Is the 6.7 Cummins a V8? No — It Is an Inline-Six

# Is the 6.7 Cummins a V8? No — It Is an Inline-Six The 6.7-litre Cummins Turbo Diesel is an inline-six. Six cylinders in a single row, not two banks of four. The question comes up constantly, and the reason is that the trucks this engine goes in *also* offer a V8 — just not this one. That settles the search in a line. The rest of this page is the part that changes anything: what the manufacturer's own documents say, the dimensions that sit behind the layout, why the federal certification system treats cylinder arrangement as part of an engine's identity rather than as a badge, and what any of it does for a truck with a trailer on the back. ## The answer, in the manufacturer's own words Ram's published material describes the engine at its introduction without ambiguity: The high-performance 408-cubic-inch inline-six intercooled turbo-diesel is clean and quiet.oem-29 Read that line closely. The displacement is given in cubic inches rather than litres, and the cylinder arrangement is stated outright in the same breath — not implied, not left to the badge on the bonnet. Later material drops the long form and uses the shorthand. Ram describes the torque milestone as reached with the Cummins I-6 Turbo Diesel, now rated at a best-in-class 1,075oem-30, oem-31 lb-ft. A separate Ram release states that the output comes from the Cummins I-6 Turbo Diesel engine. "I-6" and "inline-six" are the same thing. One is the abbreviation, and it is the one that turns up in press material, spec tables and forum posts alike. ## Where the V8 confusion comes from Here is the actual source of the misunderstanding, and it is a reasonable one: the same Ram Heavy Duty trucks offer a V8 — as the **gasoline** engine. Ram's own material names it in the same documents, alongside the diesel: of torque with 6.4-liter HEMI® V-8 engine.oem-27, oem-28, oem-30 That engine is published at 410 horsepower at 5,600 rpm and 429 lb.-ft. of torque at 4,000 rpm.oem-27, oem-28, oem-31 Put the two side by side and the shape of the difference is obvious before you get anywhere near cylinder count. The petrol V8 reaches peak power almost at the top of its rev range and peak torque more than halfway up it. The diesel's headline figure is torque, it is roughly two and a half times larger, and it arrives nowhere near the top of the range. Those are two different tools sold under one bonnet line. So a buyer looking at one truck can be shown a V8 and a Cummins on the same page. Both are engines in that truck. Only one is the diesel, and it is not the V8. Which of them is the common one is not close. Cummins puts it plainly: Nearly 70% of all Ram 2500 and 3500 Heavy Duty pickup owners choose the Cummins Turbo Diesel as their powertrain of choice.oem-12 That is worth being precise about, because the two engines have almost nothing in common beyond the vehicle they sit in. Different fuel, different layout, different cylinder count, different aftertreatment, different service requirements entirely. A maintenance schedule, a parts list or a repair estimate written for one is not a document about the other, and treating them as interchangeable is how people end up buying the wrong filter or quoting the wrong interval. ## What the layout looks like in the engine's own dimensions Cylinder arrangement is not the only thing configuration decides. The proportions of each cylinder matter more to how an engine behaves, and Cummins publishes them. The bore is listed as 107 mm, which the same page gives as 4.21 in. The stroke is longer: 124 mm, or 4.88 in. A stroke longer than the bore is what engineers call undersquare, and it is the most useful pair of numbers on the page for predicting how an engine will feel. A longer stroke means the piston travels further for every rotation and pushes on the crankshaft through a longer lever. That trades away the ability to rev — long-stroke engines run out of breath early — in exchange for turning effort at low engine speed. The launch specification sheet puts a number on the trade: maximum high-idle engine speed is given as 3,500 rpm. For comparison, the petrol V8 in the same truck is making its peak power at a crank speed the diesel is not built to reach. The geometry is also why the diesel's block is tall and narrow rather than short and wide, because six long-stroke cylinders in a row take up length and height instead of width. An inline layout has one bank of cylinders by definition, so the engine carries one cylinder head and one exhaust manifold run rather than a pair of each. Fewer of those parts means fewer of the joints, gaskets and sensors that come with them, which is part of why the layout persists in heavy-duty applications long after passenger cars abandoned it. None of that comes for free. Cummins publishes the engine wet weight of the 2021 version at 1,070 lbs. The block material is part of the answer to that weight. Cummins describes the change this way: A Compacted