Before pushing this Duramax L5P past 800 horsepower, we verified the engine was still healthy where it counts: leakdown, cylinder walls, piston condition, valve condition, and oil condition. The leak numbers were solid, and the borescope showed clean bores with crosshatch still visible and no obvious damage. The next limit wasn’t just whether the engine would take more fuel. At 711 horsepower and 3,300 rpm, injector duration was already at 39.3 degrees of crank rotation. Our rule is to stay out of the danger zone beyond 40 degrees, because excessive duration drives piston heat and EGT up and puts more stress on the turbine, housing, and exhaust valves. That’s why the engine got larger modified Denso injectors from S&S: more fuel in less time. There’s also a cylinder-pressure reason to raise rpm instead of only leaning harder at lower speed. With the engine already around 230 bar cylinder pressure, moving the power higher in the rpm range can make horsepower with less pressure per firing than trying to force the same gain at 3,300 rpm. That’s the balancing act here: confirm the engine is healthy, increase injector flow, and chase the next power step without crossing the limits that start breaking parts.
The engine was checked cylinder by cylinder before further testing. Leakdown measured between 5 percent on the low side and 7 percent on the high side, which was judged healthy enough to continue pushing the engine harder. The glow plugs were removed so leakdown could be performed through custom adapters machined for that purpose.
During reassembly, a Kistler cylinder-pressure transducer was installed in cylinder number six through the glow plug hole. Gale noted that each of these sensors costs about $4,000, and the Kistler tie-box system can support all eight cylinders. Outfitting the full engine with pressure sensing therefore represents roughly $32,000 in sensors alone. The significance of the setup is that it allows in-cylinder pressure development work in a running vehicle, whether in a car, truck, or even a farm tractor, rather than limiting this kind of measurement to a dyno cell.
After the leakdown work, the engine was inspected internally with an Olympus borescope. The probe contains a camera in the tip and allows the viewing angle to be adjusted from 45 degrees downward to 90 degrees straight out and to 115 degrees, which is 25 degrees upward. The camera can also be rotated 360 degrees, making it possible to inspect multiple surfaces from a single access point.
In this case, the borescope was inserted through the injector bore. That gave a clear view of the piston crown, the combustion cup in the piston, the critical radius area that Gale specifically worries about, and the cylinder wall. The bore still showed substantial crosshatch and was described as looking excellent. With the piston moved through its travel, the inspection could follow the full cylinder wall and check for scuffing or other damage. The same process also allowed inspection of valve condition and valve seats by rotating the engine to open and close the valves. Gale pointed out that this method works on both diesel and spark-ignition engines, although he prefers diesels with a center-mounted injector because they provide a more symmetrical view than the often awkward angles of spark plug holes.
Once the borescope inspection was complete, the final injector was installed. The engine was being fitted with what Gale referred to as a last 50-over injector. The injector itself is a Denso unit modified by S&S for increased flow.
The modification was not described as a simple increase in fuel delivery alone. Gale emphasized that the injector retains good idle quality, responds cleanly and crisply off idle, and keeps smoke to a minimum. In his view, that reflects a well-engineered injector modification rather than a crude high-flow change. The goal is to expand the injector's usable flow range without sacrificing drivability or combustion quality.
The injector change was driven by data from the previous power run. The engine had already produced 711 horsepower at 3,300 rpm, but doing so required the injector to remain open for 39.3 degrees of crank rotation. That is very close to Gale's self-imposed limit of 40 degrees.
He explained that he does not want injector duration to exceed 40 degrees of crank angle because excessive duration can damage pistons and drive exhaust gas temperature high enough to threaten the turbine, turbine housing, turbocharger, and exhaust valves. That 40-degree threshold is his rule of thumb for staying in a safer operating range.
A larger injector is therefore needed for two separate reasons. First, if the goal is to increase power beyond 711 horsepower, more fuel mass must be delivered, and that becomes difficult when injector duration is already crowding the 40-degree limit. Second, if the engine speed is increased from 3,300 rpm to 3,800 rpm, the available time for injection becomes shorter even though crank-angle duration is measured in degrees. To deliver the required fuel quantity in that shorter real-time window, injector flow capacity must increase. In other words, a bigger injector is necessary both to support more horsepower at the same rpm and to maintain fuel delivery as rpm rises.
The next target was an increase of about 90 horsepower, bringing the combination to roughly 800 horsepower. Gale framed that as the immediate next step in development. He also made clear that the injector upgrade was not only about chasing a peak number at 3,300 rpm. Even if the engine were held at that same speed, reaching 800 horsepower would still require more injector because the fuel quantity needed would exceed what he is comfortable delivering within the 40-degree crank-angle limit.
At the same time, he wanted to extend the engine speed to 3,800 rpm. That rpm increase is part of the broader strategy for making more power while managing cylinder pressure. Rather than relying solely on higher pressure per firing event, increasing engine speed raises the number of firing events per minute. That can allow the engine to produce the same horsepower with less cylinder pressure, or produce more horsepower while holding cylinder pressure near the current level.
Cylinder pressure was already at 230 bar during the 711-horsepower pull at 3,300 rpm. Gale compared that with the engine's original design figure of about 183 bar and acknowledged that he was already operating well beyond the stock design point. The unanswered question is how far the structure can be pushed before something fails, whether by lifting the heads or damaging another major component.
His reasoning for moving toward 3,800 rpm is tied directly to that concern. If the engine turns faster, it produces more combustion events per minute, which can reduce the pressure required per event for a given horsepower level. Gale believed that by going out to 3,800 rpm, the engine might achieve the desired output with less than 230 bar cylinder pressure. He was confident enough in that possibility to proceed, even while remaining cautious about the engine's structural integrity. He also noted that if cylinder pressure could be held near 230 bar at 3,800 rpm, the resulting horsepower could be substantially higher, though he stopped short of making a firm prediction.
The final part of the engine health check was an oil analysis. A sample was taken in the container used for shipment to Blackstone in Fort Wayne, Indiana, a lab Gale said they had worked with for many years and trusted for this kind of evaluation. The oil analysis serves as another safeguard before continuing with more aggressive testing, helping identify wear metals or other signs of distress that may not yet be visible through mechanical inspection alone.
The engine was then refilled with AMSOIL Competition Diesel Oil in 20W-50 viscosity. Gale described this lubricant as a key part of keeping their engines alive under severe operating conditions. With leakdown complete, borescope inspection finished, injectors upgraded, and oil sampling handled, the engine was cleared for the next phase of testing: the push toward 800 horsepower.