exhaust gas temperature, and reduced thermal stress throughout the engine. Banks specifically notes that piston temperature dropped, which helps explain why less heat was transferred into the engine oil. Because the piston is a major source of heat to the lubrication system, lowering piston temperature directly reduces oil heat load.
Coolant load also fell, which means the engine could maintain the same horsepower on a hotter day with less risk of overheating. Banks presents this as an across-the-board improvement in efficiency and thermal management. At the 510-horsepower test point, he sees no downside in steady-state operation: less fuel, lower EGT, better volumetric efficiency, and less heat rejected to both oil and coolant.
The one likely drawback is transient response. Variable-geometry turbos exist to improve low-speed response by changing nozzle area and gas vectoring at the turbine wheel. That lets the turbo accelerate quickly at low flow, then open up for higher flow at peak power. The Precision 7675 has no variable geometry, and it is physically larger, so it carries more rotating inertia and likely somewhat more bearing drag.
To quantify the difference, Banks devised a response test on the dyno. Accelerator pedal position and pedal rate of change were matched between tests. The engine was taken from 50 horsepower to 320 horsepower at 100 percent throttle, with air-fuel ratio held at 17:1 through the pull. Under those conditions, the stock variable-geometry turbo reached 320 horsepower in 2.9 seconds. The larger non-VGT Precision took 4.3 seconds. Banks says this is the turbo lag people talk about, and it approximates a wide-open-throttle acceleration through a gear from about 1,000 rpm and part load.
He adds that if this turbo were used on a street-driven L5P or similar engine, torque-converter calibration would need attention. A looser converter would let the engine move through the low-rpm range faster and get the turbine into a more favorable operating zone. Simply installing the turbo without recalibration would likely produce sluggish low-speed response followed by a very hard hit once the turbo came on. Even so, Banks concludes that the Precision turbo is massively capable and efficient. At this operating point, both turbos were in roughly the 70 to 73 percent efficiency range, but the stock unit was moving off its efficiency island while the Precision was only beginning to come into its own. The next step, he says, is to push the setup well beyond 600 horsepower.