Graphite Iron (CGI) material for the block allows us to increase the cylinder pressure capability while reducing the overall weight by almost 60 lbs. Theoem-10 same 2019 revision brought a new 29,000 psi (2,000 bar) fuel pump.oem-10 Higher cylinder pressure capability and higher injection pressure are the two levers behind most of the output this engine has gained without growing, and a stiffer block is what makes both survivable. ## Why the six runs smoother than a cylinder count suggests The inline-six is the layout that cancels its own primary and secondary shaking forces without a balance shaft, because each cylinder has a mirror image on the other side of the crank. That is a large part of why the configuration survived in trucks: the balance comes from the arrangement rather than from parts bolted on to fix it. Ram's engineers still spent real effort on the rest of the noise, and the launch release counts it up. The engine incorporates several new features that contribute to a 50-percent quieter engine and a 3 Dba reduction in cabin sound levels.oem-29 The list of what did it is specific: New engine mounts, a constrained-layered oil pan, intake silencer and engine-block shield create a quiet cabin environment.oem-29 Even the crankshaft was part of the acoustic work: machined crankshaft counter weights significantly contribute to quiet acceleration.oem-29 That last detail is the layout showing through. Counterweight machining matters on a six-in-a-row because the rotating assembly is long, and a long crank is where residual imbalance turns into noise you can hear from the driver's seat. ## Federal rules treat cylinder arrangement as identity, not marketing There is a place where "is it a V8" stops being a matter of opinion and becomes a formal definition. Federal emissions certification groups engines into families, and the rule sets out what engines must share to belong to the same one: To be classed in the same engine family, engines must be identical in all the respects listed.oem-14 One of those respects is architecture, and the regulation supplies its own examples: The cylinder block configuration (air-cooled or water-cooled: L-6, 90 deg., V-8, and so forth).oem-14 So layout is one of the attributes the certification system uses to decide whether two engines are the same engine at all. An inline six and a V8 cannot be certified as one family however similar the rest of the hardware is. The regulator's shorthand for an inline six-cylinder is "L-6" — one more abbreviation for the same architecture, this one sitting in the Code of Federal Regulations rather than in a brochure. Certification is not an abstraction for an owner either. At the engine's introduction Ram published the design target for its aftertreatment: The emissions system is designed for a government-certified 120,000 miles.oem-29 What that system consists of was published alongside it, and it is the other half of why the engine is built the way it is. On the intake side, decreasing NOx emissions is a next-generation cooled Exhaust Gas Recirculationoem-29 system, and Ram describes how it is arranged: EGR system is water cooled, with the air reintroduced into the intake system, keeping turbo and intercooler components clean, which increases the engineoem-29's durability. On the exhaust side, reducing particulate matter is achieved through a self-cleaning Diesel Particulate Filter (DPF).oem-29 Both of those sit in one exhaust stream and one intake path, which is a simpler routing problem on a single bank of cylinders than on two. Architecture does not stop mattering once the exhaust leaves the head. ## The lineage is inline-six too This is not a recent layout choice, and it is not one Cummins made only once. Cummins engines have been an option in RAM pickups since 1989.oem-06 The engine that established the name in these trucks was the same configuration: the Cummins 5.9-liter I-6 in heavy duty applications.oem-29 The first one made an impression on the strength of the same characteristic the current engine is bought for: With 400 pounds-feet of torque, it roared past the competition.oem-06 Cummins describes the arc of it in one line: From our 5.9-liter, 160-horsepower beginnings to the 6.7-liter, 400-horsepower engines available in today's RAM pickups.oem-06 The 5.9 became the 6.7 by growing both cylinder dimensions rather than by changing shape. Ram's introduction release gives both sets: featuring a 107mm bore by 124mm stroke (versus 102mm bore by 120mm stroke for the 5.9-liter engine). Theoem-29 two engines are closely related in hardware, not only in layout: The new engine retains more than 40 percent of its components from the 5.9-liter engine.oem-29 That is the useful way to read the history. The current engine is a bored-and-stroked development of the one before it, which was an inline-six, which is why anyone who has owned these trucks across generations has owned the same architecture the whole time. ## The rival diesel is