5.9 Cummins 12V vs 24V Cummins Turbodiesel

Dodge Ram 5.9 Cummins engines are often regarded as some of the most reliable light truck turbodiesel engines ever. The 12v Cummins, also known as the 6BT, has a legendary reputation for reliability. However, the newer 24v 5.9 (ISB 5.9) Cummins is still an excellent, reliable diesel engine with a few pros of its own. Point is – the 12v vs 24v Cummins discussion isn’t easy. There’s not really a correct answer as to which one is better.

In this article, we compare the two 5.9L Cummins engines along with the benefits of each engine. We will discuss reliability, factory and aftermarket performance, and engine specs comparing the 12-valve to the 24-valve.

12v vs 24v Cummins: Which Is Better?

We quickly addressed this topic in the intro to the article. There isn’t always a right answer as whether the 12v vs 24v 5.9 Cummins is better. They’re both great engines all around, and which is better will vary from person to person. There are, however, some things that each engine does better than the other. Below we’ll quickly dive into the pros of the 12 valve and 24 valve Cummins diesel engines. Afterwards, we’ll expand on those topics and explore performance, reliability, etc.

Pro’s of the 12v:

• Simplicity
• Minimal electronics
• More reliable
• High HP potential

One of the biggest benefits for the 12 valve 5.9 Cummins engine is simplicity. The engine was built in an era before endless electronics and complex systems. Cummins 12v engines use the P7100 injection pump, which is known to be more reliable than the 24v Bosch VP44 injection pump. However, we want to be clear here. When it comes to the 12V vs 24V Cummins they’re both very reliable turbodiesel engines. The earlier 12v engines have a small advantage.

The same can be said when it comes to mods for the two variants. They both have their advantages for mods. 12v Cummins engines have the advantage of simplicity, but tuning isn’t as simple.

Pro’s of the 24v:

• Higher stock output
• Newer truck & engine
• Still very reliable
• “Chip” tunes

24 valve Cummins engines are still awesome all-around engines. Many enthusiasts give a slight edge to the older 12v, but the 24v does have its pros. For one, it has higher output in stock form. This is a good factor for those not looking to tune and mod the 5.9 Cummins engine. The 24v Cummins is also a newer engine and truck in general. They offer some extra features, an improvement to the interior, etc.

Tying into that – as we said about the two Cummins engines above – the 12v has a slight edge in reliability. However, the 24v is the newer engine and might not require quite as much TLC simply due to age. Added electronics also allows the ability to use “chip” tunes. It makes tuning a little more convenient.

Specs Comparison

5.9 Cummins Specs	12-Valve	24-Valve
Engine	Inline-6 Turbodiesel	Inline-6 Turbodiesel
Known As	6BT	5.9 ISB
Production	1984-1998.5	1998.5-2007
Displacement	359 cu. in. (5.9L)	359 cu. in. (5.9L)
Configuration
Bore x Stroke	4.02 x 4.72 in. 102.1 x 120.0 mm	4.02 x 4.72 in. 102.1 x 120.0 mm
Compression	17.0:1	16.3:1 17.2:1 (HO)
Firing Order	1-5-3-6-2-4	1-5-3-6-2-4
Block	Cast Iron, deep skirt	Cast Iron, deep skirt
Head	Cast Iron	Cast Iron
Crank	Forged Steel	Forged Steel
Pistons	Cast Aluminum	Cast Aluminum
Injection	89-’94: Bosch VE44 ’94+: Bosch P7100	98-’02: Bosch VP44, Electric ’03+: Bosch CP3, electric
Turbocharger	89-’93: Holset H1C ’94: WH1C and HX35 ’94-’98: Holset HX35	98-’99: Holset HX35W ’00-’04: Holset HY35W ‘04.5+: Holset HE351CW
Valvetrain	OHV, 2-valves/cylinder	OHV, 4-valves/cylinder
Oil Capacity	12 quarts	12 quarts
Horsepower	160-230hp	215-325hp
Torque	400-440lb-ft.	420-610lb-ft.
Weight	890lbs. Dry	1,150 lbs. dry

Performance Comparison

The newer ISB 5.9 24v Cummins gets the win for bone stock performance. Early ISB engines beginning in 1998 get 235hp and 460 lb-ft. Only a small increase from the outgoing 6BT 12 valve Cummins. However, in 2003, the 24v received common rail fuel injection. This assisted in boosting power to 305hp and 555 lb-ft. A year later this was further increased to 325hp and 600 lb-ft.

Of course, years of technical and engineering improvements allow newer engines to offer better performance. For the 12v vs 24v Cummins, the later 24v engines definitely offer better bone stock performance. That’s not to say the 12v Cummins is a slouch by any means.

Tuning & Aftermarket Potential

For those looking to keep things simple with a tune and minor bolt-ons the 24v engine is still the better choice. It’s a lot simpler to tune as the electronics allow for use of “chip” tunes. However, the VP44 injection pump is a major limiting factor for those looking to push the engines far.

