6.4L vs 6.7L: Which Power Stroke is Really Better?
There might not be a better way to illustrate polar opposites within the same engine lineage than by comparing the 6.4L Power Stroke with the 6.7L Power Stroke. Thanks to its high volume Siemens common-rail fuel system and factory-installed compound turbocharger arrangement, the Navistar-built 6.4L was a great platform for making horsepower, but it shared much of its architecture with the 6.0L that preceded it. On the other hand, Ford’s 6.7L Power Stroke was a ground-up undertaking from FoMoCo that practically reinvented the wheel in light-duty diesel enginery. The all-new V8 introduced the pickup segment to a compacted graphite iron crankcase, reverse-flow cylinder heads, water-to-air intercooling and reached the 400hp and 800 lb-ft of torque thresholds before GM and Dodge could.
The 6.4L Power Stroke was the third, and last, Navistar-built turbo-diesel engine for Ford. The cast-iron block came from the factory putting out 350hp and 650 lb-ft of torque, with its 3.87-in bore and 4.13-in stroke creating 390 ci of displacement. Ford fully took over the reins of the 6.7L Power Stroke, creating it in-house. For it, they upped the power numbers to 390hp and 735 lb-ft of torque from a deep-skirt, compacted graphite iron block that created 406 ci of displacement.
With nearly 12 years of seat time in the 6.4L and roughly nine years since the 6.7L’s debut, both power plants have been out long enough to draw a host of conclusions. As far as its emissions system, fuel system and overall engine longevity is concerned, the 6.4L leaves a bit to be desired—and repairs can be extremely spendy. However, in performance applications where big horsepower is the name of the game, the 6.4L is tough to beat. We’ve seen 700rwhp squeezed through the OEM turbochargers and the factory connecting rods capable of handling 900rwhp. As for the 6.7L, turbo failure was common on ’11-’14 engines, the connecting rods are a known weak link (with 650 to 700rwhp being their generally accepted limit) and emissions system-related issues come and go. That said, hard-part failures are few and far between on the 6.7L Power Stroke, and the engine seems to age much better than the 6.4L does.
For an apples to apples comparison between the two most powerful Power Strokes to ever grace Ford Super Dutys, keep scrolling.
Let’s Compare the Hard Facts
|6.4L Specs||6.7L Specs|
|Displacement:||390 ci||406 ci|
|Bore:||3.87 inches||3.90 inches|
|Stroke:||4.13 inches||4.25 inches|
|Block:||Cast iron with bed plate||Deep-skirt, compacted graphite iron (CGI)|
|Heads:||Cast iron with four 16mm head bolts per cylinder (with sharing)||Cast-aluminum, reverse-flow with six 12mm head bolts per cylinder (with sharing)|
|Valvetrain:||Overhead valve, four valves per cylinder||Overhead valve, four valves, four rocker arms and four pushrods per cylinder|
|Injection System:||Siemens high-pressure common-rail||Bosch high-pressure common-rail|
|Factory Horsepower:||350hp at 3,000 rpm||390hp at 2,800 rpm (initial), 400hp (H.O. reflash), 450hp (‘18+)|
|Factory Torque:||650 lb-ft at 2,000 rpm||735 lb-ft at 1,600 rpm (initial), 800 lb-ft (H.O. reflash), 935 lb-ft at 1,800 rpm (‘18+)|
6.4L Cast-Iron Block With a Bed Plate
One of the best parts on the 6.4L is something it shares in common with its 6.0L predecessor: a bed plate. By tying directly in to the block itself, traditional main bearing caps are eliminated, as is main cap walk. Main cap walk was highly common on the 7.3L Power Stroke and it also occurs on the 6.7L Power Stroke, although not until roughly 700rwhp. The 6.4L’s bed plate is cinched down via four bolts per main.
A Segment First: CGI Crankcase
Cast from compacted graphite iron by Tupy, the 6.7L Power Stroke’s deep-skirt block reduces both weight and NVH over the 6.4L’s cast-iron crankcase yet is considerably stronger. The CGI block, in addition to the aluminum cylinder heads (more on that in a bit) is a big reason why the 6.7L weighs in some 160 pounds less than its predecessor. There is no bed plate to secure the crankshaft, but nodular iron six-bolt main caps are employed. All 6.7L Power Stroke engines are assembled in Ford’s Chihuahua, Mexico plant.
