L5P Duramax vs 6.7 Cummins: What Each Manufacturer Actually Publishes

The headline numbers, from each manufacturer

GM publishes the L5P rating with its engine speed attached: HORSEPOWER: 470 HP (350 KW) @ 2800 RPM*^[1]

The torque figure is given the same way: TORQUE: 975 LB-FT @ 1600 RPM*^[1]

GM's own summary of the engine puts both together: L5P is even more capable with an impressive power rating of 470 horsepower and 975 lb.-ft. of torque^[1]

The asterisk on those numbers is not decoration, and GM says what it covers: *As tested in the Chevrolet Silverado Heavy Duty and GMC Sierra Heavy Duty.^[1]

For the 6.7L Cummins, the current pickup rating on the latest-generation engine page reads: Power 430 hp^[2]

And the torque figure: Torque 1075 lb-ft^[2]

So on published peaks the Duramax leads on horsepower and the Cummins leads on torque, by roughly forty of one and a hundred of the other. That is the whole of what the headline numbers tell you, and it is less than most comparisons claim from them.

Displacement: the badges are almost a coincidence

The two engines are sold as a 6.6 and a 6.7, which sounds like a tenth of a litre between them. In cubic inches the gap is smaller still.

GM publishes the L5P at Displacement: 6604 CC (403 CI)^[1]

Ram's specification sheet for the Cummins at its 2007 introduction gives 408 cubic inches, or about 6,690 cubic centimetres.^[3]

Five cubic inches apart, out of more than four hundred. Whatever separates these engines in behaviour, capacity is not it — which is worth establishing early, because "bigger engine" is the most common shorthand in this argument and it is not describing anything real.

Bore and stroke: where the two designs actually part company

The dimensions of each cylinder tell you far more than the total, and both manufacturers publish them.

GM lists the L5P at Bore x Stroke: 103 x 99 mm^[1]

Cummins gives the 6.7L bore as 107 mm, or 4.21 in.^[4]

Its stroke is considerably longer: 124 mm, or 4.88 in.^[4]

Read those together and the two design philosophies separate cleanly. The Duramax's bore and stroke are within four millimetres of each other — near enough to square, a shape that breathes well and is happy turning higher revolutions. The Cummins' stroke is seventeen millimetres longer than its bore, which is what engineers call undersquare: the piston travels further per rotation and pushes on the crankshaft through a longer lever.

That single difference predicts most of what the headline numbers then show. A longer stroke produces more turning effort at low engine speed and runs out of breath earlier; a squarer engine gives up some of that low-speed effort and keeps making power further up the range. The Duramax leading on horsepower while the Cummins leads on torque is not a coincidence of tuning. It is the geometry showing through in the results.

Where the peaks arrive, and how wide the plateau is

The more useful comparison is engine speed, because it describes how each engine behaves with a trailer behind it.

GM publishes the L5P's peak torque at 1600 rpm and its peak power at 2800 rpm, and it goes one better than a peak by publishing the width of the plateau: ninety percent of peak torque is produced from 1,550 rpm thru 2,850 rpm^[1]

That is the most practically useful number on either page. A 1,300 rpm window holding ninety percent of maximum torque means the engine does not need to be kept in a narrow band to pull well, which is what actually matters when a grade steepens and the transmission has to choose a gear.

GM also states where the engine stops: Maximum Powered Speed: 3450 rpm^[1]

The Cummins figures that carry an engine speed come from Ram's launch specification for the engine, which describes the hardware as introduced rather than a truck on a lot today. At its introduction, the 6.7L Cummins was published at 350 horsepower at 3,000 rpm and 650 lb.-ft. of torque at 1,500 rpm — replacing the 5.9-litre engine that came before it.^[3]

The same specification sheet gives the ceiling: maximum high-idle engine speed of 3,500 rpm.^[3]

Those two ceilings — 3450 rpm and 3,500 rpm — are close enough to be treated as the same answer, and it is a low one by petrol standards. Both of these engines do their work in the bottom half of a rev range that a gasoline V8 would consider idle-adjacent. The torque peaks, at 1600 rpm for the current L5P and 1,500 rpm for the Cummins as introduced, are close as well. Where the engines genuinely differ is above those peaks, and that is the part of the curve that gets you up a long grade without a downshift.

