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6BT vs. ISB: Comparing the P-pumped 12-Valve Cummins To Its 24-Valve Successor

By Featured Writer: Mike McGlothlin

The promise of medium-duty capability in a light-duty application has always made the 12-valve 6BT 5.9L Cummins an easy sell for Dodge Ram 2500 and 3500 owners. The cast-iron inline-six is about as robust as an anvil, features a simple 2-valve per cylinder design, and—thanks to having the kind of mechanical injection technology you see on farm tractors— it never seems to die. However, with tougher federal diesel emission standards approaching for 1998, Cummins was forced to redesign its industrial-strength 5.9L to meet the January 1st deadline. The tighter particulate matter and NOx limits were met by switching to a 24-valve cylinder head, an electronically controlled injection pump, and revised injectors. The new engine also dropped the 6BT nomenclature in favor of ISB (Interact System B-Series).

 

6BT Cummins vs ISB Cummins

6BT Cummins vs ISB Cummins

As we begin to take more and more orders for ISB 24-valve 5.9L long-block builds here at Big Bear Engine, there appears to be no better time to explain the key differences between the venerable 6BT and the ISB Cummins power plants. In some ways these engines are directly comparable (displacement, crankshaft, connecting rods, turbocharger), but in others they are simply worlds apart (injection pump, lift pump, valvetrain, pistons). Below, we’ll compare the P-pumped 12-valve version of the 6BT found in Dodge trucks from 1994 to 1998 to the VP44-equipped 24-valve that powered them from mid-1998 to 2002.

 

Emission Standards Expedite Change

12 Valve Cummins 24 Valve Cummins Comparison

12 Valve Cummins 24 Valve Cummins Comparison

The P-pumped 12-valve 6BT Cummins definitely made its mark in the light-duty domestic truck market (left), but by the late 90s the walls were closing in on the on-highway emissions front. Cummins’ answer for meeting the new, federally-mandated particulate matter and NOx standards was the ISB 24-valve 5.9L (right). In addition to doubling the intake and exhaust valves present in the cylinder head, the mechanical Bosch P7100 was replaced with the electronically controlled Bosch VP44, an injection pump that made it possible to vary injection pump timing independent of engine speed. Not only did this make the ISB capable of meeting stricter emission standards, but it did so while making more power (235 hp vs. 215 hp and 460 lb-ft vs. 440 lb-ft, respectively), along with offering improved drivability over the 12-valve engine.

 

12-Valves

12 Valve Cummins Cylinder Head

12 Valve Cummins Cylinder Head

One of the easiest ways to distinguish a 6BT from the ISB 5.9L Cummins exists in the valve covers. The 6BT’s 12-valve cylinder head is capped off with six individual, modular valve covers while the ISB’s features a single piece with a reusable gasket. You’ll also find the injection lines positioned on the exhaust manifold side of the 6BT vs. the driver side of the engine on the ISB—along with the injector’s physical location being off-center in the 6BT, and directly centered above the cylinder on the ISB. Both cylinder heads employ six 12mm diameter head bolts per cylinder for optimum head-to-block fastening.

 

 

24-Valves

24 Valve Cummins Cylinder Head

24 Valve Cummins Cylinder Head

Despite the added complexity it brought to the table, the ISB’s 4-valve cylinder head allowed the engine to breathe more efficiently. Valve adjustment intervals were also stretched to 150,000 miles, a big difference from the 24,000-mile (or 24-month) interval recommended for the 12-valve head. On top of the added intake and exhaust valves’ ability to move more air, the exhaust ports were redesigned for improved flow, and coolant flow throughout the head was enhanced, too. A cam-in-block camshaft was still employed, but it featured wider lobes and activated the valves via longer push tubes than what you’d find in the 6BT.

 

P7100

Bosch P7100 Diesel Injection Pump

Bosch P7100 Diesel Injection Pump

Bosch’s mechanical inline P7100 injection pump debuted on the 6BT in 1994 to ensure the 12-valve Cummins could meet the changing particulate matter emission standards at the time. Its capacity to provide quicker injection rates and higher injection pressures made this possible, along with the corresponding injectors, which had a higher pop-off pressure (260 bar or 3,770-psi), and pistons with a revised fuel bowl. The P7100, perhaps better known as the P-pump, features six plunger and barrel assemblies (one per cylinder) driven by the pump’s camshaft. But while the P7100’s cam was in charge of the firing order, the pump’s injection timing was fixed (typically set near 12-degrees BTDC from the factory).

 

VP44

Bosch VP44 Cummins Injection Pump

Bosch VP44 Cummins Injection Pump

For its ability to vary injection timing independent of engine speed, the electronically controlled Bosch VP44 got the nod for fueling the ISB, which had to meet even more stringent emission standards beginning in 1998. However, that isn’t the VP44’s only job. It’s still tasked with pressurizing the low-pressure fuel it receives from the tank and then supplying ample volume to the injectors. The distributor style, rotary injection pump utilizes a high-pressure solenoid valve, three internal pumping plungers, and an integrated computer (PSG), which serves as the brain of its operation. While the VP44 is vastly different from the mechanical P7100, the ISB Cummins still makes use of mechanical injectors down wind of it—albeit with versions spec’d for a higher pop-off pressure (4,500 psi vs. 3,770 psi).

