In 2008 Ford Motor Company decided to develop a new diesel engine in house and no longer outsource their Super Duty diesel. The new engine, code named Scorpion; would become a ground breaking power plant that could lay the foundation for Ford’s next generations of diesel engines to come. After a year of extensive development, the engine was put into its new home (the Ford F-series) and the engine code named Scorpion, was designated as the 6.7 Power Stroke.
When the 6.7-liter Power Stroke was introduced, so too was the 6R140 Torqshift transmission.
Years later, it is easy to look back and see how obvious it was that this new engine would be a huge success, but it was quite a risky proposition. The current (at that time) diesel market was undergoing extreme change. RAM had been partnered up with diesel engine powerhouse Cummins for 15 years and Cummins was certainly able to wade through all up coming emissions requirements. The Cummins engine has proven it’s self as not only a torque powerhouse, but a very reliable engine. GM was partnered with Isuzu on their Duramax power plant that had been in production since 2001. The Duramax was, also, a proven engine that could produce good horsepower and torque. The industry experts didn’t foresee the Duramax not passing the new upcoming emission levels. Both of these competitors were continuing with legacy programs that customers appreciated, whereas Ford, would be on its third engine in seven years.
Ever since Ford and Navistar introduced the 6.0 Power Stroke in 2003, Ford was plagued with high warranty issues and decreasing customer confidence. “We know that durability and reliability is uppermost in the minds of our Super Duty customers, along with torque and horsepower. So we took a conservative approach that ensured that the new Power Stroke would be absolutely bulletproof while delivering significantly improved power,” said Chris Brewer, chief engineer of the 2011 Super Duty. But starting from scratch, meant they were risking their dominance within the diesel light duty market which translates to a huge amount of revenue.
What was the new engine
Starting with a clean sheet, there were no legacy programs that needed to be continued. So, every aspect of the engine was reviewed under a microscope. Beginning with the foundation of the engine, the block. It was apparent that Ford wasn’t looking to create something similar to what was already in the market.
6.7 Power Stroke Specs
The engine’s basic specs are:
Bore: 3.90 in
Stroke: 4.25 in
Cylinder Head: Aluminum
Valves per cylinder: 4
Turbo: Single Sequential (GT32)
HPFP: Bosch CP4.2
Injectors: Bosch Piezo
Initial Horsepower: 390 hp
Initial Torque: 735 lb.-ft
Adjusted Horsepower: 400 hp
Adjusted Torque: 800 lb.-ft
The current market diesel engine blocks are/were made out of heavy cast iron. While very durable, cost effective, predictable, and fairly easy to work with, Ford, opted to use a new material called compacted graphite iron (CGI). This was the first use of CGI in a diesel engine, but Ford had been using it in some of their gas engines all over the world with great results. CGI has a higher tensile strength than regular cast iron because of the graphite that is added. The graphite particles create a stronger bond, resulting in a material that is almost twice as strong.
Ford was not only looking to increase the power and reliability of the engine, they were hoping to reduce the weight. This could give them better fuel economy but also the ability to increase the trucks payload capacity. With a stronger material, Ford was able to optimize the block design utilizing the stronger material. (Overall engine is about 160 pounds lighter than the previous generation 6.4.)
Instead of using a cradle to hold the crank, the block itself features deep skirts and 6 bolt mains. The crankshaft is made out of steel and features unique treatments to relieve stress and increase durability. The connecting rods are forged and have an end cap that is rotated 45 degrees to increase strength. The cast aluminum pistons went through a number of designs to optimize the combustion chamber for power, noise and emissions.
When it comes to the top end, Ford opted to make the cylinder heads out of aluminum. This reduces weight, but historically aluminum heads have not fared well on a diesel (with the major exception being the current Duramax.) The heads do feature 4 valves per cylinder and Ford retained a glow plug to help with emissions.
One of the biggest and more radical designs of the engine is the actual flow of the cylinder head. A standard “V” engine has the air entering the cylinder heads from the engine valley and the exhaust exits from the outside of the engine. This configuration makes a ton of sense for a naturally aspirated engines as the exhaust just needs to be directed down the frame rails and out.