Engine braking is half of towing, and both pages address it

Going down a grade with weight behind you asks a different question of an engine, and each manufacturer answers it in its own vocabulary.

GM publishes a second speed limit specifically for it: Maximum Braking Speed: 4800 rpm^[1]

That figure is far above the powered limit because the engine is being driven by the truck rather than driving it, and it tells you how much retarding work the hardware is rated to absorb.

Ram approached the same job through the turbocharger. At the engine's introduction an integrated exhaust brake is available direct from the factory.^[3]

The launch release published what that bought: Testing has shown more than a 30 percent improvement in retarding torque at 2000 rpm compared with traditional brake exhaust methods.

The hardware doing it is a variable-geometry unit, described at launch as follows: VGT utilizes 16 fixed vanes and a sliding yoke^[3]

Two different framings of one problem. GM gives a rated speed for the braking mode; Ram gives an improvement in braking torque at a working engine speed. Neither converts into the other, which is why descent performance is worth a test drive on a real hill rather than a spec-sheet argument.

Each figure is a configuration rating, not an engine's identity

This is the caution that sinks most online comparisons, and it applies to both brands.

On the Cummins side the top rating is not available across the range: This high-output (HO) rating is offered exclusively in the RAM 3500 equipped with the Aisin transmission option.^[4]

The 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.^[4]

And the top figure itself is a transmission-specific one: The new high-output engine boasts 420 hp and 1,075 lb-ft of best-in-class torque.^[4]

A fourth rating exists for the commercial chassis: In 2019, RAM 3500, 4500 and 5500 Chassis Cab models equipped with a Aisin automatic, see an increase in horsepower and torque to 360 hp and 800 lb-ft.^[5]

Four published numbers, one engine. The transmission is doing more of that work than people expect, and the launch release said as much about the gearbox it arrived with: The new six-speed automatic transmission features a best-in-class gear ratio spread of 5.16:1^[3]

A wide ratio spread lets the same engine cover a launch gear and a highway overdrive, which is why a rating quoted without its transmission is only part of a description.

GM's asterisk does the same job from the other direction, naming the two trucks the rating was measured in. So a Ram 2500 owner and a Ram 3500 owner can quote different Cummins figures and both be reading the manufacturer correctly, while a Silverado owner is quoting a number measured in their own truck.

Comparing one manufacturer's best rating against another's mid rating produces whatever answer you started with. Any comparison worth anything states the chassis and the transmission alongside the number.

A peak is also not a duty cycle. Sustained load is a different question from a maximum, and it is the one a work truck asks every day. Both of these engines are rated for heavy-duty pickups, and a peak figure does not settle which of them suits a given job.

Both engines have moved, and not from the same starting point

The Cummins' current rating is a long way from where the 6.7 began, and that context is easy to lose in a comparison of today's numbers.

Set the launch figures against the current rating and the scale of the change is clear: torque is up by more than four hundred lb-ft over the engine's life, on the same displacement and the same bore and stroke.

Some of how it got there is published. The 2019 revision changed the block itself: 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.^[5]

The same revision raised injection pressure with a new 29,000 psi (2,000 bar) fuel pump.^[5]

Higher cylinder pressure capability and higher injection pressure are how a diesel gains output without gaining size, and both of those changes are the reason a modern 6.7 is a different engine from a 2007 one wearing the same badge.

The class threshold it crossed on the way is worth stating: The 2019 model year introduced HD pickup buyers to 1000 lb-ft of torque for the first time ever.^[6]

GM frames its current engine the same way, against its own earlier versions rather than against a rival, describing the latest L5P as more efficient compared to the previous 2023 and older L5P engines.^[1]

Which means a comparison assembled from figures found in two different years is not comparing these two engines. It is comparing one manufacturer's current hardware against another manufacturer's history.

A published rebuild interval

One durability figure is stated outright rather than implied. At launch Ram published life-to-major overhaul intervals of 350,000 miles.^[3]

That is a design interval to a major overhaul rather than a coverage promise, and it is not a prediction about any particular truck. It is still a useful number, because it tells you what the engine's bottom end was engineered around, and it is the sort of figure that separates a heavy-duty diesel from a light-duty one.