Strongest OEM Power Stroke Rods Ever Offered (6.4L)
The powdered-metal connecting rods used in the 6.4L are some of the stoutest you’ll find in any light-duty diesel engine. They’re capable of handling an impressive amount of abuse in stock form (think 900rwhp and torque in excess of 1,500 lb-ft). However, on the small end of the rod you’ll find the weakest link in the 6.4L: its pistons. They’re known to crack due to age, accumulative abuse and the fact that they feature a lip design in the fuel bowl that retains heat.
Powdered-Metal, Cracked-Cap Rods (6.7L)
While nowhere near the size of the rods used in the 6.4L, the 6.7L’s Mahle-supplied connecting rods are still good to 650 to 700rwhp before they give. They feature a 45-degree, cracked cap design and are made from powdered-metal. The 6.7L’s cast-aluminum pistons come from Federal Mogul and entail a 16.2:1 compression ratio—the lowest of any Power Stroke engine ever offered.
Reverse-Flow Heads (6.7L)
Ford’s 6.7L Power Stroke wasn’t exactly first-to-market with its use of cast-aluminum cylinder heads, but its design is a first for the truck segment. The reverse-flow heads take in air via the valve covers and expel exhaust gases into exhaust manifolds located in the lifter valley (where you would find an intake manifold on a conventional V8). With the exhaust manifolds mounted in the valley, shorter up-pipes are required to feed the turbo, which aids both responsiveness and emissions. On the intake side, the 6.7L’s heads outflow any Power Stroke heads ever produced and are a big reason why 6.7L-powered Fords can make 650rwhp without the need to upgrade the injectors.
Busy Valvetrain (6.7L)
Inside the 6.7L Power Stroke’s reverse-flow heads you’ll find a hive of activity. Every valve has its own rocker arm and pushrod. This means four valves, four rockers and four pushrods per cylinder—no more floating valve bridges (direct your attention to the bottom left in the image above). In addition to quieter valvetrain operation, each rocker having its own pushrod reduces side loading and decreases pivot wear on the rocker arm itself.
Siemens VDO K16 High-Pressure Fuel Pump (6.4L)
The injection pump is what makes or breaks every diesel, and here the 6.4L definitely has an advantage. With its K16 high-pressure fuel pump said to flow as much as 20-percent more fuel than the Bosch CP3 (’01-’10 Duramax and ’03-current Cummins), and the Bosch CP4.2 (6.7L Power Stroke) flowing approximately 20-percent less than the CP3, the K16 moves a boatload more fuel than the pump used on the 6.7L Power Stroke. The K16 is only capable of producing 24,650 psi, but the massive volume it can supply the rails is how the 6.4L-powered Fords are able to clear as much as 590rwhp with aggressive tuning.
Bosch CP4.2 High-Pressure Fuel Pump (6.7L)
Where the CP4.2 on the 6.7L Power Stroke makes up ground on the K16 is in its ability to produce as much as 30,000 psi of injection pressure (although the CP4.2 did receive more internal stroke for ’15 that yielded 11-percent more fuel volume). Unlike the K16, which is buried within the block and beneath the turbo arrangement on the 6.4L, the CP4.2 is easily accessible at the front of the lifter valley on the 6.7L. Unfortunately, both the CP4.2 and K16 are known to fail any time water (i.e. rust) makes it into the high-pressure system, and when they do the pump can send shrapnel through the rails and into the injectors. This type of failure, which is more common on a 6.4L, typically runs $6,000 to $8,000 for a new fuel system.
6.4L Piezo Vs. 6.7L Piezo
Another key piece to the power-making puzzle exists in the quick-firing piezoelectric injectors used on both the 6.4L and 6.7L. Both engine’s injectors are capable of pulling off five events per combustion cycle (pilot, main and post events), but the 6.4L is fitted with Siemens injectors equipped with six-hole nozzles, while the 6.7L sports Bosch units utilize eight-hole tips. With minimal duration commanded via the PCM (less than two milliseconds of on-time), either injector can support twice the factory horsepower level the engine originally came with.