an inline-six as well If the inline-six were a Cummins quirk, the V8 question would make more sense. It is not. GM's own light-duty diesel is described on its engine page as follows: The LZ0 Duramax 3.0L turbo diesel is the second generation inline-six turbo diesel over the previous LM2.oem-32 Two manufacturers, two classes of truck, the same cylinder arrangement — because the reasons for it are physical rather than stylistic. The heavy-duty Duramax is worth a moment as well, because its numbers make the badges look misleading. GM publishes that engine's displacement as 6604 CC (403 CI). Cummins' figure at the 6.7's launch, from the specification sheet in the same release, is 408 cubic inches, or about 6,690 cubic centimetres. So the engine badged 6.7 displaces roughly five cubic inches more than the one badged 6.6, which makes capacity the least interesting difference between them. Proportion is the interesting one. GM lists its heavy-duty diesel at Bore x Stroke: 103 x 99 mm. That engine's bore and stroke are within four millimetres of each other, near enough to square. The Cummins' stroke is seventeen millimetres longer than its bore. Two roughly equal capacities, two different decisions about where in the rev range the effort should appear — and that difference in geometry says more about how each truck drives than the cylinder count does. ## Why the layout matters with a trailer behind you Configuration is not trivia here. It is part of why the engine behaves the way owners describe. Torque low in the rev range is what pulls a loaded trailer away from a stop and up a grade without a downshift. It is why a long-stroke six is the standard answer in this class of truck, and it is the number the manufacturers lead with. Ram's 2021-generation material puts the top engine at 420 hp and 1,075 lb-ft of best-in-class torque.oem-07 That rating is not universal across the range, which is where most disagreements about "what the 6.7 makes" come from: This high-output (HO) rating is offered exclusively in the RAM 3500 equipped with the Aisin transmission option. Theoem-07 volume configuration is rated lower: For RAM 2500 and 3500 (equipped with a 68RFE Auto) HP and torque have increased to 370 hp and 850 lb-ft to provide the right power for these models.oem-07 Cummins' current page for the engine in the Ram 2500 and 3500 lists Power 430 hp. Two owners can both be right about very different figures, because the rating follows the chassis and the transmission rather than the engine alone. Reading a specification without its configuration attached is how most comparisons go wrong before they start. What the engine is asked to do with that torque is published as well: a towing capacity of 35,100 pounds and a payload capacity of 7,680 pounds.oem-27, oem-28, oem-30 The class milestone behind those numbers is worth the context: The 2019 model year introduced HD pickup buyers to 1000 lb-ft of torque for the first time ever.oem-12 The layout does work on the way down a grade, too. The integrated exhaust brake uses the turbocharger rather than a separate device, and Ram published the result: Testing has shown more than a 30 percent improvement in retarding torque at 2000 rpm compared with traditional brake exhaust methods.oem-29 The turbocharger that does it is described this way: VGT utilizes 16 fixed vanes and a sliding yoke.oem-29 A single exhaust manifold run feeding one turbocharger is straightforward plumbing on an inline-six. On a V-configuration engine the exhaust leaves in two directions and has to be collected before it can drive anything, which is why V8 diesels of the era went to more elaborate turbocharger arrangements to do the same job. Longevity is the other thing the architecture buys. At launch Ram published life-to-major overhaul intervals of 350,000 miles. ## The short version Six cylinders, one row. The V8 in a Ram Heavy Duty is the petrol engine, and it is a different tool for a different job. If you are looking at a diesel in one of these trucks you are looking at an inline-six, and you have been since the 5.9. The layout is stated in the manufacturer's own launch material, carried in the "I-6" shorthand ever since, used by the rival brand's diesel, and treated by federal certification rules as part of what makes an engine that engine. --- ## Sources 1. Stellantis North America media newsroom — 6.7L Cummins introduction release and specifications, https://media.stellantisnorthamerica.com/newsrelease.do?id=6229&mid=97 2. Stellantis North America media newsroom — Ram Heavy Duty model-year fact sheets and press kits 3. Cummins Inc. — Cummins 6.7L Turbo Diesel (2021) engine page 4. Cummins Inc. — 6.7L Turbo Diesel for Ram 2500 and 3500, the pickup truck engine overview, and the 2019 Chassis Cab engine page 5. Cummins Inc. — Cummins-RAM history 6. GM Powered Solutions — L5P 6.6L Duramax and LZ0 3.0L Duramax engine pages 7. Electronic Code of Federal Regulations, Title 40 Part 86 — engine family criteria