Power Upgrades for 12-Valve Cummins 
There’s no denying just how impressive today’s new trucks are, they’re comfortable to drive, they’re quiet and they make ridiculous power for ‘off the showroom floor’ vehicles. While anyone can appreciate all the amenities found in today’s $70,000 trucks, not everyone will appreciate that monthly payment. With that said, it seems more and more guys are willing to put up with a little steering wheel slop, a CD player that hasn’t worked for ten years, and a little engine noise from a pre-high pressure fuel injected diesel. Take this old flatbed for example, while the owner has a 2019 Ram Mega Cab sitting in the garage more than capable of doing everything he’d ever want from a truck, he still finds need for this 280,000 mile 12-valve Cummins on a weekly basis. That fully loaded 6.7L Cummins Laramie is an impressive truck that tows like a dream, but when it comes to weekend work, a pair of muddy boots and dirty clothes leads to the old flatbed where there will always be a soft spot in a blue-collar guy’s heart.

This old truck has already had the timing increased and some minor pump work done, including the addition of an Attitude Adjuster for on-the-fly fuel plate adjustments and makes a solid 300hp but it was time for more. To further increase the fueling and power potential of the old 12-valve engine, Ducky Fuel Injection set us up with some of their 5x.014 SAC injectors, that will make injector for heavy towing with the right turbo.

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.

We opted for the SAC style injector nozzle for a couple reasons. While the VCO nozzles, like the factory used are tried and true, the SAVC style is used on all the newer injectors on the market as it offers cleaner emissions and increased injector life.

Fuel Injectors
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.

While it’s seen over 280,000 miles of road, the 5.9L Cummins under the hood of this 97’ truck still runs very healthy and after a few common repairs to seal some oil leaks and some modifications to the P7100 injection pump, with the injector swap it should be more than capable of producing 450- 500hp without issue.

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.

This truck has had a few owners before it’s current, so we were unsure whether or not the injectors we were replacing were the originals, but in the quest for more power while towing, it was time for them to go. Using brand new Bosch body injectors from Ducky Fuel, these weren’t required as cores and didn’t need to be returned.

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.

The new Ducky Fuel injector is ready to be slid down into its bore. Be sure the copper sealing washer around the nozzle is in place or you’ll run into some issues on first start-up. To optimize performance from the injector, Ducky modified the inside with some extensive needle work that allows for more fuel flow from the nozzle, while keeping smoke output under control.

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.

The old mechanical fuel injection system sure makes an injector swap simple. With no electronics or valve covers to mess with, the injector swap was completed in about an hour. Obviously, you need to pay close attention to be sure each injector is seated, and the lines all get installed correctly to prevent any leaks.

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.

With plans of getting up in to that 450-500hp range and the new 5x.014 DFI injectors, there was no way the factory HX35W was going to move enough air so it needed to be swapped out for something with a more flow, the new Stealth 64 from Calibrated Power Solutions was just the ticket.

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.

With a larger 67mm 10-blade high flow turbine wheel and 12cm wastegated turbine housing, the new Stealth 64 will offer much better control over EGT’s without sacrificing the response and quick spool-up you need while towing.

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.

As a completely brand new unit, we won’t need to worry about shipping back a core and can enjoy the ease of installing all new parts without fighting rusted and worn out bolts and adapters. Calibrated Power includes both the factory style 5-bolt elbow or flanged outlet to fit multiple applications and model years.
This is where the real magic happens, on the compressor side of the Stealth 64, you can see how much larger that compressor wheel size is. Calibrated Power tested multiple wheel styles for this 2nd Gen 5.9L platform and settled on the 11-blade wheel which offered the best spool-up, broadest torque curve and surge protection.
The factory turbine wheel worked fine at stock horsepower levels where it was designed to perform, but any increase in fueling could soon produce more exhaust flow than it could handle. The results are higher EGT’s and back pressure. Increasing turbine flow is extremely important when it comes to allowing that engine to work as efficiently as possible at higher power levels.
Going through the disassembly process of getting that stock turbo removed the T-3 gasket showed us something that’s common, exhaust leaks. Notice the soot build-up around the external side of the sealing surface on the gasket, to get the most out of your engine, you want to be sure the exhaust stays inside the turbine chambers and exhaust, where it belongs.
The old, worn out HX35 has been removed and sent to the scrap pile, where it’ll make a great paper weight. Working on these old motors every once a while makes you remember a simpler time when it didn’t take an electrical engineering degree to make more horsepower.
The Stealth 64 is a complete stock replacement with no modifications needed to install. It mated right up to our original manifold, which you can see has been upgraded to a cast 3-piece at some point in life and the supplied cast elbow bolted right up to the 4” exhaust.
That shiny new turbocharger looks a little out of place on an old work truck, but the new compressor wheel and high-flow turbine will be exactly what this engine wants to keep the heavy trailers moving down the highway.
Seems a shame to hide such a beautiful work of art behind an intake tube and air filter, but we can’t wait to hear this big 63.5mm compressor wheel singing away at 25-pounds of boost as we crest a long 6% grade without excessive EGT’s to worry about.

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