EGR System Improvements
The EGR systems used on Power Stroke engines have come a long way since the days of the 6.0L. Starting with the 6.4L, two EGR coolers were employed, and the EGR valve saw some operational and durability improvement thanks to a stronger DC motor. However, sludge buildup eventually hampers flow (especially the driver side cooler), and EGR system failures still run rampant. While it too sees its fair share of issues, the EGR system on the 6.7L is the most robust to date. It’s comprised of a hot-side EGR valve, which manages exhaust flow before it enters the EGR coolers (not the case on the 6.4L) in order to reduce soot buildup. Two floating core EGR coolers are packaged together on the passenger side valve cover (shown above), which are designed to expand and contract without rupturing.
6.7L Adds SCR, Sees MPG Bump & Improved Engine Longevity
Just like EGR, both the 6.4L and 6.7L make use of a diesel particulate filter (DPF) to trap soot in the exhaust system. The difference in the 6.7L’s case is that selective catalytic reduction (SCR) is also part of the equation, which means a chemical mixture composed of urea and water called diesel exhaust fluid (DEF) is injected down stream (post-engine) to lower NOx emissions. A key benefit of SCR is that less fuel is used during active regeneration—the process required to incinerate the trapped particles in the DPF, performed by injecting fuel on the engine’s exhaust stroke—which means better fuel economy, lower emissions and reduced dilution of the engine oil.
Compounds From the Factory (6.4L)
From a performance standpoint, the biggest gift any OEM has given Ford was the compound turbo setup for the 6.4L. Called a series sequential turbo system, it was manufactured by BorgWarner and makes use of a fixed geometry, 65mm charger for the atmosphere unit (low-pressure) and a 52mm variable turbine geometry (VTG) for the high-pressure turbo. While considered small for a set of compounds, when an ’08-’10 Ford is aggressively tuned it can produce more than 50 psi of boost and allow the truck to send as much as 590hp to the wheels. On nitrous, we’ve seen the compact turbo configuration support just over 700rwhp.
Chokehold Turbo (’11-’14 6.7L)
Transitioning from the 6.4L’s awesome compound turbo system into the 6.7L, Ford fans’ expectations were high—but the single sequential turbo aboard ’11-’14 engines hardly delivered. The Garrett GT32 SST packed two 46mm inducer compressor wheels into a dual inlet compressor housing and made use of a tiny 64mm turbine wheel on the other end of the shaft. To try to get the charger to live, Garrett employed a ball bearing center cartridge to support the turbine shaft, as well as added an externally actuated wastegate. Although the small GT32 provided great transient response and tremendous low-end torque, at stock power levels the 6.7L could zing the charger to 130,000 rpm and once an aggressive tune was involved shaft speed could increase to 160,000 rpm or more. Overspeed failures are very common with this turbo. Unlike the power potential of the 6.4L’s turbo arrangement, on its best day it can hardly support 500rwhp.
Back in the Game (’15-Present 6.7L)
The second biggest gift any OEM has given Ford is the Garrett GT37 family variant that replaced the tiny, failure-prone GT32 SST. It brought a traditional, single compressor wheel design (61mm) back into play, along with a larger turbine wheel, while doing away with the wastegate and ball bearing center section. Not surprisingly, the factory turbo upgrade was coupled with a higher-flowing CP4.2 high-pressure fuel pump and optimized injector nozzles, which yielded a bump in both horsepower and torque for model year 2015 (400hp to 440hp and 800 lb-ft to 860 lb-ft). The GT37 has proven much more reliable than its GT32 SST predecessor and can support 540rwhp—not very far off the pace of the compound turbo’d 6.4L.
Water-to-Air Intercooler & Two Separate Cooling Systems (6.7L)
Up until the 6.7L Power Stroke came along, sled pulls were really the only place you could find water-to-air intercoolers being used on diesel pickups. That all changed when Ford brought its diesel-powered Super Dutys to market in 2011. The water-to-air intercooler is cooled via a secondary cooling system separate from the engine’s. This powertrain secondary cooling system (as Ford calls it) is also tasked with cooling the EGR circuit, transmission fluid, fuel cooler and operates at a relatively low 122 degrees F. The primary cooling system is dedicated to the engine and runs at approximately 194 degrees F. Both cooling systems have their own radiator, water pump, thermostats and degas bottle (secondary refill shown above). While added complexity isn’t always a good thing, so far the dual cooling systems on the 6.7L have proven fairly robust. The primary system radiators leak from time to time, but then, that was an even bigger problem on the 6.4L.