On a single turbocharged “V” engine, the exhaust is routed to the back of the engine, then up to the engine valley where the turbocharger is mounted. This extra plumbing/volume creates a delay in turbocharger response. It also heats up the engine compartment.
Ford identified this issue and designed a reverse flow head. The 6.7 Power Stroke is the first diesel engine to be produced with the exhaust exiting towards the engine valley (where the turbocharger is located) and the fresh/boosted air enters the outside of the cylinder heads. As mentioned above, this reduction in exhaust volume, results in a much more dynamically responsive turbocharger. “The physical size of the system is smaller, but more importantly, the air-handling part of the system is considerably smaller and that translates directly into the responsiveness of the engine,” said Adam Gryglak, lead 6.7-liter diesel engineering manager.
The turbocharger itself is a pretty radical design in its own right. Ford has been trying to achieve more low end torque and top end horsepower for the past 10 years. To achieve that, they initially started with a variable geometry turbocharger on the 6.0-liter Power Stroke. While this gave the turbocharger a much wider operating range, there were still some limits on its performance. With the 6.4-liter Power Stroke, Ford partnered with BorgWarner to design a sequential turbocharger system. This used a variable geometry turbocharger and a fixed geometry turbocharger in series. This system was designed slightly different than a compound system due to the high pressure turbocharger’s variable geometry. The concept was that the high pressure charger would only flow the boost and volume of air that the low pressure turbocharger was pushing but not compress it further. thus no compounding.
While this sequential system worked well, the cost was high, as the system consisted of two turbochargers. For the 6.7, Garrett, designed a single turbocharger that worked as a sequential turbocharger. The exhaust side of the turbocharger works as any variable geometry turbo. While the exhaust inlets are not side by side, the functionality of the exhaust is the same.
On the compressor side of things, everything is different. The compressor cover is really two compressor covers in one. This is due to the unique design of the compressor wheel or should we say wheels. What Garrett did was to utilize a single turbine shaft to power a single compressor wheel with Siamese profiles.
With basically having two compressor wheels, they essentially act as a twin turbo system. They will flow twice the mass or volume at the same pressure of a single turbo. When you take into account the reverse flowing heads, and variable geometry turbo, this system is able to provide massive flow at the top end, but also respond extremely well at the lower RPM range.
In trying to obtain the best possible performance, Ford opted to use the first air to water intercooler in the diesel market. An air to water intercooler is much more efficient than a standard air to air.
Traditionally, the boosted air is sent through an air to air intercooler that is in front of the truck. This intercooler has fresh air pass flow between the fins. As the fresh air is moving across the fins, heat is being transferred from the intercooler to the air (which is removing heat from the boosted air). Once the boosted air is cooled, the air is directed into the cylinder heads.
In this new air to water system, boosted air is sent to the air to water intercooler. The intercooler still removes heat from the boosted air, but the heat is transferred to the water that is flowing through the intercooler. The thermal conductivity of water is about 24 times higher than air. So, the water is able to remove much more heat from the air. What this mean is that Ford can reduce the size of the intercooler by almost 75 percent. By reducing the size of the intercooler, there is less parasitic loss throughout the system. This makes the engine much more responsive.
To cool the water, Ford uses another heat exchanger (like a radiator) in front of the radiator. As fresh air passes through the fins, heat is removed from the system.
Reliability has been Ford’s “Achilles heel” for the past few years. There are millions of Power Strokes on the road. While the percentage of them having issues is low, the actual number of failures is quite high (five percent of 3 million is 150,000 for example.) With the 6.7, they knew they needed an extremely reliable engine.
This started with holding down the cylinder heads. No longer would they rely on four bolts per cylinder. The 6.7 features six bolts per cylinder. Another key change made in the head was the rocker arms. There is a dedicated rocker arm for each valve. Previous generations of Power Strokes had one rocker pushing on a bridge, actuating two valves. The cylinder heads also feature dual water jackets.
