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Rail Pressure Boxes

While performance tuning and recalibrating the Engine Control Module (ECM) is one of the simplest ways to add major horsepower to your diesel truck, for some reprogramming the computer can be a bit intrusive or overkill for their needs. Or there are those truck owners who need to meet factory warranty or CARB compliancy tests and their state won’t allow tampering with the ECM unless they find CARB compliant products capable of doing so. For those looking for just a little extra mid-range torque for passing on the freeway, or a little extra pep without leaving a trace in the ECM for warranty, there are some options available. For well over a decade, aftermarket companies have been making what’s referred to as a Rail Pressure Box to do just that, increase drivability and power, without hacking the ECM or leaving traces of performance programming. So, what exactly is a rail pressure box?

A rail pressure box, like the Stealth Module shown here for the 2011-2016 LML Duramax, is one of the easiest performance upgrades you can install. In most applications all it takes is unplugging the factory rail pressure sensor, piggybacking the module in-line with the harness and mounting the box under the hood.

The latest diesel engines, from about 2003 on use a high-pressure common rail injection system that uses a high-pressure injection pump to feed a common fuel rail that then feeds the injectors. In some of these systems, it’s not uncommon to see upwards of 30,000 psi of fuel pressure. Since diesel fuel requires high compression and heat to create a combustion cycle, the engine depends on a consistent delivery of high pressure fuel to run properly. In today’s diesel engine, the higher the fuel pressure, the more power it can make and the cleaner it can run. Obviously, there is a point when that pressure can be too extreme and create problems within the system, but fuel rail pressure at a given throttle position and engine rpm is vital to how the truck will accelerate and make power. At idle, the engine will see around 5,000 psi, but as you roll into the throttle, the ECM will command for more pressure, but in some cases the actual pressure seen is often delayed or below desired.

An aftermarket rail pressure box ties into the factory rail pressure sensor where it can read and interpret these signals from the fuel rail to help ramp the pressure up much faster, sending the rail pressure to its max commanded quicker for a better power/torque curve throughout the rpm range while staying within the safe peak pressure parameters. So how does this affect power and drivability? By reaching peak pressure sooner, typically the engine benefits from reduced low-rpm lag, better mid-range power, and most will see 10-20% increases in fuel mileage. The higher injection pressure equates to a more efficient burn within the cylinders, meaning better power and engine efficiency.

Of course, because these are recognized as “performance” modules everyone wants to prove horsepower gains with dyno charts to back the claims. But with a rail pressure box, because the commanded rail pressure at wide-open throttle isn’t going to differ much from the factory settings, a peak horsepower number doesn’t really tell the whole story. With most rail pressure boxes, like the Stealth Module, the performance is more something you’ll feel when driving it on the street. The power and torque curves come on so much sooner, at peak gains will be minimal, but it’s not uncommon to see 70+ horsepower and 100+ torque gains at low and midrange rpm, as this is where the new rail pressure ramps are different than stock.

On the LML Duramax the factory rail pressure pigtail is located on the back of the driver-side cylinder head, making the installation of a rail pressure box super simple and literally a 5-minute job. Deciding to route the adjustable switch into the cab for on-the-fly adjustment will take a little more time, but some will opt to just leave the switch under the hood and leave the pressure box on their preferred setting all the time.
While the 3.0L Power Stroke diesel in the 2018 Ford F-150 has only been on the market a few months, because remapping the rail pressure curve is simply done and the ECM doesn’t have to be cracked or new code written to adjust all the other engine parameters, Stealth already has a performance rail pressure box available for these cool little motors.
On Stealth Performance’s 2018 F-150 3.0L Power Stroke the first version of their Stealth Module netted 17 hp and 45 lb-ft of torque. While that may not sound like much, consider this is a motor that only made 218 hp stock—and notice how much sooner all that torque comes on in the rpm range. That’s performance you’ll feel when driving.

The typical diesel owner will never see a chassis dyno, but tasks like towing, commuting, or just stop-and-go in town is where you’ll really notice the benefits of a rail pressure box. In speaking with Eric Madrid, owner at Stealth Performance Products, he likes to compare a stock turbo diesel power curve to a two-stroke engine. You’ll experience significant bottom end lag followed by a sudden surge in power as rpm rises. By transforming the power curve into something more like a four-stroke engine, power comes on a lot more smoothly and is more predictable in daily driving and towing situations. The changes from adding a rail pressure box enables you to roll into the throttle, ramp up rail pressure sooner and have instant power when you need it. This keep your rpms and throttle input lower, so you can do the same job with less work.

Tuning the ECM is a great option for those looking to get a lot of peak power out of their truck or to get everything out of it possible, as this allows changes to fueling curves, timing tables, boost pressures, transmission strategies, etc. For a large portion of diesel truck owners, that isn’t a priority or even an option for some. These rail pressure modules are designed for the silent majority that are trying to avoid modifying and monitoring their truck but want that bump in performance for normal driving situations.


Stealth Performance

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