As you might’ve seen in previous articles, we’ve compared the 7.3L and 6.0L Powerstroke as well as the 6.0L and 6.4 Powerstroke, but today we’re going to 6.4L and the 6.7L Powerstroke. What makes this comparison interesting is that the 6.7L Powerstroke was developed in-house by Ford, rather than Navistar who developed the early Powerstroke engines.
I think it’s safe to say the 6.4L Powerstroke is one of the most hated diesel engines of all time. It had a ton of issues, many of which caused catastrophic failure. For the most part, the 6.4L was designed a throwaway engine, plus it was an absolute pain in the rear to work on and the cab had to be removed for many basic services.
Before the 6.4, Ford and Navistar’s relationship was already on the rocks because of the massive amount of issues caused by the 6.0L Powerstroke. Ultimately, all the issues with the 6.4L Powerstroke led Ford to abandon Navistar and develop its Powerstroke engine in-house. For the most part, there aren’t that many similarities between the 6.4L and 6.7L Powerstroke, mostly because they were designed by different companies entirely.
- Specs: 6.4L vs 6.7L
- Bore: 3.87inches vs 3.90inches
- Stroke: 4.13inches vs 4.25inches
- C/R: 17.5:1 vs 16.2:1
- Block: Cast Iron vs Compacted Graphite Iron
- Heads: Cast iron with four 16mm head bolts per cylinder (with sharing) vs Cast-aluminum, reverse-flow with six 12mm head bolts per cylinder (with sharing)
- Valvetrain: 4V vs 4V with 4 pushrods per cylinder
- Injection: Siemens high-pressure common-rail vs Bosch high-pressure common-rail
- Stock HP: 350HP vs 390 – 450HP (depending on year)
- Stock Torque: 650LB-FT vs 735 – 935 (depending on year)
Not too surprisingly, the bore and stroke are bigger on the 6.7L, which is what gives it that extra 300cc of displacement compared to the 6.4L. Increasing displacement is pretty much the best way to increase power throughout the entire rev-range, so it makes sense why Ford went to the larger displacement. Interestingly enough, Ford went with a lower compression ratio on the 6.7L.
As far as the block goes, the 6.4L is cast-iron and the 6.7L is compacted graphite iron, also known as CGI. (made by Tupy). The CGI material used to construct the 6.7’s block makes it lighter and stronger compared to the 6.4’s cast-iron block. Unfortunately, there is no bed-plate on the 6.7’s bottom end, but we’ll cover that a bit later.
Heads and Injection
Looking at the heads, the 6.4L uses cast iron with four 16mm head bolts per cylinder and the 6.7L uses cast aluminum heads which flow in reverse compared to the 6.4L’s heads. Overall, the 6.7’s heads are lighter and flow better than any other Powerstroke heads, plus the reverse layout leads to a much better throttle response.
On top of the cast aluminum construction and reverse flowing design, the 6.7 uses a fairly unique valvetrain setup. Both engines use four valves per cylinder, however, the 6.4L only uses two pushrods per cylinder with rocker arms with floating valve bridges. The 6.7L uses four pushrods per cylinder with four rocker arms. Ultimately, this makes the 6.7’s valvetrain very quiet.
Taking a closer look at the heads also shows the difference in the injection systems. The 6.4L uses a Siemens K16 high-pressure fuel pump, where the 6.7L uses a Bosche CP4.2 high-pressure fuel pump. Both fuel have their pros and cons, however, the K16 pump on the 6.4L flows well over 20% more than the CP4.2 pump.
Where the CP4.2 makes up for its lack of total flow is with its ability to produce up to 30,000 psi of injection pressure. Another pro of the CP4.2 pump is that it’s way easier to replace than the K16 pump which was located under the compound turbocharger system on the 6.4L. Unfortunately, both pumps are known for failing any time water gets in the system and can lead to catastrophic failure.
