Power Upgrades for 12-Valve Cummins
More and more truck owners are hanging on to their old trucks and getting more use out of them by reinvesting in some repairs and upgrades like never before. While it may not be as comfortable or as quite to drive, the 1994-1998 12-valve Cummins will always be a workhorse when properly taken care of. Used for odd jobs on the weekends, from landscaping to hauling cattle, this old truck still gets to see its fair share of heavy loads. The engine is getting up there in mileage, but after a complete reseal to eliminate some pesky oil leaks, some timing adjustments and P7100 injection pump upgrades it was producing a very usable 300hp. Being spoiled by that 400hp/1000tq 6.7L Cummins in his 2019 though, the old 12-valve needed a little more, some fueling and air upgrades were in order to pump up this 5.9L to keep up with today’s latest rigs.
First off, to increase power from here, we needed to get more fuel into those combustion chambers. The injection pump had already been upgraded to move more fuel volume to the injectors, now it was time for larger injectors to get that increased flow into the cylinders quicker. Ducky Fuel Injection has been specializing in Cummins fuel injectors for quite a few years now and prides themselves in offering not only higher output injectors, but injectors that run clean and efficient while doing it. With custom internal modifications, like fine needle modifications, they can maximize fuel flow through the nozzle without always have to step up nozzle size to make the power you’re after. This means more power with less smoke.
With hopes of getting around 450 rear wheel horsepower, Chris Luttrell of Ducky Fuel Injection suggested one of his 5x.014 injectors built from brand new Bosch injector bodies. This 5x.014 nozzle size would offer around a 140hp increase over stock, but still be great for heavy towing when paired with the right turbocharger. As that nozzle size continues to increase, fuel entering the cylinder can be hard to clean up with air and make for a lot of uncontrollable smoke and EGT’s while towing. They did receive the needlework upgrade to maximize their potential and could possibly flow enough to push upwards of 500hp. Being new, there was no need to return our old injectors as cores and of course they included brand new copper sealing washers and new injector line seals.
In the diesel engine, all it needs to make power is fuel and air. With the fueling side of things taken care of we needed to address the air side. The factory HX35W turbocharger is a decent little turbo and has been used for decades now on various Cummins engines from trucks to school buses and at factory power levels it works quite well. But at our previous 300hp level, smoke output and EGT’s could get pretty high, so upping to a larger injector, we knew we’d need more. When picking a turbocharger, it’s important to know exactly what your plans are and what you’ll be using the truck for. While a bigger turbo is the obvious need here, there is a such thing as going too big. For a towing application like ours, the low end performance is key, we need airflow down low in the RPM range to get into the increased power and torque curve soon. Quick spool-up will keep smoke output to a minimum and allow you to get that load moving with ease.
A brand new option on the market for the 1994-2002 5.9L Cummins is the Stealth 64 from Calibrated Power Solutions (also known as Duramaxtuner.com). While not new to the turbocharger market, as they’ve been building modified stock replacement turbos for the Duramax and 2003-2007 Cummins market for a few years now, this HX35W replacement is their first one available for the 2nd Gen trucks. Drawing from their experience on those other platforms, this new Stealth offers a much larger compressor wheel and a high flow 10-blade turbine to not only increase airflow to the engine, but increase exhaust flow getting out.
As a complete stock replacement unit, the Stealth 64 is built from all new components, so you won’t need to return a core or have to worry about getting a remanufactured 300,000 mile core with an unknown history. Build for the average diesel guy, the Stealth fills that need for a great daily-driver turbo with enough air to support 500hp without losing that quick off-idle response that makes it easy to drive. The tight wastegated exhaust housing paired with the larger high flow turbine wheel keep EGT’s and back pressure under control while an all new 11-blade compressor wheel design offers a broad power curve with great surge control while towing. For the truck that gets worked hard, this turbo will pair perfectly with the 5x.014 injectors we’ve installed. The Stealth 64 is also upgraded with a 360-degree thrust bearing and covered by an unlimited mileage one-year warranty.
While we couldn’t get the truck back to a chassis dyno to confirm new power numbers before this article went to print, we did get to do what this truck was built for, and that’s heavy towing. After a little fine tuning on the injection pump with the new higher flowing injectors, we were able to dial in low-end fueling to keep smoke at a minimum and could run the Attitude Adjuster (on the fly adjustable fuel plate) on Level 5 while towing with next to no smoke. Running this injector and turbo with that fuel plate setting should be right around 425 horsepower and with 11,000 pounds in tow, the truck had zero issues getting down the highway. Boost comes up super easy and quick, and in the 5-speed manual, would respond quickly between shifts.
Towing over some long 6% grades, we were able to maintain a strong pulling RPM with boost in the mid 20’s and EGT’s hovering around 1200-degrees. We had more throttle to work with if we needed it, but it really had no issues maintaining speed limits setup like this. The Stealth turbo offered seamless power, with no strange drivability issues regardless of the situation we put it in. We did find with the tire size, gearing and weight of the trailer, that the engine did prefer to run a little higher RPM, but that could be timing-related as it’s set a tad higher than what a normal ‘tow rig’ would be at 21-degrees. Overall, the results with this combo make it one of the nicest power combos we’ve driven in an old 12-valve for quite some time. It makes great power, much more than the stock turbo/injector did, while still drivable on the street.