The stock L5P turbo was already past its safe working range at 567 horsepower. It needed excessive shaft speed, high compressor discharge temperature, and heavy drive pressure just to get there. With the Precision 7675, the engine made 711 horsepower at the same 18:1 air-fuel ratio because the turbo moved more air mass into the manifold with less restriction on both sides of the wheel. Boost went up, turbine inlet pressure dropped, and the boost-to-backpressure relationship finally got into a usable range. That solved the airflow problem, but it exposed the next limit: stock injectors. Even with more lift-pump pressure helping the high-pressure pump, rail pressure and injection duration were right on the edge. At full power, once injection stretches much past about 40 degrees of crank rotation, the fuel plume can climb out of the bowl and start torching the piston radius and cylinder wall. That’s why this combination stopped at 711 horsepower. The turbo had more left. The injectors didn’t.
This episode continues the effort to push a stock L5P Duramax far beyond its original limits. Earlier testing with the factory turbocharger ended at 567 horsepower, but only after driving the turbo past its safe operating range. Exhaust gas temperature reached the redline, and shaft speed climbed to 134,000 rpm, which was beyond the turbo's approximate limit. At that point, the stock unit was effectively finished.
To continue the program, the stock turbo was replaced with a Precision 7675 turbocharger and a Turbosmart wastegate. Fuel supply was also upgraded after the team ran out of lift-pump capacity. Holley supplied a 12-890 billet inline fuel pump, a 12-851 high-pressure regulator, and a 7-micron fuel filter. With those parts installed, the next goal was to see whether the truck could cleanly reach 600 horsepower and then possibly 700.
During the dyno session, the focus was not just peak power but the full set of operating conditions that determine whether the engine and turbocharger are still in a survivable range. The team monitored turbocharger shaft speed, turbine inlet temperature, air-fuel ratio, turbine inlet pressure versus boost pressure, ambient conditions, compressor discharge temperature, intercooler inlet temperature, intercooler outlet temperature, and manifold air density.
Gale emphasized that manifold air density is the key metric because it represents the actual air mass entering the cylinders. It was measured in pounds per thousand cubic feet. That air mass is then mixed with diesel fuel at roughly 18:1 air-fuel ratio. Higher manifold air density allows more fuel to be burned while maintaining the target mixture, which directly supports higher power output.
The first target was 600 horsepower. As the pull came up through the midrange, manifold air density continued to climb through the 180s, 190s, and beyond 200 pounds per thousand cubic feet. At 3,300 rpm, the engine reached 600 horsepower, which Gale identified as the sweet spot for evaluating the setup.
At that point, turbine inlet temperature was about 1,230 degrees Fahrenheit. The air-fuel ratio was very lean at roughly 22:1. Drive pressure was about 30 psi, while boost pressure was about 34 psi. Torque at that power level was around 1,000 lb-ft. The overall impression was that the combination was comfortable and stable, with no immediate sign that the new turbocharger was being pushed into the same danger zone as the stock unit. Because the system still looked healthy, the decision was made to continue toward 700 horsepower.
The next pull carried the truck well beyond the original 600-horsepower target. The dyno showed 711 horsepower from a stock L5P Duramax long-block configuration, a result that exceeded expectations. Torque at the horsepower peak was in the 1,100 lb-ft range, while exhaust gas temperature climbed past 1,500 degrees Fahrenheit.
Gale noted that the truck had reached 567 horsepower with the stock turbocharger, but now made 711 horsepower with the Precision unit. The run was stopped not because the turbocharger had run out of capacity, but because the fuel system had reached the injector limit. That distinction mattered: the new turbo still had room left, while the injectors had become the next bottleneck.
The comparison between the two turbochargers revealed why the new setup worked so much better. With the stock turbo, boost was a little over 32 psi at 134,000 rpm, which was about 2,000 rpm over its approximate 132,000-rpm redline. Compressor discharge temperature at that point was 431 degrees Fahrenheit. Gale's rule of thumb is that compressor discharge temperature in work engines or endurance racing engines should stay below about 400 degrees Fahrenheit. At 431 degrees, the stock turbo was clearly in a danger zone and not a sensible tuning target.
With the Precision 7675 at 711 horsepower, boost increased by 22 percent to 39.6 psi, yet shaft speed was only 101,000 rpm. That meant the turbo still had substantial speed overhead. Compressor discharge temperature was also lower, at 399 degrees Fahrenheit, despite the higher boost level. That combination of more boost, lower shaft speed, and lower discharge temperature showed a major improvement in compressor efficiency and operating margin. Gale even raised the question of whether the Precision unit might safely tolerate compressor discharge temperatures around 430 degrees Fahrenheit for short periods, because he was already thinking ahead to an 800-horsepower attempt.
