The 12 valve, 24 valve and common rail Cummins are three generations of the B-series inline-six diesel used in Dodge and Ram pickups. The 12 valve 5.9L (1989 to early 1998) has two valves per cylinder and a mechanical injection pump, the 24 valve 5.9L (1998.5 to 2002) has four valves per cylinder and an electronically controlled Bosch VP44 pump, and the common rail engines (5.9L from 2003 to 2007, then 6.7L from 2007.5) keep fuel at very high pressure in a shared rail feeding electronically controlled injectors.
12 valve Cummins (1989 to 1998)
The 12 valve 5.9L, often called the 6BT, uses two valves per cylinder for twelve in total. Fuel injection is controlled mechanically rather than by an engine computer: early trucks used a Bosch VE rotary pump, and from 1994 the engine used the Bosch P7100 inline pump. That mechanical simplicity is a large part of why the 12 valve remains popular with owners who value easy field repair.
24 valve Cummins (1998.5 to 2002)
The 24 valve 5.9L, known as the ISB, moved to four valves per cylinder and an electronically controlled Bosch VP44 rotary injection pump managed by an engine control module. Electronic control allowed more precise fueling and timing. The VP44 relies on fuel supplied by the lift pump for cooling and lubrication, and low supply pressure is widely blamed for VP44 failures, so fuel supply health matters on these trucks.
What is the difference between a 12 valve and a 24 valve Cummins?
The 12 valve has two valves per cylinder and purely mechanical injection; the 24 valve has four valves per cylinder and an electronically controlled injection pump. Both are 5.9L inline-sixes, but the 24 valve’s cylinder head, injection system and electronics are different, so the two are not interchangeable part for part.
Common rail Cummins (2003 and later)
In a common rail system, a high-pressure pump (a Bosch CP3 on the 5.9L common rail) feeds a shared fuel rail, and the engine computer fires each injector electronically. Because injection timing and quantity are no longer tied to a mechanical pump, the engine can use several injection events per combustion cycle, which makes it quieter and gives finer control over emissions and power. The trade-off is more complexity and sensitivity to fuel quality: water and contamination can damage high-pressure parts, so good filtration and timely filter changes are essential.
5.9L common rail (2003 to 2007)
The 5.9L common rail kept the displacement of earlier engines while switching to the common rail injection system and electronic injectors.
6.7L common rail (2007.5 onward)
The 6.7L arrived for the 2007.5 model year with larger displacement, a variable geometry turbocharger and new emissions aftertreatment, including a diesel particulate filter and exhaust gas recirculation. Later model years added selective catalytic reduction, which uses diesel exhaust fluid (DEF). For 2019, Ram introduced a revised 6.7L with a compacted graphite iron block. Replacement engines and parts for these trucks should be matched to the factory emissions equipment, which needs to stay in place and working.
Side-by-side comparison
| Engine | Ram model years | Valves per cylinder | Injection |
|---|---|---|---|
| 12 valve 5.9L | 1989 to early 1998 | 2 | Mechanical (Bosch VE, then P7100) |
| 24 valve 5.9L | 1998.5 to 2002 | 4 | Electronic Bosch VP44 rotary pump |
| 5.9L common rail | 2003 to 2007 | 4 | High-pressure common rail |
| 6.7L common rail | 2007.5 onward | 4 | High-pressure common rail |
Replacement common rail Cummins engines
The Cummins short blocks and long blocks we list are built for common rail trucks. Examples include the D&J Essential short block for 2003-2007 5.9L, the D&J Heavy Hauler common rail long block for 2007.5-2018 6.7L and the D&J Precision Plus remanufactured long block for 2019-2024 6.7L. Not sure how much engine you need? Read short block vs long block. If you own an earlier 12 valve or 24 valve truck, contact us before ordering.
See every option by model year on our Cummins 5.9 and 6.7 engines (or go straight to 5.9 Cummins engines and 6.7 Cummins engines) page.
More guides: 5.9 Cummins problems · 5.9 vs 6.7 Cummins · Cummins killer dowel pin.
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