At 711 horsepower, the stock injectors were out of room. Injection duration had stretched to the edge of our limit, exhaust temperature was up, and there was no headroom left. Swapping to 50-over injectors fixed more than fuel flow. The faster-opening, faster-closing injectors let us make the same 711 horsepower with less fuel, less air, lower turbine inlet temperature, lower cylinder pressure, and a much better injection window. That moved peak cylinder pressure to a more useful crank angle and improved engine efficiency instead of just pouring in more fuel. Then we turned it up and made 852 horsepower on a stock-internal L5P. At that point, the data showed the next restriction clearly: not just injector size, but air system capacity. Air-fuel ratio was getting richer, compressor outlet temperature climbed hard, and exhaust manifold pressure rose much faster than manifold air density. That means the turbocharger and exhaust plumbing were doing too much work for too little gain, and cylinder fill started going backward. That’s why the real takeaway isn’t just the 852-horsepower number. It’s what the data proved. Better injectors improved combustion efficiency. After that, bigger exhaust-side piping, more turbo airflow, and eventually more injector headroom are what it takes to keep going without opening the engine. Banks iDash logging made that visible in real time, so the limits weren’t guesswork.
This episode continues the L5P Duramax dyno program with a specific objective: replace the injectors and see whether the stock long-block can reach 800 horsepower. Gale Banks explains that the engine had previously made 711 horsepower, but at that level the combination was already out of fuel-system headroom. S&S Diesel Motorsport supplied a set of Denso-based 50 percent over injectors for the test, and the expectation was that the added flow would reduce injector on-time enough to support more power.
Banks frames the test around one of his long-standing rules of thumb: he does not want the injection event to occupy more than 40 degrees of crank rotation. In his experience, pushing beyond that point drives exhaust gas temperature up, hurts efficiency, and leads to smoke and other undesirable behavior. On the earlier 711-horsepower pull, the engine was already at 39.3 degrees of injection crank angle at 3,300 rpm, essentially at the limit. Turbine-inlet temperature was also high at 1,522 degrees Fahrenheit, so the injector swap was expected to improve both fuel delivery margin and thermal behavior.
With the new 50 over injectors installed, the team first repeated the previous 711-horsepower operating point rather than immediately chasing a bigger number. Banks emphasizes that the engine remains stock internally. The turbocharger and oil cooler have been changed, but the rotating assembly and major internal hardware are untouched, which makes the test as much about the durability of the stock L5P as it is about fuel delivery.
The repeat pull landed at 711 horsepower again, giving the team a clean before-and-after comparison at essentially the same rpm. The engine survived the run cleanly, and Banks immediately turned to the logged data rather than the headline number. That comparison would determine whether the larger injectors were actually improving combustion efficiency and reducing stress, or merely adding fuel capacity.
At the repeated 711-horsepower point, the new injectors produced a substantial improvement in injection behavior. Maximum cylinder pressure was 225 bar, down slightly from the earlier figure, and the injection crank angle dropped from 39.3 degrees to 31.5 degrees. That put the engine back into Banks' preferred 30 to 35 degree range and represented nearly a 20 percent reduction in crank rotation during the injection event. Injector on-time dropped by about 20 percent as well.
The cooling system data also looked healthy. At 3,300 rpm, coolant flow was about 135 gallons per minute. Coolant entered the engine at 204 degrees Fahrenheit and exited at 214, for a 10-degree delta across the engine. Banks says he likes to see an 8- to 12-degree temperature drop across the engine, so this result was right in his preferred window.
Air-fuel ratio at 711 horsepower was surprisingly lean at 19:1, which indicated useful air-side headroom. Compressor outlet temperature was 412 degrees Fahrenheit, a number that made Banks somewhat uneasy, although turbo speed was only 100,000 rpm. Turbine-inlet temperature improved to 1,486 degrees Fahrenheit. He notes that 1,472 degrees is the maximum allowed by the stock L5P ECM, so the engine was still operating above that stock threshold, but the injector change had clearly reduced exhaust temperature compared with the earlier setup.
Before attempting the higher-power pull, Banks set explicit alarms around the parameters he considered critical. He targeted an air-fuel ratio no richer than about 17:1 and set an alarm at 16.7:1. He also set a turbine-inlet-temperature alarm at 1,600 degrees Fahrenheit. Those limits, along with turbo speed monitoring, defined the stop conditions for the run.
Banks was less concerned about cylinder pressure as an immediate abort criterion, even though he expected it might exceed the nominal 250-bar rating of the pressure sensor. The sensor manufacturer rates it to 250 bar and says it can survive to 300, so he accepted that the next pull might push beyond the formal measurement range. The larger concern was whether the stock head bolts, head gaskets, pistons, oil pump, and otherwise stock internal hardware could tolerate the added cylinder pressure and heat.
The full-power run exceeded the original 800-horsepower target. The engine reached 852 horsepower, with the team seeing no obvious signs of failure: no smoke, no oil on the wall, and no parts on the floor. During the pull, however, the engine was clearly approaching the preset limits. Air-fuel ratio moved down toward the rich side, turbine-inlet temperature climbed toward 1,600 degrees Fahrenheit, and cylinder pressure reached roughly 255 bar.
The result proved that the stock-internal L5P could survive an 852-horsepower top-end pull in this configuration, but it also showed that the combination was no longer comfortably within its efficiency window. The engine made the number, yet the data suggested that both the injectors and the turbo system were nearing their practical limits.
Banks then compared three states of tune: 711 horsepower with stock injectors, 711 horsepower with the 50 over S&S injectors, and 852 horsepower with the same 50 over injectors. The first comparison was especially revealing because it isolated the injector change. At essentially the same 3,300 rpm and same horsepower, torque differed by only about 4 lb-ft, so the operating points were directly comparable.