When it comes to serious power and performance the 12v 5.9 Cummins takes the win. These engines are known to hold up better at extreme power levels when compared to the 24v engines. P7100 injection pumps on the 6BT are easier to mod and support the extra power.

In summary, those looking to keep things stock or modest the 24v is the better street engine. If you’re looking for extreme power and performance the older 12v Cummins engine is likely the better choice. It’s not cheap to take either engine and transmission to 600+whp, though.

5.9 Cummins 12v vs 24v Reliability

Again, both versions of the 5.9 Cummins engines offer excellent reliability. With proper maintenance both the 12v and 24v have a good chance of making it beyond 300,000 miles. However, each have their share of common problems. Although, most are pretty minor issues in the grand scheme.

Killer dowel pins are a well known common problem on the 12 valve 5.9 Cummins engine. They also run into some issues with the P7100 overflow valve, transmission, throttle position sensor, and heater grid failure. On the newer ISB 5.9 24 valve Cummins the 53 block is one of the more concerning issues. The easy solution there is simply avoiding the 53 blocks. Otherwise, the VP44 pump is a problem that can be an expensive fix. Fuel injectors, exhaust manifold leaks, and pedal position sensor failure are among some other issues with the 24v Cummins.

Ultimately, the 12-valve is usually regarded as the more reliable engine. However, the 24-valve comes in a close second but does tend to have some problems that are a bit more expensive to fix. Outside of the #53 blocks, there aren’t any concerns with longevity on these engines either,.

12-valve Common Problems

Despite having a better reputation for reliability, the 12v does have a number of common problems. We cover them more in-depth in our 5.9 Cummins 6BT Problems guide, but we’ll give a high-level rundown here. The common problems on the 12-valve engines are:

• Killer Dowel Pin
• Heater Grid
• TPS Failure
• P1700 Overflow Valve

The killer dowel pin problem is the biggest highlight on the list. The issue is with the dowel pin but got the addition of “killer” in its name because of the likelihood this problems destroys the engine, requiring a new engine. The problem isn’t quite as common as the internet hypes it up to be, but it’s about $50 in parts to preventatively fix it. With the KDP issue fixed on the 5.9 Cummins 12v it is bulletproof.

The other issues with the heater grid, throttle position sensor, and fuel pump are very minor and don’t cause any serious problems.

24-valve Common Problems

While not held as high in regard as the 12-valve, the 24v doesn’t have any catastrophic problems if you avoid the #53 blocks. Common problems on this engine are however a bit more expensive to fix and include:

• Lift pump failure
• Leaking injectors
• Exhaust manifold cracks
• Accelerator pedal sensor
• #53 blocks cracking

The block manufacturer TUPY in Brazil had a bad batch of blocks causing them to crack. The likelihood of failure is pretty high on these ones but fortunately they weren’t the only block manufacturer and it only happened to a few production runs.

Outside of this potentially serious issue the remaining problems aren’t that big of a deal, but can be expensive. Lift pumps and injectors are relatively expensive to replace but generally won’t cause any serious issues.

Overall, avoid the bad blocks and there is no reason this engine isn’t as reliable as the 12-valve. It just has a lot more electrical components that can fail compared to the more mechanical and simplistic nature of the earlier engine.

Engine Age

Another factor to consider is age of these engines. Both are 14+ years old at this point so some standard maintenance, repairs, and TLC will be needed. However, the ISB 5.9 24 valve diesel engine does have the age advantage. While the 12v is considered more reliable this may not be the case simply due to age and mileage. All else equal, it’s likely a 2007 ISB 5.9 will be more reliable than a 1995 6BT engine simply due to age.

Summary

Dodge Ram 5.9 Cummins engines quickly earned a legendary reputation in the light truck world. Early 12 valve 6BT versions are still known as one of, if not, the best turbodiesel engines. They offer an excellent balance of performance, reliability, towing, aftermarket potential, etc. Most importantly, the simplicity of the 12v Cummins wins over many people. It was built in an era before endless electronics and emissions equipment were commonplace.

The 24 valve ISB 5.9 Cummins is no slouch, either. It may not have quite the same legendary rep as the 12v, but the 24v Cummins is still an awesome diesel engine. 24v diesel engines are easier to tune and come from the factory with better performance. The 24v is known to have a couple more expensive problems vs the 12v Cummins. However, it does have the benefit of being the newer engine.

We don’t believe there’s a right or wrong answer as to which 5.9 Cummins engine is better. A lot of the extreme performance builds use the 12v engine and it’s a very reliable diesel. However, most people aren’t swinging for 800+whp and 10 second 1/4 miles with their Dodge Ram trucks. As such, the 24v engine still offers plenty of potential for modest builds. It also offers great reliability, but isn’t as simple or straight-forward as the older 12v.