The same release put a design life on the emissions hardware as well: The emissions system is designed for a government-certified 120,000 miles.^[3]

Those two numbers describe different things and are worth keeping apart. One is when the engine is expected to want a rebuild; the other is what the aftertreatment was certified to survive, and it is the shorter of the two by a wide margin.

What the pages say about living with each engine

Peak figures are the least of what an engine costs its owner, and the Cummins pages carry a few numbers that bear on that directly.

Weight is one of them, and it is published per application rather than once. The 2021 pickup engine is listed at Engine Wet Weight 1,070 lbs^[4]

The 2019 chassis-cab version of the same engine family is listed at Engine Wet Weight 1026 lb (465 kg)^[5]

Several hundred pounds of engine over the front axle is a real part of how a heavy-duty truck rides and steers unloaded, and it is one of the trade-offs that comes with a long inline-six rather than a shorter engine.

Two service items changed in a way owners notice. The first removed a recurring labour cost: 2019's Hydraulic Lash Adjusters completely eliminate the need to adjust the overhead.^[5]

The second stretched a consumable: The interval for the Fleetguard breather filter has been extended to 75,000 miles.^[5]

Both of those are the kind of change that never appears in a horsepower comparison and shows up on every invoice. Valve-lash adjustment on a six-cylinder diesel is a labour job, and removing it from the schedule removes a whole shop visit from the ownership arc.

The fuel side got attention in the same revision, and it is worth knowing what the filtration on the truck actually is: the new fuel-delivery system uses Cummins Filtration “filter in filter” NanoNet® technology.^[5]

The 3.0 Duramax is a different class

One clarification, because the name causes genuine confusion: GM's 3.0-litre Duramax is not part of this comparison.

Its published power rating is HORSEPOWER: 305 HP (227 kW) @ 3750 RPM*^[7]

Its torque rating is TORQUE: 495 LB-FT (671 NM) @ 2750 RPM*^[7]

It is also a much smaller engine than the name suggests to anyone thinking in heavy-duty terms: Displacement: 2999 CC (183 CI)^[7]

Its cylinder dimensions are undersquare in the other direction, at Bore x Stroke: 84 x 90 mm^[7]

And its rev ceiling tells you what it was designed for: Maximum Engine Speed: 5000 rpm^[7]

That ceiling is around fifteen hundred rpm above either heavy-duty engine's limit, which is the clearest single indication that this is a light-duty half-ton engine rather than a small heavy-duty one. It is a good engine in that role, and GM's own description of it is worth noting for anyone who assumes the layout follows the badge: The LZ0 Duramax 3.0L turbo diesel is the second generation inline-six turbo diesel over the previous LM2.^[7]

It also delivers its torque low, with 94 percent of that max twisting force available by 1,500 rpm.^[7]

So a "Duramax vs Cummins" argument that quotes 3.0-litre figures against heavy-duty Cummins figures is comparing different classes of truck, and usually different jobs entirely.

The honest summary

On the numbers each manufacturer publishes: the L5P leads horsepower, the 6.7L leads torque, the L5P's torque peak is published at a low 1600 rpm with ninety percent of it held across a 1,300 rpm plateau, and the two engines are within five cubic inches of the same size. The Cummins is the longer-stroke design and its figures follow the chassis and transmission it is fitted to; the Duramax is the squarer one and its figures were measured in the Silverado and Sierra Heavy Duty. Anything more specific than that — sustained-load behaviour, real-world towing, longevity on a particular truck — is decided by how the truck is used rather than by either engine page.

---

Sources

1. GM Powered Solutions, L5P 6.6L Duramax diesel engine — https://poweredsolutions.gm.com/products/engines/l5p-diesel-engine 2. GM Powered Solutions, LZ0 3.0L Duramax engine — https://poweredsolutions.gm.com/products/engines/lz0-engine 3. Cummins Inc., 6.7L Turbo Diesel for Ram 2500 and 3500 — latest generation engine page 4. Cummins Inc., Cummins 6.7L Turbo Diesel (2021) and 6.7L Cummins Turbo Diesel (2019) for Chassis Cab engine pages 5. Cummins Inc., pickup truck engine overview 6. Stellantis North America media newsroom, 6.7L Cummins introduction release and specifications — https://media.stellantisnorthamerica.com/newsrelease.do?id=6229&mid=97