At 912 horsepower, the Precision 7675 had reached the point where both the compressor and turbine were essentially tapped out on this stock Duramax L5P combination. The plan is not to chase power by driving cylinder pressure through the roof. It’s to move the horsepower peak higher in the rpm range, keep heat under control, and make more power with better breathing. That is why the turbo step matters. A camshaft change can help the engine move more air per cycle, but if the turbo cannot support that added airflow, the combination stalls out. The Precision 8085 brings more compressor and a substantially larger turbine wheel, giving the engine more airflow capacity where the smaller turbo runs out. Turbine housing size also matters here: a smaller A/R responds quicker, while a larger A/R carries more mass flow with less restriction at high output. The real point is simple. Durable diesel power starts with air. Get enough cool, dense air into the manifold, keep exhaust temperature in check, and improve cylinder fill without turning the engine into a smoke-making, overfueled grenade. This turbo change is about adding the airflow headroom needed to push a stock L5P toward four-digit horsepower the right way.
In this installment of Engineering Unboxed, Gale lays out the next step in his effort to make 1,000 horsepower with a single turbocharger on a stock Duramax L5P. The plan is not simply to add boost and fuel, but to reshape where the engine makes power. He wants to move the horsepower peak from roughly 2,800 rpm to 3,800 rpm while keeping maximum cylinder pressure under control. That approach is intended to make substantially more power without overstressing the bottom end or lifting the cylinder heads.
The reasoning is straightforward: if the engine can maintain the same cylinder pressure at 3,800 rpm that it had at 2,800 rpm, horsepower rises because the engine is doing that work more times per minute. Gale emphasizes that controlling cylinder pressure is critical on a stock engine. His goal is to avoid catastrophic failure such as damaging the crankcase or forcing the heads off the block while still extracting much more output.
To reach that higher-rpm power target, the stock camshaft had to go. The replacement cam is intended to improve breathing on both the intake and exhaust sides of the cycle, even while retaining stock valve springs. Better airflow per cylinder event should help the engine continue making power at higher engine speed and reduce the tendency to rely on excessive cylinder pressure to achieve the desired output.
At the same time, Gale says the current turbocharger combination is nearly out of capacity. The existing Precision 7675 setup reached 912 horsepower, and at that point both the compressor and turbine were effectively maxed for the application. Before replacing it, however, he plans one more test with that same 7675 after the camshaft change to isolate what the cam alone contributes.
That final 7675 test will be tightly controlled. He intends to limit compressor discharge temperature to about 450 degrees and turbine-inlet exhaust gas temperature to about 1,650 degrees. Even with those limits, he believes there is still shaft-speed margin left. The turbo had been running at about 110,000 rpm, and he estimates another 20,000 to 22,000 rpm remains available. His expectation is that if the camshaft allows the engine to ingest and expel more air each cycle, the 7675 may still support additional power within those thermal and speed limits, but only up to a point.
With the 7675 nearing the end of its useful range for this project, Gale unboxes the next round of Precision turbochargers sent over for testing. The shipment includes an 8075 and an 8085, along with turbine housings for the 75 mm turbine version. These represent a meaningful step up in both compressor and turbine size.
Compared with the 7675, the larger units increase compressor inducer diameter from 76 mm to 80 mm. On the turbine side, the jump is even more significant: the exducer grows from 75 mm to 85 mm on the larger option. Gale notes that this means substantially more turbine wheel and substantially more compressor wheel, which should extend the airflow range well beyond what the 7675 could support.
He is immediately struck by the machining quality of the compressor wheel, describing it as a piece of art. The 8075 appears to share the same general form factor as the current 7675, which suggests it may fit in the same location without major packaging changes. That would make it an attractive next step if the goal is to increase airflow while preserving the existing installation layout.
The package also includes multiple turbine housing options to tune the turbocharger's behavior. Gale identifies a 1.28 A/R housing and a smaller 1.15 A/R housing. He explains that turbine housing size changes the relationship between shaft speed and mass flow. As the housing gets larger, more mass flow can pass through the turbine wheel per revolution, which shifts the turbo's operating range toward higher airflow and higher power.
The tradeoff is response. A smaller A/R housing generally spools faster and improves throttle response, but it becomes more restrictive as engine speed, mass flow, and horsepower rise. A larger housing tends to support better peak-power efficiency because it reduces restriction at high flow, but it usually gives up some low-speed response. Gale frames this as a tuning balance: if the engine were intended to live at peak power all the time, the most efficient larger A/R housing would usually be the preferred match. For a broader operating range, the choice becomes more nuanced.
One of the most interesting pieces in the shipment is a custom-shop turbocharger from Precision that contains an experimental turbine wheel. Gale is careful not to reveal details that Precision asked him to keep confidential, but he does share a few important points. Unlike the standard wheel, whose impeller surfaces are cast, the experimental turbine wheel is said to be fully machined on the impeller surfaces.
He handles it cautiously because the turbine is exposed and he does not want to put any load on it or show more than he has permission to disclose. Even with the limited reveal, it is clear that this turbo is part of a development effort rather than a standard catalog part. Gale also credits Precision's custom-shop capability and specifically notes that the company's operation is strong enough that this kind of special work is routine for them.
After seeing the new hardware, Gale suggests that the project may be capable of more than the original 1,000-horsepower target. In fact, he says the combination is beginning to look like it could support 1,100 horsepower. That estimate is based on the airflow potential of the larger turbochargers and the expectation that the camshaft will improve the engine's ability to use that airflow effectively.
He contrasts this approach with other high-horsepower diesel builds that rely on multiple turbochargers, heavy smoke, and questionable thermal control. Without naming detailed calibration data from those builds, he points out that headline power numbers mean little if the engine is operating in a destructive state. He specifically criticizes combinations that run excessive smoke, inadequate intercooling, and unknown air-fuel or temperature conditions. By comparison, he notes that his stock-engine L5P already made 912 horsepower with a single turbocharger, and he expects the camshaft change alone to push the combination to or beyond the upper-900-horsepower range others celebrate with much more aggressive and less sustainable setups.
A major theme of the episode is the difference between durable power and what Gale calls "on kill" power. He dismisses horsepower figures achieved in a state where the engine is visibly smoking and clearly overfueled, arguing that such numbers are essentially meaningless if the engine cannot survive repeated use. In his view, if the engine is smoking heavily, it is being pushed into a destructive operating condition.
His objective is to find the actual limits of the stock L5P, then back away from those limits to create durability headroom. That reserve margin is what allows a finished combination to be used hard without immediately consuming the engine's safety margin. He describes many other high-output diesel setups as hand grenades: every time the throttle is applied, the operator is effectively pulling the pin. By contrast, his tuning philosophy is to establish a repeatable, sustainable power level that can be used continuously rather than briefly displayed on a dyno sheet.
Gale closes by summarizing the diesel tuning principles guiding the project. In his view, a diesel cannot really be made too lean in the same way a gasoline engine can, but it can absolutely be made too rich. When the mixture becomes too rich, exhaust gas temperature rises sharply, which can destroy the turbine and also threaten the exhaust valves and pistons. If the engine is too lean, the penalty is simply reduced horsepower.
That is why he prioritizes air before fuel. The engine needs cool, dense air in the intake manifold, and the turbocharger must be matched so the cylinders achieve a high fill percentage. He refers to this as cylinder fill percentage: the fraction of the available manifold air density that actually makes it into the cylinder. The larger turbochargers and the camshaft change are both intended to improve that result. The next phase of testing will show how much gain comes from the Comp camshaft and whether the revised airflow package can carry the stock Duramax L5P to the next power level without sacrificing the durability margin he considers essential.