11 Reasons Why the 12-Valve Cummins Is the Ultimate Diesel Engine

A simple design with unmatched reliability, tremendous performance potential and rugged, million-mile durability sums up the appeal of the 12-valve 5.9L 6BT Cummins. To thousands of diesel lovers, this 1,100-pound hunk of iron is the patriarch of the modern diesel performance era. The ¾-ton and 1-ton Dodge Rams they grace can be made to produce 500 rwhp with relative ease, rack up more than 20 mpg on the highway and easily last more than a half a million miles. In addition to being the power plant of choice in the truck pulling game, the 12-valve is a regular choice in the engine swap world as well, powering countless Jeep, rat rod, muscle car and dragster projects.

But exactly why is a 20-year-old diesel — straddled with ancient injection technology — so high on everyone’s priority list? Scroll along as we pinpoint all of its strong suits. From free horsepower to a near-indestructible design, to the immense parts interchangeability that exists across all model years (including on-road, off-highway and marine applications), the following 11 reasons spell out why the 12-valve version of the 5.9L is so legendary.

1. Simple Design

Meet the 12-valve version of the 5.9L Cummins, produced from ’89-'98. A cast-iron block and head, forged-steel crankshaft and connecting rods, an inline-six design and mechanically controlled direct injection all play into the hands of a power plant built for maximum reliability and longevity. A stroke of 4.72 inches (accompanied by a 4.02-inch bore) yields 359 cubic inches, incredible low-rpm torque and remarkable fuel efficiency.

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Along with it being in an inline engine’s nature to produce gobs of torque, they’re also easier to work on than the V8 competition. You can pull the turbo in minutes and a novice mechanic can replace the water pump in well under an hour. The one drawback is that with very few performance modifications, the 12-valve is known to wreak havoc on transmissions and axles. So while added power comes easy, the rest of the powertrain often requires reinforcement in order to cope with what the 5.9L can dish out.

2. Stout Connecting Rods

The forged-steel connecting rods found in the 12-valve 5.9L (and ’98.5-’02 24-valve engines) are of an I-beam design and capable of easily handling 800 rwhp in stock form. For drag race and sled pull applications, a host of aftermarket companies offer polished, shot-peened and balanced versions of the factory rod, which can be made to withstand 1,200 rwhp (give or take) before bending.

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3. Heavy-Duty Rod Bolts

Even though the factory rods can handle north of 800 rwhp, the stock rod bolts are on borrowed time past this point as they can back out with age and increased engine speeds. Luckily, ARP manufactures heavy-duty rod bolts for the ’89-’02 5.9L, which offer approximately 23 percent more tensile strength than the factory units (PN 247-6303).

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4. 6 Bolts Per Cylinder

With six 12-mm diameter head bolts per cylinder, the 5.9L Cummins is rarely ever at risk of blowing a head gasket, even with serious boost and cylinder pressure in the equation. In fact, the stock head bolts can stand up to as much as 100 psi of boost before stretching! For this reason, a lot of 5.9L gurus simply re-torque the factory hardware (vs. adding head studs) before pushing big boost.

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In this photo, the owner of a ’94 Dodge Ram re-tightened the head bolts (from the center out) to 150 ft-lb at the same time he added a compound turbo arrangement. His fuel and air mods would eventually subject the stock head bolts to 80 psi of boost — and the head never lifted.

5. P7100 (The Holy Grail)

While the 12-valve was produced from ’89-’98, most folks seek out the ’94-’98 version. These engines were equipped with the mechanical Bosch P7100 injection pump (also known as “P-pump” or inline pump), which features six plunger and barrel assemblies, a cam and delivery valves. When the camshaft rotates (the cam is in charge of the firing order), its lobes move the six plungers up and down in their respective barrels (thereby creating injection pressure).

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As you can imagine, there is a lot of room for improvement with so many moving parts inside the P7100. Larger diameter plungers and barrels, bigger delivery valves, quick-rate cams, different rack plugs and performance fuel plates are all available.

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In addition, advancing the timing of the P7100 improves performance considerably. Most factory P-pumps were set at a conservative 12.5 degrees (BTDC), but bumping up into the 18-19-degree range is always good for a noticeable boost in power, and is also considered about as far as a daily driven setup should be taken (before cold start and drivability issues surface). In competition sled pull engines, it’s not unheard of for pumps to be set at 40+ degrees of timing.

6. Free Horsepower

Being a mechanically-injected engine, you’re not beholden to electronically interfacing with the ECM when you make fueling changes on the 12-valve Cummins. This means you can add horsepower with a few simple hand tools and your own two hands. The free mods begin with the AFC (air fuel control) housing shown below, which sits at the rear (top) of the aforementioned P7100 injection pump.

