By Featured Writer: Mike McGlothlin
There’s no question that diesel swaps have become commonplace in cars, trucks, and off-road culture in recent years, and the Cummins name is at the top of the most desirable candidates you’ll find. But of the six-cylinder or four-cylinder B-series variety, which mechanical-injection power plant is a better choice? For some, the decision is easy. For others, it’s not as cut and dry. After all, both the 4BT and 6BT benefit from the same, simple design, enjoy reputations of providing hundreds of thousands of trouble-free miles, and—thanks to being developed for industrial, marine, and construction use—seem to thrive in any type of environment. Their abundant low-end torque is valuable off-road, their fuel efficiency is welcomed by those seeking more range, and their robust, overbuilt composition means they can handle tasks such as towing, hauling, or additional horsepower without skipping a beat.
However, despite their similarities, there are several key differences between these two iconic oil burners. In the following 4BT versus 6BT Cummins face-off, we’ll explain why your specific vehicle’s application, its intended use, and your budget are vital in determining which version is best for you. Establishing how much real estate you have to work with under the hood, being honest about your own mechanical and fabrication abilities, and making plans for a driveline and suspension that can support the Cummins you choose all play a role in the engine selection process. So before you launch into that ambitious 6BT/Wrangler swap or decide to finally do something with that 4BT taking up space in the corner of the shop, give the following analysis a careful read.
Intended purpose
Both choosing the right vehicle and knowing how you plan to use it once the Cummins swap is complete should be the driving force behind any of your project plans. If you’re going off-road, be it rock-crawling, sand-blasting, or digging through the mud, either the 4BT or 6BT will thrive. For all-out horsepower, a 6BT graced with a Bosch P7100 injection pump is the way to go. If it’s fuel economy you’re after, the 4BT is the ideal candidate—just be careful not to drop the four-cylinder into a vehicle that’s too heavy or you’ll negate the fuel savings you thought the smaller B series would afford you. For towing lighter loads, neither Cummins will complain. If you need to move more than 8,000 pounds with authority, the larger 6BT should be your choice.
Your Budget
Anything is possible on a budget without limitations, but those instances are few and far between. If you’re on a tighter budget, be honest with yourself about your own fabrication and mechanical skill sets. Will you have to outsource your fab work or wrenching? If that’s the case, keep the swap as straightforward as possible. Leaving fabrication and mock-up work to a professional can get expensive fast. Due to space constraints, this may mean holding off on the 6BT idea and opting for a 4BT instead. Or it could mean you’re looking at a body swap rather than an engine conversion. Countless 6BT projects have been pulled off by first sourcing a donor Dodge Ram 2500 or 3500. One of the most successful 6BT Cummins swaps we’ve seen entailed a four-door JK Wrangler body sitting atop a modified Dodge frame.
Engine Affordability And Availability
Believe it or not, finding a used 4BT can cost more than sourcing a take-out 6BT these days. Perhaps, due to being so many years removed from the step van liquidations that once took place, the market is no longer saturated with 4BT’s. Or perhaps it’s because the 6BT is simply more widely available, thanks in large part to more than half a million of them making their way into first and second-generation Dodge Rams from ’89-‘98. No matter the reason, it’s both easier and cheaper to get your hands on a used 6BT Cummins at present. And as we alluded to above, buying a 6BT Cummins in conjunction with an entire donor truck (even if it’s wrecked) gives you a heavy-duty drivetrain or suspension option you may come to need.
Wow Factor Calls For Deep Pockets
When it comes down to it, your vehicle’s intended purpose matters most. After all, we’ve seen a 5.9L stuffed into a four-door JK, along with the NV4500 transmission, AAM axles (9.25 and 11.50), and even the ¾-ton Ram’s frame to support it, but the owner needed an off-road vehicle that was virtually bulletproof and wasn’t afraid to break out the plasma cutter to make it happen. That’s obviously an extreme example, but it illustrates what components are needed to support a 1,100-pound 6BT in that particular application (i.e. ¾-ton pickup frame, suspension, and axles).