The turbine-side comparison was just as important. On the stock turbo, turbine inlet pressure, or drive pressure, was over 53 psi, with turbine inlet temperature at 1,549 degrees Fahrenheit. Gale referenced BorgWarner's rating for that turbocharger: 1,472 degrees Fahrenheit continuous service, with brief spikes to 1,508 degrees Fahrenheit for only a minute or two. In other words, the stock turbo was already beyond what he considered acceptable for sustained operation, and the engine control system normally trims fueling to protect against that range.
With the Precision turbo, turbine inlet pressure dropped by 37 percent even though horsepower rose from 567 to 711. Turbine inlet temperature also dropped to 1,522 degrees Fahrenheit. Gale imposed his own 1,600-degree Fahrenheit redline on the Precision unit, although he said Precision had indicated 1,750 degrees Fahrenheit might be allowable. Even so, he preferred to stay conservative.
He also compared boost pressure to back pressure using what he called the boost-to-back-pressure ratio, or B2B. With the stock turbo, the ratio was only 61 percent, meaning exhaust pressure greatly exceeded boost pressure. Gale considers anything below 100 percent undesirable, and values far below that are effectively off the table. With the Precision turbo, dividing 39.6 psi boost by 33.6 psi drive pressure produced a ratio of 118 percent. That was 57 percent higher than the stock-turbo result and represented a dramatic improvement in breathing efficiency. Both tests were run at 18:1 air-fuel ratio, which Gale considers a practical lower limit before smoke and exhaust temperature rise too sharply.
After establishing the power results, Gale turned to the reason the test had to stop: injector limitation. The recommended lift-pump pressure feeding the Denso HP4 high-pressure injection pump on the L5P Duramax is 40 to 60 psi. During the 567-horsepower stock-turbo pull at 3,000 rpm, the system was supplied with 60 psi of lift pressure. Even then, the HP4 could produce only about 1,985 bar, or 28,800 psi, of rail pressure.
Gale's rule of thumb is that, at full power, fuel should be injected within a maximum of 40 degrees of crankshaft rotation. Beyond that point, exhaust gas temperature rises and piston damage becomes likely. The reason is geometric. The injector sprays from the center of the cylinder into the piston bowl. As the piston approaches top dead center and then starts back down, the spray remains safely in the bowl only for a limited crank-angle window. If injection continues too long after top dead center, the fuel plume begins to strike the bowl-to-crown radius and eventually the cylinder wall. That superheats the piston edge and can destroy the piston.
To illustrate the risk, Gale described an earlier Banks drag-racing project. In 2007 or 2008, the team built a Pro Stock-style diesel truck by installing a road-race engine into an S10 chassis and naming it the Sidewinder Type-D. By 2009, at Speedworld in Arizona, the truck lowered the record to 7.77 seconds at 180 mph.
That record came at a cost. The injectors were not large enough, so the engine was tuned aggressively and total injection duration was pushed to nearly 50 degrees of crank rotation. Gale expected piston damage but accepted the risk in pursuit of the record. The result was exactly what the theory predicted: the seven spray plumes came out of the bowl and melted the aluminum at the piston radius. Metal sprayed onto the cylinder walls and passed through the turbochargers. The team got the record, but also had to buy a set of pistons. His point was clear: this is not something anyone should tolerate in a street-driven truck, even though some tuning strategies effectively do exactly that by extending injection duration through stock nozzles while allowing excessive EGT.
On the current L5P test, 1,985 bar rail pressure at 3,000 rpm required 36 degrees of crank rotation to supply enough fuel for the 18:1 air-fuel ratio at 567 horsepower. To make more power with the stock injectors, the system needed more injection pressure. The team therefore increased Holley lift-pump pressure from 60 psi to 90 psi to see whether feeding the HP4 harder would improve its output.
It did. With 90 psi lift pressure, the HP4 reached 2,300 bar at 3,300 rpm. That allowed the engine to make 711 horsepower at the same 18:1 air-fuel ratio. Even so, injection duration had stretched to 39.3 degrees of crank rotation, which was already very close to Gale's 40-degree limit. That left almost no safe margin for additional power on the stock injectors.
The conclusion was straightforward. The Precision turbocharger unlocked a major airflow improvement and carried the engine to 711 horsepower without reaching its own limit, but the injectors had become the controlling restriction. If the goal is to push this Duramax toward 800 horsepower-or finally kill it-the next step is larger injectors, specifically S&S 50-over units.