5.9L Cummins 12 Valve Or 24 Valve: Which Is Better

5.9L Cummins 12 Valve Or 24 Valve: Which Is Better

Diesel truck owners are always up for a good debate. Put a couple of Ford, GM, and Ram owners in a room and wait to see where the discussion leads. But what would happen if we put just several Ram fans in the same room? What if the discussion focused solely on the Cummins engine? Let’s imagine the debate was between the 12-valve and the 24-valve 5.9L Cummins and which is the better engine. I know the 24-valve engine has grown to 6.7 liters, but for this discussion, let’s keep it apples to apples and limit the discussion to 5.9 liters.

Many might not consider it a fair discussion, as technology elevated the 24-valve engine to capabilities never dreamed of by 12-valve owners. And even though the first 5.9L Cummins engine was released in 1989, making it 35 years old, both the 12- and 24-valve engines are still known to be very reliable.

5.9-Liter Cummins: From The Beginning

In stock form, the first-generation 5.9-liter Cummins engine delivered 160 horsepower and 400 lb-ft of torque. It was aided by a Holset H1C fixed-geometry turbo. The cast-iron block and forged-steel rods are well known for their durability and the assembled engine delivered 359 cubic inches of displacement.

5.9L Cummins

When you hear someone mention a Cummins swap, nine times out of ten, it involves a 6BT like this 12-valve.

The turbocharged inline-six utilized direct injection and definitely became the standard in the diesel truck segment for years to come in terms of power, fuel efficiency, and long-term durability.

While the first-generation 5.9L Cummins’ 160 horsepower might not seem impressive by today’s standards, it was right on par with GM and Ford’s engines at the time of its release. However, the 6BT would handily beat out GM and Ford’s torque output. With its 4.72-inch stroke, the 5.9-liter engine delivered 400 lb-ft at just 1,700 rpm. The original 12-valve was equipped with a camshaft-driven, rotary-style injection pump (VE pump) that has proven very reliable over the years, but also has the least power potential.

The second-generation overhead-valve inline-six engines were produced between 1994 and 2002. They too feature cast iron engine blocks and cylinder heads, cast aluminum pistons, direct fuel injection, and a single Holset turbocharger. Not a lot changed through the 12 valve’s lifespan. However, In 1994, the 5.9-liter Cummins saw the introduction of Bosch’s P7100 inline injection pump (P-pump), which replaced the VE pump.

When we get to the 24-valve engine, a single Holset HX35W turbocharger was utilized through the 1999 model year. Beginning in 2000, the engine used a Holset HY35W turbo, which had a more restrictive exhaust setup. The 1998 through 2000 engine produced 215/235 horsepower and 420/460 lb-ft of torque (manual transmission versions offered higher horsepower numbers). For 2001 and 2002, the 24-valve power increased to 235/245 horsepower and 460/505 lb-ft of torque.

5.9L Cummins

By the late 2000s, Cummins’ replaced the mechanical Bosch P7100 fuel pump (left) with the electronically controlled Bosch VP44 (right).

Halfway through 1998, an electronically controlled fuel injection system debuted. The system featured a high-pressure, Bosch-branded VP44 injection pump. The change was the result of stricter emissions standards. This new injection pump made it possible to vary injection timing independent of engine speed. Not only did this make the ISB capable of meeting stricter emission standards, but it also allowed the engine to make more power while offering improved drivability over the 12-valve engine.

There was also a high-output version of the 24-valve engine. When bolted to a New Venture NV5600 six-speed manual transmission, the high output 24-valve 5.9 made 10 more horsepower and 45 more lb-ft of torque than the standard version.

1994 Through 1998 12-Valve 5.9L Cummins

The 12-valve 5.9L Cummins — also known as the 6BT engine — offered better emissions and performance than the first generation 5.9-liter engine thanks in part to the new turbo, intercooler, injectors, and pistons.

With its single Holset HX35W turbocharger, the early 6BT delivered 160/175 horsepower and 400/420 lb-ft of torque, depending on which transmission was ordered. From 1996 through 1998, output increased to 180/215 horsepower and 420/440 lb-ft of torque.

Pros Of The 12-Valve 5.9L Cummins

Minimal electronics, high horsepower potential, and overall simplicity. The Bosch P7100 fuel injection pump can easily be modified for higher flow and better performance. With a plunger for each cylinder, the P-pump was installed to improve the engine’s emissions by decreasing the amount of particulate matter in the cylinders. In addition, the engine’s fuel injectors were designed to endure higher fuel pressures.

Cons Of The 12-Valve Cummins

One of the biggest issues with the 12-valve is the small dowel pin located in the front of the engine. Installed during assembly, the “killer dowel pin” helped with aligning the timing cover. The engine’s vibrations can cause the dowel pin to come out and end up behind the front engine cover, between timing gears, causing fatal damage.

5.9-liter Cummins

The killer dowel pin. If you are looking at an early 5.9 engine that has not had this repair done, make sure you take care of it before you run the engine.

The 12-valve also can have issues with the injection pump’s overflow valve, which can leak or end up with a broken spring or an eroded seat. The throttle position sensor can also fail, resulting in unbalanced air-to-fuel ratios.

1998.5 to 2002 24-Valve Cummins

The 5.9-liter 24-valve Cummins engine (the ISB engine) offers higher performance numbers in stock form and responds favorably to tuning.

Pros Of The 24-Valve Cummins

With more valves, it’s no surprise the 24-valve engine has better airflow compared to the 12-valve version. The engine also offers better coolant flow and features a reusable gasket for the one-piece valve cover, a definite improvement over the 12-valve’s six individual covers.

Instead of a mechanical fuel pump, the injection pump utilized on a 24 valve is electronically controlled and can be easily tuned with a tuner. Working with the truck’s engine control unit, the rotary Bosch VP44 injection pump efficiently controls fuel flow and injection timing. The 24 valve also has an electric fuel lift pump to help provide consistent fuel pressure.

The Cons Of The 24-Valve Cummins

A major downside of the 5.9-liter 24-valve Cummins is the thin-wall cast iron engine block. Specific to the earlier ISB engines (1999 through 2001) stamped with the number “53,” the thin blocks are susceptible to cracking by the freeze plugs, resulting in coolant leaks.

Also, while the Bosch VP44 injection pump is a strong unit, it is also one of the more common problem areas of the 24-valve engine and can be expensive to repair. Other problematic parts include the engine’s exhaust manifold, which can crack or leak, and fuel injectors that are known to go bad.

Which Is The Better 5.9L Cummins Is A Matter Of Opinion

There really is no right or wrong answer as to which 5.9L Cummins is better. Both engines are known for their reliability, and even though they are now more than 19 years old, which means they will require some maintenance and repairs, they are highly sought after by those looking to do a Cummins swap. The reason? The ease at which they can be installed into almost any truck.

Those who want high-performance numbers may prefer the 12-valve version as it can be built to exceed 1,000 horsepower with small upgrades. On the other end of the spectrum, the newer 24-valve engine may be ideal for those who are hoping to make upgrades using a handheld tuner or who want to keep their truck closer to stock for street use.

Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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