Size and Packaging (Advantage 4BT)
Although 4BT’s are becoming harder to find and often carry a premium price tag, they are smaller—and that makes them ideal for tight engine bays. Their shorter overall length is ideal for Jeep Wranglers, Ford Rangers and Broncos, old Toyotas, and even Dodge Dakotas, where hacking up the firewall isn’t a prerequisite. They’re also lighter, making life much easier for front suspension systems and axles. The one caveat is the 4BT Cummins’ similar overall height to the 6BT, which makes oil pan-to-axle clearance a concern in some applications.
Things a 6BT Swap Needs—Space!
In addition to a 3/4-ton or larger frame, suspension, and axles being ideal, the 6BT calls for roughly 38-inches of clearance between the vehicle’s firewall and radiator. It can be squeezed into 35-36-inch openings, but an electric fan will likely be in store (as well as deleting the factory fan idler pulley). Of course, an air-to-air intercooler shrinks available space even further. The clear options here are to either go non-intercooled or invest in a water-to-air alternative. Highly efficient, we’ve seen the latter intercooler option used in a wide array of tight engine bays, and even on a ’39 Ford DeLuxe sedan. Custom air boxes, radiators, and tightly packaged or remote-located heat exchangers are also par for the course when shoehorning a 6BT into place.
| Performance Potential (Advantage 6BT) Dimension Comparison | ||||
| Engine: | Length: | Width: | Height: | Weight: |
| Cummins 4BT | 30.6-inches | 24.6-inches | 37.7-inches | 745 - 782 lbs |
| Cummins 6BT | 35-inches | 24.9-inches | 37.7-inches | 1,100 lbs |
Performance Potential (Advantage 6BT)
With 300 hp being obtainable with a few simple hand tools, 500hp with minimal investment, and 800 hp before the factory rod bolts become a concern, the 6BT is the clear winner when it comes to chasing horsepower. Add in the inline-six mill’s inherent balance and you’ll generate added horsepower smoothly vs. the I-4’s rougher idle and operation. In addition to its larger displacement, the extra cubic inches of the 6BT Cummins allow it to spool a larger single turbocharger much more effectively. Thanks to arguably being the most popular engine in diesel motorsports, a lively aftermarket supports the 6BT in virtually every way. With billet-aluminum blocks and even heads on the market now, the sky is essentially the limit for making horsepower with the 6BT.
Replacement Part Availability
Even though many parts are interchangeable, the 6BT’s much heavier on-road presence and higher production quantity mean running to a local salvage yard for a replacement part will likely be met with greater success than if you’ve got a 4BT. For example, while the word is definitely out (and has been for 20 years) that the Bosch P7100 is the most desirable injection pump for either application, six plunger cores are much more commonly found at the salvage yard. Further, the road-going to an industrial ratio of 4BT engines isn’t nearly as tilted toward on-highway applications, which means sourcing the right parts to make an industrial 4BT road-worthy can add time, complexity, and cost to any swap project.
No Matter What, Beware of The Junkyard 4BT/6BT
Trust us, a 30-year-old take-out engine might not get you very far as a direct drop-in. No matter what, have your junkyard Cummins checked out by a professional before you drop it into your engine bay. At a minimum, plan on a compression test, a re-seal, and a thorough once-over before you deem your 4BT or 6BT ready for operation. This is one of the reasons we at Big Bear Engines do so much business with the Jeep and off-road community. Our long-block and extended long-block programs, available for either Cummins power plant, allow you to start with a clean slate and removes all guesswork from the equation.
All The Rest Of It (Transmission, Transfer Case, Axles, Suspension)
A heavy-duty chassis, suspension, and drivetrain can’t be stressed enough in any Cummins swap, especially if you’re opting for a 6BT. This is where the right donor truck makes so much sense. For instance, scoring a second-generation (’94-‘98) Ram 2500 or 3500 could potentially yield you a Dana 60 front axle and coil springs (if four-wheel drive), a leaf spring Dana 80 rear, the strongest pickup truck frame from that era, and a transmission that’s ready to accommodate the Cummins (be it the 46RH, 47RH, 47RE autos, or the NV4500 manual). If obtaining a donor truck isn’t an option, the components that came on these Rams will give you a great idea as to the kind of components you’ll need to support your engine choice.