Along with head gasket failures cooling system concerns are pretty high on people’s lists. Ford opted to use two cooling systems for the 6.7 Power Stroke. The primary cooling system is for the engine vitals (heads, block, etc). The other cooling system is to cool the EGR, fuel, transmission fluid, and charge air cooler (air to water on the 6.7 instead of air to air). By separating the cooling systems into two, Ford doesn’t have to worry about the hot exhaust gasses adding additional heat into the engine cooling system. They are also able to run them at different temperatures to optimize cooling.
Ford did extensive durability testing on this engine. In addition, they did extensive testing in vehicle. Here is a shot from one of the 40 below tests.
Now before we get into the problems areas of the 6.7 Power Stroke, lets first acknowledge that is has proven to be a very robust engine and extremely durable. The engine has been wildly successful and has the Best In Class horsepower, with a huge increase in fuel economy, reduced warranty costs, reduced NVH (noise, harshness and vibration), and increased durability.
The issues: “The 6.7 Power Stroke is pretty solid,” says Crede Young, owner of Diesel Dynamics. “The major things that go wrong with them are the turbos and the power steering pumps.” As with any mass produced vehicle, there will be other components that fail, but in the grand scheme, those are really the only consistent failures. Well, I guess we could include rod failures at around 700 horsepower, but that isn’t really Ford’s fault.
Revisions for 2015
MORE POWER! There is no telling whether Ford will change these numbers in the coming months or not, but when Ford released the horsepower and torque numbers for 2015, the 6.7 Power Stroke had more horsepower and torque (440 hp and 860 lb.ft.) Since then, RAM has announced they will be increasing the torque output on the 6.7 Cummins. No telling whether Ford will pump the number up slightly or not. When the 6.7 was first released, Ford increased the numbers after a few months to capture both crowns. So, how did they achieve that?
We’ve dramatically improved performance while reducing overall engine complexity by focusing on the turbo system- David Ives, Ford Motor Company
It all starts with air management. The turbocharger itself is being upgraded. Ford will replace the previous Garrett GT32 with a GT37. The GT37 features a larger 88 mm compressor wheel (exducer) that replaced the GT32’s siamese wheel. On the turbine side of things, the wheel is increased to 72.5 mm up from 64 mm. In addition, the wastegate and wastegate controls have been eliminated. “The original designers of the current Power Stroke V8 diesel forecasted the need for higher output. This facilitated the larger turbocharger, increasing airflow and creating more power,” said David Ives, Power Stroke technical specialist. “We’ve dramatically improved performance while reducing overall engine complexity by focusing on the turbo system.”
A few months back an announcement was made about the downpipe being updated. That might sound a little odd until you really think about it. As these engines continue to produce more power, they must flow more exhaust. The standard elbow was causing too much turbulence. The new cobra head design reduces that and flow more.
With the improvements to the turbocharger and exhaust, Ford is now able to increase engine braking. The previous generation had some but it was rather limited due to the concern of overspinning the turbocharger.
If you have more air, you can use more fuel. So, the fuel pump and injectors have been upgraded as well. The fuel pump now has a larger stroke and the injectors have a redesigned nozzle. One change that you may or may not like is the new exhaust temperature sensor. This will allow the ECU to know how hot things are getting (especially important when towing) and change the fueling/timing to prevent damage.
With the improvements made to the fuel system and turbocharger, the engine is able to produce more power. This increase means more stress on the cylinder heads. To compensate additional material was added to the cylinder heads and a new five layer MLS (Multi-layer Steel) gasket created.
Other key improvements are:
- New polymer coating on main bearings for improved wear- and-tear durability and lower friction.
- Improved crankshaft fillet design – the transition point between the crankshaft’s journals – to increase crankshaft power carrying capability.
- Increased crankshaft damper weight to reduce rotating forces on the crankshaft when the engine is performing at peak power levels.
- Added material to the exhaust manifold and valvetrain to handle increased power levels.
- Redesigned turbo oil and cooling lines to improve sealing.
- Piston assembly upgrades to increase load-bearing capability.
- New four-layer exhaust manifold gasket for improved durability.
For Ford fans, these improvements illustrate Ford’s commitment to keeping this engine in house. Hopefully, a family of Power Strokes will soon emerge in some of the half-ton options as well as other vehicles sharing this same architecture!