Cummins is one of the best-known diesel engine manufacturers in the world, and many automotive enthusiasts know them for their engines which are used in Ram trucks.
The 5.9L Cummins is partially responsible for the success of diesel pickup trucks, and throughout its years it had a few updates. To put it simply, there are two different versions of the 5.9L: The 12 valve and the 24 valve, and today we’re going to compare them to find out which one is better!
Before we start, I would like to clarify that there are a handful of different versions for the 12 valve and 24 valve. We’re going to look at the 1994 to 1998 P-pump 12-valve and the 1998 to 2002 ISB 24 valve engines.
We’re going to ignore the earlier 12-valve engines and the later 24-valve engines. This will make everything a lot easier to understand and a lot less complicated.
For those who aren’t intimately familiar with the 5.9L engine and its history, let’s have a quick history lesson. Dodge began offering the 5.9L in their trucks all the way back in 1989.
It might’ve been used in the outdated AD Dodge chassis, but the Cummins option helped increase sales of the Ram dramatically.
Eventually, competition caught up to the performance of the 5.9L and stricter emissions standards being set in place, Cummins updated the 5.9L in 1994. The engine went pretty much unchanged until it again needed to be updated for more power and better emissions in 1998.
Eventually, the 24-valve engine wasn’t able to keep up with emissions standards and was replaced by the 6.7L Cummins in 2007.
Like I said earlier, there are a handful of different variants of the 12 valve and 24 valve engines, but the variants were comparing today are arguably the most popular versions of the engine.
Both engines share the same bore, stroke, architect, and more. Most of the changes revolving around the fueling system, cylinder head, turbocharger, and electronics.
The 12-valve is famous for its lack of electronics. With a fully mechanical fuel system, there are zero electronics involved with how the engine runs, it’s completely mechanical.
A fully mechanical fuel has its pros and cons, with the biggest pro being how insanely reliable it is. The specific 12-valve engine we’re looking at uses the Bosche P7100 injection pump, also known as the P-pump.
The P-pump was originally installed with the goal of improved emissions by cutting down on particulate matter produced in the cylinder, however, it didn’t take long for people to notice the performance benefits of the P-pump.
While the VE pump on the early Cummins only had one plunger, the P-pump has six plungers, one for each cylinder. Nearly every internal component on the P-pump can be modified for increased power.
With some basic modifications to the P-pump, fueling can be massively increased which ultimately increases power significantly. The 24-valve engine, on the other hand, uses an electronically controlled fueling system.
There are a lot of benefits with an electronically controlled fuel system, such as reduced emissions output and more precise air/fuel ratios, but unfortunately, many Cummins owners still prefer the P-pump for its superb reliability.
The nice thing about the electronically controlled injection system is being able to tune it through a handheld tuner. While it’s not very hard to get more power from the mechanical injection system on the 12-valve, being able to just stick a tuner on your truck to increase power is nice to have.
Another major change between these two engines is the valvetrain. As you probably guessed by their names, the 12-valve engine has a 12-valve head and 24-valve engine has a 24-valve head. The four-valve head massively improved airflow and coolant flow.
The 24-valve head featured 60 PPI valve springs, reshaped exhaust ports for improved exhaust flow, and six 12 mm head bolts per cylinder. A one-piece valve cover with a reusable gasket also replaced the 12-valve’s six individual units, which simplified servicing and accessing the valvetrain.
Interestingly enough, many sled pullers and drag racers still prefer to 12-valve head, even with all the improvements of the 24-valve head.
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 head offers insane airflow. (note there are other minor changes including camshafts).
Both engines use the Holset HX35W, better know as the HX35. It should be noted that 2001 and 2002 24-valve ISB engines used the HY35W which is a little bit more restrictive on the exhaust side.
This turbo is known for being super reliable, even when pushing twice as much boost as it was originally intended for.
Underneath the cylinder heads, you’ll find that these engines are surprisingly similar. With the different injection systems, the pistons are shaped differently to maximize efficiency.
Under the pistons, you’ll find that both these engines use the same forged I-beam connecting rods which have been proven to easily withstand 800whp or higher.
Unfortunately, every engine has its flaws and both the 12 valve and the 24 valve engines have some major problems. The biggest program with the 12-valve is the Killer Down Pin.
This is a small dowel pin pressed into the front of the engine during assembly to enable proper alignment of the timing cover. Over long periods, the vibrations of the engine can cause this dowel pin to dislodge.
Should it dislodge, there is a fair chance that the pin will bind between a series of timing gears behind the front cover and cause significant, if not catastrophic damage.
For the 24-valve, the number block problem is the weak point. Approximately 100,000 crankcases, produced from 1999 to 2001 were cast with thinner water jacket walls than what was found on earlier and later model engines.
These thinner blocks are prone to cracking beneath the freeze plugs. These blocks are not guaranteed to crack, but it was a big enough problem that Dodge ended up replacing quite a few cracked blocks under warranty.
Which One is Better?
Both engines have massive potential if you were to stick a larger turbo on them and both can serve as great tow vehicles or daily drivers. Deciding which one is better depends on what you value most.
While the 12-valve is a little bit more reliably with the mechanical P7100 pump, tuning it requires modified parts of the injection pump. The 24-valve might not be as insanely reliable, but it’s still very reliable and can easily be tuned with a handheld tuner.
If you’re buying a truck with the goal of making huge power numbers, the 12-valve might be a little bit better, but if you want something simple for the street which can easily be tuned, the 24-valve might be better.
There are some other small changes from the 12 valve to the 24-valve, but what we’ve talked about in this video covers all the major differences.