Further down the injection system, we have the injectors. Both the 6.4L and the 6.7L use piezoelectric injectors which are capable of supporting upwards of double the factory power output. The 6.4L is fitted with Siemens injectors equipped with six-hole nozzles, while the 6.7L sports Bosch units utilize eight-hole tips. Either way, both engine’s injectors are pretty good.
Block and Internals
Taking a closer at the block, the big differences are the materials used and the design. As stated earlier, the 6.4L uses a cast-iron block with a bedplate and four bolts per main, which makes the bottom end pretty rigid. The 6.7L uses a deep-skirt CGI block without a bed-plate with nodular iron six-bolt main caps.
The connecting rods are one of the strong points for the 6.4L since it uses insanely strong and beefy powdered-metal rods. The 6.4’s rods are capable of withstanding close to 1000hp. The 6.7L, on the other hand, uses powdered-metal cracked-cap rods. While they might not be as insanely strong as the 6.4L’s rods, they’re of withstanding over 700whp just fine.
On top of the rods, you’ll find the pistons, which is one of the weak points for the 6.4L. To put it simply, the rods on the 6.4L are known for cracking due to age, abuse, and the fact that they feature a lip design in the fuel bowl that retains heat. The 6.7L’s pistons don’t have this cracking issue at all and they’re decently strong.
Like pretty much all modern diesel engines, both the 6.4L and the 6.7L are turbocharged. The 6.4L uses a compound turbocharger setup, which combines two turbos in series. This compound system was manufactured by BorgWarner and combined a 65mm fixed geometry turbo with a 52mm variable geometry turbo. When combined these turbos produced over 40psi of boost in totally stock form. To put it simply, the compound setup on the 6.4L is well known for being able to produce a massive amount of power.
The 6.7 has had two turbos throughout its lifetime. The first turbo is the Garrett GT32 SST, which is a very interesting single sequential design, using two 46mm compressor wheels into a dual inlet compressor housing and made use of a tiny 64mm turbine wheel on the exhaust side. A cool feature of this turbo is that it is a wastegated variable geometry turbocharger. Traditionally, variable vane turbochargers do not require a wastegate system, but rather prevent turbocharger over-speeding by opening vanes in the turbine housing.
Ultimately, the Garrett GT32 SST provided great throttle response and a massive amount of low-end torque, but it lacked top-end power and once an aggressive tune was involved, shaft speed could increase to 160,000 rpm or more, resulting in overspeed failures.
In 2015 to current 6.7L Powerstroke’s you’ll find a Garrett GT37 turbo. The newer GT37 turbo is more reliable than the GT32 SST and can support more power, however, it still can’t quite match the insane potential of the 6.4’s compound turbo setup.
The EGR systems used on Powerstroke engines have come a long way since the days of the 6.0L. Starting with the 6.4L, two EGR coolers were used and the EGR valve saw some operational and durability improvement, however, sludge buildup eventually reduces flow and can lead to failure.
The EGR system on the 6.7L is the most robust to date, however, it’s not perfect. The 6.7’s EGR system is comprised of a hot-side EGR valve, which manages exhaust flow before it enters the EGR coolers to reduce soot buildup.
Just like EGR, both the 6.4L and 6.7L make use of a diesel particulate filter 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 diesel exhaust fluid is injected downstream to lower emissions output.
Which One is Better?
Overall, both engines have a lot of potentials when modified correctly, however, the 6.4L is plagued with various issues that can lead to catastrophic failure. At the end of the day, the 6.4L is one of the most diesel engines ever used in a modern diesel truck, solely because it’s a pain to work on, and it’s designed as a throwaway engine.
The 6.7 was designed in-house by Ford since they got sick of all the issues caused by Navistar’s design choices on the 6.0L and the 6.4L. I think it’s pretty safe to say the 6.7L is undoubtedly better than 6.4L in almost every way. It makes more power in stock form, it’s more efficient, its lighter, its quieter, and its way more reliable.