 

Mechanical Vs. Electric Lift Pump

6BT Cummins Diesel Lift Pump

6BT Cummins Diesel Lift Pump

The manner in which low-pressure fuel is supplied to the injection pump from the tank is different between the 6BT and ISB engines. On the 6BT, a mechanical transfer pump (left) is actuated via a dedicated lobe on the camshaft. On the ISB, a self-priming electric lift pump is employed (right). Each lift pump was mounted to the driver side of the block from the factory, which meant constant exposure to engine vibration (and eventual failure). However, when the electric version on the ISB engine fails it’s notorious for taking the VP44 out with it.

 

“53” Blocks

5.9L ISB Cummins 53 Block

5.9L ISB Cummins 53 Block

Irreparable engine damage isn’t something that’s normally associated with Cummins power plants, but there is no getting around a cracked block—and on select ’99, ’00, and ’01 model year ISB’s with a “53” cast into the front, driver side of the crankcase that is a very real possibility. In this three-year span, roughly 100,000 blocks were cast by TUPY with thinner water jacket walls than what was found on both early and late crankcases. Over time, the 53 blocks are infamous for cracking and spreading under the freeze plugs, allowing coolant to escape. Back in the day, Dodge dealerships addressed the 53 block issue by replacing entire short-blocks. Today, the in-frame repair known as “Lock N Stitch” is the preferred remedy for dealing with the problem, and the most effective way of sealing the crack(s).

 

Offset Bowl Pistons vs. Center Bowl Pistons

Cummins 12 Valve Piston Cummins ISB

Cummins 12 Valve Piston Cummins ISB

With emission standards dictating much of any engine’s design, the fuel injector, the injector’s physical location, and the direct injection piston beneath it all had to be reworked on the ISB Cummins. Instead of being offset in the head like they had been in the 6BT, the ISB’s injectors were centrally located in the head, and also made accessible from the driver side (intake manifold side) of the engine. This central positioning of the injector meant the ISB piston’s fuel bowl was centrally located, too (right). Further, its fuel bowl is specifically designed to match the spray pattern from the 7-hole injector nozzle directly above it.

 

Similarities

Same Bore and Stroke

5.9L Cummins Cast Iron Block

5.9L Cummins Cast Iron Block

Though the blocks were cast slightly different, the ISB shares the same 4.02-inch bore and 4.72-inch stroke as the 6BT. Interestingly enough, the 6BT’s forged-steel crankshaft was carried over to the ISB—along with the same 12mm diameter main cap bolts that were introduced on later versions of the 6BT. Prior to ’97.5 model year engines, larger 14mm main cap bolts had been employed.

 

Carryover (And Overkill) Rods

12 Valve Cummins Forged Steel Connecting Rod

12 Valve Cummins Forged Steel Connecting Rod

Because it shares the same crankshaft as the 6BT, the same rod journals are in play. This means that the exact same forged-steel connecting rods found in the 6BT are used in the ISB Cummins. On the small end of the beefy I-beam rods, you’ll find a sizeable 1.57-inch diameter floating wrist pin. When equipped with ARP rod bolts, the forged rods are known to easily handle four-digit horsepower, and when treated to micro-polishing and shot-peening, they’re known to hold up to as much as 1,400 hp in extreme performance applications. As you might’ve already guessed, connecting rod failures are few and far between on either the 6BT or ISB engines.

 

A Turbo As Reliable As Its Engine

Holset HX35 Turbocharger

Holset HX35 Turbocharger

Like all Cummins engines, the Holset turbochargers that feed them boost are some of the toughest in the business. In particular, the HX35W model—a fixed geometry and internally wastegated turbo with a T3 turbine inlet flange—is a particularly durable unit. It was found on ’95-’98 6BT models but also used on all ’98.5-’02 ISB engines with the NV4500 manual transmission option. The HX35W was also found on ’98.5-’00 automatic transmission versions of the ISB (’01-’02 automatics received the HY35W) and the ’01-’02 high output, version of the engine that produced 240 hp and 505 lb-ft. Even today, the HX35W is known as a highly durable, budget-friendly turbocharger capable of handling boost pressure in excess of 35 psi.

 

KDP: The Achilles Heel

Cummins Killer Dowel Pin

Cummins Killer Dowel Pin

Even though the B-series and ISB Cummins mills are overbuilt in nearly every possible way, they both share a common flaw that can prove catastrophic. Called the killer dowel pin (or KDP), it’s a small steel dowel pin that was pressed into the block on the assembly line to properly locate the timing gear housing. With age, vibration, heat cycles, and even just by chance, it can work itself free from the crankcase. When or if the dowel pin backs out, it can make contact with the cam gear, injection pump gear, and even the crank gear, the results of which can be disastrous (think piston-to-valve contact and a cracked timing gear housing to name a few). To keep the dowel pin from backing out, many aftermarket companies offer kits to permanently lock the pin in place.

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