5.9L VS. 6.7L: WHICH CUMMINS IS REALLY BETTER?
Which Cummins, the 5.9L or the 6.7L, is really superior?
When it first debuted for Dodge Rams of the '07.5 model year, the 6.7 Cummins had a lot to prove. The 5.9L it replaced was highly regarded because to its million-mile reliability, great performance potential, and endless availability of aftermarket parts. Fortunately, more than 40% of the 6.7L's internal parts were carried over from the 5.9L, and the injection system practically remained the same.
The popular 5.9 Cummins has a fixed-geometry turbo as a feature. It had a 359 cc displacement at the conclusion of its run, a bore and stroke of 4.02 in and 4.72 in, and 325 horsepower and 610 lb-ft of torque. For the Ram pickups the 6.7L Cummins engine came standard with 350 horsepower and 650 lb-ft of torque. It had a displacement of 408 cubic inches, a bore of 4.21 inches, and a stroke of 4.88 inches. It used a turbo with variable geometry.
The 5.9L owners were unaware of the full range of emissions control equipment that the 6.7L would need to be equipped with. The 6.7L Cummins came equipped with a diesel particulate filter and exhaust gas recirculation (EGR) (DPF). Rams would also add selective catalytic reduction (SCR) to the mix. As a part of the exhaust after treatment system, it would need the use of diesel exhaust fluid (DEF) to reduce NOx emissions.
The compromises have already begun! Due to the aforementioned pollution control components, 6.7L-equipped vehicles' fuel efficiency and general dependability are significantly inferior. However, because of the variable geometry turbocharger it employs, the 5.9L is significantly less responsive than the 6.7L at low rpm. The 6.7L engine also features a highly effective exhaust brake function. The 6.7L's larger displacement also helps it produce considerably more torque, which is ideal for truckers who routinely tow a trailer.
Although many still believe that the outdated 5.9L is the only Cummins that matters, the 6.7L is essentially a bigger, better version of the 5.9L. However, is the 6.7L engine actually better? To try to balance this 50/50 split, we've listed the key advantages and disadvantages of each engine below.
The 5.9L engine was manufactured with a number of different configurations (12-valve with mechanical injection, 24-valve with electric-over-mechanical injection, and 24-valve with common-rail injection), but for the purposes of this article, we're concentrating on the 24-valve, common-rail version.
The Common-Rail Made For High Pressure
A high pressure common-rail fuel system is used by all 6.7L mills and 5.9L Cummins engines.. The system's core component is a Bosch CP3 injection pump, which is in charge of generating and maintaining a fuel pressure of more than 23,000 psi. The CP3 that graces the 6.7L is said to flow more, and even Bosch admits that this is only a rumor. We have never been able to confirm this, though.
According to Bosch statistics, the 5.9L and 6.7L CP3 pumps flow 180 to 185 lph at 3,500 rpm. However, the 6.7L Cummins' common-rail system's fuel rail, fuel rail feed lines, injector lines, and crossover tubes are all 50% greater than those on the 5.9L. The 6.7L's injection system operates at a little bit more pressure than the 5.9L's does, hence an additional 3,000 psi pressure release valve is used.
Similar Injectors
The injectors used in the 5.9L and 6.7L Cummins are actually manufactured and rated for higher pressure despite having similar dimensions (and also requires lower voltage to operate them).
Unlike the 5.9L units, the injectors used in the 6.7L are programmed to the ECM of each specific truck and cannot be shifted into different cylinders (which was possible on the 5.9L).
Fixed Turbo Geometry (5.9L)
For the sake of comparing the stock and stock turbos, the Holset HE351CW found on 5.9L Cummins mills produced between '04.5 and '07 is far less intricate and technologically advanced than that which adorns the 6.7L.
The HE351CW has a 58mm turbine wheel (exducer), a 58mm compressor wheel (inducer), employs an internal wastegate, and is quite durable as long as it is not frequently subjected to 45 psi of boost or more. Nevertheless, turbo lag at low rpm cannot be prevented because to the fixed form design.
High-Speed Variable Geometry (6.7L)
At all engine speeds, variable geometry turbochargers, like the Holset HE351VE on the 6.7L Cummins, give good responsiveness. This turbo has a 60mm compressor wheel (inducer), variable nozzle technology from Holset, and no wastegate. By adjusting the exhaust flow across the turbine wheel, the turbo can run as a smaller unit at lower engine rpms and as a much larger turbo at higher rpms to increase responsiveness (via the nozzles). In reality, this offers the engine's best performance from idle to redline across the whole power range.
Even though it seems like the best of all possible worlds, the HE351VE has significant reliability issues. Additionally, any variable geometry style turbo design would experience the same problems as the HE351VE, such as the turbine side carbon, soot, and coking that prevents the nozzles from operating (i.e. causing them to stick). If the nozzles are stuck open, the truck will be sluggish until the tachometer reads an acceptable number of revs. You're driving a vehicle that, if they keep closed, is really only helpful at lower rpms. Owners of 6.7L vehicles are well aware that the greatest prevention for developing a jammed turbo is to always maintain the exhaust brake activated (which exercises the nozzles).
Performance of Exhaust Brakes (6.7L)
We did indeed say "exhaust brake." Although the HE351VE is a turbo brake in theory, it can produce a large amount of braking power because to its variable geometry design, which minimizes wear on the truck's service brakes. A '07.5-present Ram's 8,000 pounds can be slowed down extremely fast by altering the nozzles' placements to provide more restriction.
In terms of exhaust braking performance, the Holset HE351VE leads all contemporary trucks (including Fords and GMs manufactured from 2011 to the present). Simply put, the HE351VE's powerful and effective exhaust brake function turns 6.7L engine-powered Rams into towing machines.
Emissions System Failures (5.9L Exempt)
While terminology like "EGR" and "DPF" were unfamiliar to 5.9L owners, problems like a jammed EGR valve (induced by carbon and soot buildup) or a plugged DPF were typical for 6.7L owners (unless you count a catalytic converter).
This illustration of a dirty, jammed EGR valve serves as a reminder that, in order to ensure trouble-free running, Cummins advises cleaning the EGR valve (and EGR cooler) every 67,500 miles.
The 5.9L certainly has the advantage in this regard because it was entirely produced without any emissions restrictions.
Blown Head Gasket (6.7L)
Due to its larger stroke (4.88 inches vs. 4.72 inches), the 6.7L generates much greater torque at low rpm than the 5.9L, especially when paired with the aforementioned quick-lighting variable geometry turbocharger. Regrettably, this raises cylinder pressure, which may result in head gasket rupture. Less sealing space exists between the cylinders and water jackets in the 6.7L due to its larger bore, which increases the risk of blown head gaskets.
Although modified engines frequently do it much sooner, standard towing engines have a reputation for lifting the head around the 200,000-mile mark. The 5.9L Cummins, in comparison, experienced extremely few head gasket issues. Owners of 12-valve 5.9L engines have even been known to add compound turbocharger systems, retorque the stock head nuts, and achieve 100 psi of boost without experiencing any head gasket issues.
An Effective Base
The 6.7L block has nearly finished a decade in production and has swiftly established itself as the standard basis in diesel motorsports due to its Siamese cylinder bore design and the displacement advantage it delivers.
Many companies in the upper echelons of truck pulling and drag racing add sleeves to the cylinders to prevent the tops of the cylinder bores from warping and to improve crank stroke. Because of this type of machining, 6.7L blocks often have a total displacement of 390 cubic inches, produce 1,800–3,000 horsepower, and do it in a very reliable manner.
