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.
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.
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).
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.
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.
6. Free Horsepower Being a mechanically-injected engine, you’re not beholden to electronically interfacing with the ECM when you make fuelling changes on the 12-valve Cummins. This means you can add horsepower with a few simple hand tools and your own two hands.
7. Interchangeable PartsWhile 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.
8. Affordable Injector UpgradesUnlike 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 5×12’s in Cummins-speak), can add up to 100 hp and retail in the $450 to $500 range.
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.
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.
11. Endless Performance Potential With maxed out 12-mm, 13-mm, even 14-mm P-pumps, 5×18, 5×25 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.
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.
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.
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).
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.
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.
6. Free Horsepower Being a mechanically-injected engine, you’re not beholden to electronically interfacing with the ECM when you make fuelling changes on the 12-valve Cummins. This means you can add horsepower with a few simple hand tools and your own two hands.
7. Interchangeable PartsWhile 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.
8. Affordable Injector UpgradesUnlike 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 5×12’s in Cummins-speak), can add up to 100 hp and retail in the $450 to $500 range.
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.
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.
11. Endless Performance Potential With maxed out 12-mm, 13-mm, even 14-mm P-pumps, 5×18, 5×25 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.