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Moving the AFC housing all the way forward (toward the front of the truck), turning the star wheel beneath the AFC housing toward the passenger side of the engine, removing the fuel plate and disabling the turbocharger’s wastegate are all methods of adding horsepower. In fact, performing all of the above often results in a 100-hp gain, if not more.

7. Interchangeable Parts

While not everything is interchangeable between the ’89-’93 5.9L and the ’94-’98 versions, a host of parts can be swapped over. Among the list of interchangeable hard parts includes the camshafts, connecting rods, the turbos are similar and a P7100 injection pump can be added to the ’89-’93 engines (in place of the fuel-limited rotary VE pump) with the right components and know-how. Adding a P7100 to the first generation 12-valve engine effectively takes the truck’s horsepower capability from 350 rwhp to 600 rwhp. For those kinds of gains it’s definitely worth the hassle of hunting down all the conversion parts you need.

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8. Affordable Injector Upgrades

Unlike today’s electronically controlled, common-rail diesel injectors that can run upwards of $3,000 per set, performance injectors for a 12-valve typically range from $450 to $1,000 (give or take). One common injector comes from the 370-hp version of the 12-valve used in marine applications. Made by Bosch, the marine 370 injectors feature a 5-hole nozzle with 0.012-inch diameter orifices (known as 5x12’s in Cummins-speak), can add up to 100 hp and retail in the $450 to $500 range.

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A set of Stage 4 units from Dynomite Diesel Performance (shown above) will run you just over $1,000, but due to their 5x0.014-inch nozzles can support more than 800 rwhp with the right amount of airflow. The largest streetable injector we come across are the popular 5x0.018-inch nozzle units. Without a doubt, mechanical injectors are where it’s at for making budget-friendly horsepower.

9. HX35: One Tough Customer

The Holset HX35 found on ’94-’98 12-valve engines is one of the toughest factory turbochargers we’ve ever come across. Even though it was designed to see 20 psi of boost in the 5.9L Cummins’ application, it doesn’t seem to be out of its efficiency range at double the boost. While making 35 to 40 psi (courtesy of a disabled wastegate), the HX35 not only yields more power, but lower exhaust gas temperature (EGT) as well. And with a larger (14cm) or modified (ported) factory 12cm exhaust housing, exhaust flow increases, drive pressure drops and the HX35 can support 450 rwhp — not bad considering these engines start out with 160 to 215 hp at the crank, depending on the model.

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10. High-Flow Heads

Despite all the progress that’s been made with the 24-valve head design over the years (a 24-valve head has been used on the Cummins line since ’98.5), competitive sled pullers and drag racers seeking the most horsepower possible almost always revert back to the 12-valve cylinder head. In fact, it’s not uncommon to find a 12-valve head sitting atop a 6.7L Cummins’ block in the upper echelon of diesel motorsports. With the intake shelf milled off, the intake and exhaust ports opened up and multi-angle valve jobs with huge valves added, the 12-valve unit can’t be topped in the flow department. But while a worked over factory 12-valve head can flow extremely well, due to the way they were cast, there is only so much material you can remove before you compromise the structural integrity of the head (they often crack after limited use).

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Looking to get more flow and reliability out of a competition 12-valve head, Hamilton Cams had completely new cylinder heads cast (pictured above), designed specifically to incorporate more meat around the ports (especially on the exhaust side). More material means more porting (material removed) is possible without sacrificing durability (cracking). Hamilton’s top-of-the-line Warhead cam can be made to flow a whopping 310 to 340 cfm per cylinder (vs. 150 cfm on a stock 12-valve head), and in dyno testing has shown gains of as much as 300 hp on 2,500 to 3,000 hp Super Stock class sled pull engines.

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11. Endless Performance Potential

With maxed out 12-mm, 13-mm, even 14-mm P-pumps, 5x18, 5x25 or larger injectors, and today’s highly advanced aftermarket turbochargers, the sky is the limit on what a 12-valve can do in a competition environment. This ’93 Dodge W250, owned by Cole and Cory Dow, is one of the best examples of everything a 12-valve is capable of. The truck was a competitive sled puller, ran 9s in the quarter-mile (yes, 9s!) and even did a little street driving — to the tune of more than 18 mpg.

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Their ’93 model year Cummins is graced with the coveted 215-hp variant of the P7100, a cam out of a ’94 engine, an S475 BorgWarner turbo, a water-to-air intercooler and a custom header the Dow’s fabricated themselves. At the dragstrip, nitrous is added to the equation, which culminates in 1,100-rwhp and 9-second blasts through the 1320. The old Dodge gets its power to the ground via a J&H Performance-built 47RE four-speed automatic and a Sun Coast manual valve body. Don’t believe a 6,000-pound, 25-year-old Dodge can run 9s? Check out this 9.87-second run, and then back it up by watching this 9.64-second pass at 139 mph!

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Now, learn about the 24-valve 5.9L Cummins with common-rail injection.

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