With the larger injectors, air-fuel ratio leaned out from 18.3:1 to 19:1, a 3.8 percent change. Turbine-inlet temperature dropped from 1,522 to 1,486 degrees Fahrenheit, a 2.4 percent reduction. Fuel burn fell from 271.2 lb/hr to 255 lb/hr, an 8 percent reduction in fuel consumption at the same horsepower. Banks interprets that as an 8 percent improvement in fuel economy at equal output.
The combustion phasing also improved. Maximum cylinder pressure moved from 4.8 degrees after top dead center to 8.3 degrees after top dead center. Banks explains that this matters because pressure developed exactly at top dead center has poor mechanical leverage: the piston, rod, crank throw, and main are nearly aligned, so the engine cannot convert that pressure into torque as effectively. In his experience, peak cylinder pressure occurring roughly 8 to 12 degrees after top dead center is a much better place to make torque efficiently. Even though maximum cylinder pressure dropped from 231 bar to 225 bar, it occurred at a more advantageous crank angle, so the engine produced the same power more efficiently.
The turbocharger also benefited. Compressor outlet temperature dropped from 404 to 397.8 degrees Fahrenheit. Exhaust manifold air density fell by 4.7 percent, meaning the engine needed less air density and less fuel to make the same power. Exhaust manifold pressure dropped from 39.6 psi to 38.2 psi, about a 3.6 percent reduction. That lower exhaust pressure improved scavenging, allowing more exhaust to leave the cylinder during the exhaust stroke. Banks calculates cylinder fill as the percentage of manifold air density that actually gets into the cylinder, and that figure improved from 81 percent to 84.3 percent, a 4.1 percent gain. In his view, all of these improvements trace back to the injection rate characteristics of the upgraded injectors, which open faster, close faster, and modulate better than stock.
The comparison between 711 horsepower and 852 horsepower on the same 50 over injectors showed where the combination began to lose efficiency. The higher-power pull occurred closer to 3,400 rpm rather than 3,300, and torque rose from 1,126 lb-ft to 1,322 lb-ft. Banks notes that the dyno required a torque-capacity booster and would need even more booster flow to continue climbing from there.
Air-fuel ratio fell from 19:1 to 17.2:1, which he found troubling because it indicated the engine was running out of turbocharger. Exhaust gas temperature rose by 7.5 percent. The cure for both the richer mixture and the higher exhaust temperature, in his view, is simply more air.
Injection crank angle increased from 31.5 degrees to 35.2 degrees, nearly a 12 percent increase. That was still below his absolute 40-degree ceiling, but it was moving away from the sweet spot. Fuel burn climbed from 255 lb/hr to 317 lb/hr, a 24.3 percent increase in fuel consumption for only a 20 percent increase in horsepower, showing that the engine was becoming less efficient. Peak cylinder pressure timing also moved the wrong direction, from 8.3 degrees after top dead center back to 6.3 degrees after top dead center. Banks says larger injectors would help recover that lost efficiency by shortening the injection event and improving combustion phasing.
On the air side, compressor outlet temperature rose from about 398 degrees to 452 degrees Fahrenheit, roughly 55 degrees hotter than the optimized 711-horsepower condition. Manifold air density increased from 247 to 280 lb per thousand cubic feet, a 14.8 percent increase, but exhaust manifold pressure jumped from 38 psi to 51 psi, a 34.3 percent increase. That imbalance told Banks the turbo system was being pushed too hard for too little gain. Most revealing, cylinder fill percentage fell from 84.3 percent back to 82 percent. Even though manifold density increased, the engine was actually filling the cylinders less effectively because exhaust backpressure was rising too quickly.
Banks traced much of the problem to exhaust-side restriction rather than the compressor alone. He observed more than 51 psi of exhaust manifold pressure, while turbine-inlet pressure was significantly lower, indicating substantial pressure loss in the piping between the exhaust manifold and the turbine inlet. The engine already had larger up-pipes, but he concluded that the routing and geometry on the L5P remain problematic. The turbocharger sits far forward, and the casting and direction changes in the exhaust path create losses that standard-style up-pipes cannot fully solve.
He also identified restriction after the turbine. Even with no mufflers or other obvious restrictions, the system still showed more than 2 psi of backpressure in the 4-inch exhaust that feeds into 10-inch pipes outside the cell. Banks believes a 5-inch exhaust would help. Altogether, he estimates about 7 psi of loss between the exhaust manifold and turbine inlet plus more than 2 psi after the turbine, for roughly 9 psi of avoidable backpressure. Reducing that backpressure would improve cylinder fill at the same manifold air density and allow the engine to go further without demanding as much work from the turbocharger.
The final conclusion is that the 50 over injectors were enough to push the stock-internal L5P to 852 horsepower, but they are already out of headroom at that level. Banks uses a simple scaling argument: if stock injectors support about 445 horsepower, then 50 percent over injectors imply roughly 670 horsepower of nominal support. Since the engine already made 711 and then 852 with them, the injectors are clearly being stretched.
To move toward 900 horsepower without opening the engine, Banks says the next steps are larger injectors, likely 200 over, along with revisions to the cold-air intake, a solution for the up-pipe and turbine-inlet restriction, and a larger exhaust system. He believes those changes can restore cylinder fill, reduce backpressure, and provide the airflow needed to support more fuel without driving air-fuel ratio and exhaust temperature into an unsafe range. The broader lesson from the test is that injector rate shaping improved the engine dramatically at 711 horsepower, but once power climbed into the mid-800 range, the limiting factors shifted to injector capacity, turbocharger efficiency, and exhaust-system restriction.