Plasma Transfer Wire Arc technology creates a big-bore, big-breathing 5.2
By Steve Turner
Photos courtesy of Ford Motor Company
Recently we shared a video from Ford that delivered an inside look at the high-tech Plasma Transfer Wire Arc process used to line the cylinder bores in the Shelby GT350’s Flat-Plane-Crank-equipped 5.2-liter Voodoo engine. This process uses a plasma gun to melt steel wire and spray it as a fine coating on the cylinder bores, which eliminates the need for a heavy cylinder liner in an aluminum block.
It’s definitely impressive technology. We had the opportunity to speak with two of the engineers responsible for deploying PTWA on the Voodoo engine—Ford Technical Specialist, Advanced Manufacturing Engineering Tim Beyers and Chris Palazzolo. They were able to give us some interesting perspective on where the process started and how it benefits today’s Shelby GT350 engine.
If you just came into the Ford fold with advent of the S550, you might think that PTWA is making its debut in the 5.2-liter engine. That’s not quite the case. Ford actually began developing the technology—which is often dubbed “spray-bore” for laymen—20 years ago. They pulled it out of the repair industry and refined it for the production to the point that the company holds a patent on the PTWA spray-bore process as well as the roughening process for cylinder bores.
“When we got cast-in liners into production on aluminum blocks in the mid-’90s—instead of pressing in sleeves we would put the sleeves into the mold and cast them in—we were always looking to go future,” Chris said. “So, as soon as we put them in we were figuring what’s the next process to get rid of those liners out and take that weight out.”
However, it was really an industrious Ford Motorsport (now known as Ford Performance) engineer that pulled it out of the testing lab and into performance Mustangs.
“We had been developing the technology for a number of years. We had this technology and we were developing it for high-volume applications. Andy Schwartz found out about it and, as a true racer, he said ‘I want bigger bores in the 4.6-liter engine. I want to make it a 5.0-liter.’ That was the engine of my childhood; the 5.0-liter in the Mustang, so yeah let’s go do a 5.0 modular,” Chris explained. “That was at prototype volumes, which was plenty for him. He went out and instantly had over 400 horsepower with this Cammer engine back when we were making 305 in the Cobra. That was the point at which this whole thing of using spray-bore for the specialty, SVT-type applications started to build momentum. We did it in the Cammer and along came the 5.4.”
That engine went on to power the FR500 prototype, a car that foreshadowed many of the improvements the Mustang would see as it transitioned from the SN-95 to the S197 platform. In that application, the engine wore a unique, dual-throttle body magnesium intake and produced the aforementioned 400 horsepower along with 365 lb-ft of torque.
“The specific advantage for our situation is it allows us a larger bore that otherwise would not be feasible due to the close bore spacing,” Andy told your author back in 2000. “It allows us to achieve the increased displacement.”
The goal of that displacement wasn’t just more power, but an improvement in midrange torque, which was clearly one of the goals for the GT350’s engine. However, before PTWA arrived in the modern Ford Performance Mustang, it made its production debut on the cylinder bores of the aluminum 5.4-liter engine in the 2011-2012 Shelby GT500.
“On the 5.4, you obviously lose weight, and on the SVT products we are doing everything we can to take every gram of weight out of the engine—on all the components. Those guys want to go as fast and handle as well as possible. Taking out about a pound per cylinder is value for a racing-type application,” Chris explained. “When you reduce the thickness of the iron—those sleeves are 2 to 3mm thick—so when we reduce that down to just a spray coating we get better heat transfer because we are now relying on the aluminum to carry the heat away. That allows the calibrators to tune for spark advance and more peak power. That’s another thing that’s been part of the spray technology, which is to improve the cooling at the top of the cylinder bore.”
As a result, the GT500 received a lighter, more powerful engine by employing PWTA to line the cylinders, but that was really just the beginning of the story. Not only does this process shave weight and improve block cooling, but also it enables squeezing more displacement out of an existing block architecture.
“When that 5.4 came out, Kurt Hill—who was one of us Forward Model guys—then went over to SVT and was working on the 5.4. He was always thinking ‘one day we should make this bigger just like just the Cammer and go to a 5.8,” Chris said. “We provisioned that in knowing that with PTWA we could go further and take it up to a 5.8. Using spray technology was the only way to get there. We couldn’t do it with liners.”
The result was the 662-horsepower, 5.8-liter Trinity engine, which used the extra displacement afforded by the PTWA liners, a more efficient supercharger, and numerous detail improvements to propel the 2013 Shelby GT500 to a 200-mph top speed. It obviously worked well in this application, but Ford has continued to refine the implementation of PTWA to improve its viability for production engines, and that carried on with the GT350.
“One of the major differences between the two engines, the bores on the GT500 are roughened the entire length of the bore, so this roughening process basically prepares the bore to accept the thermal spray to create the adhesion of the spray to the aluminum material,” Tim said. “With our process we can roughen bores of various sizes and we can selectively roughen areas of the bore as we desire. Many of our aluminum blocks have a cast-in liner. What we are able to do is duplicate that where we can leave an edge of aluminum at the top of the cylinder bore to help us in manufacturing with other operations we perform on the block. It allows us to not roughen and spray at the top of the bore and only leave spray in the area of ring travel.”
If you missed seeing the process in action in our previous post, you can watch it here…
The benefit of this update in the process is all about manufacturing, as the tools used to mill the decks and chamfer the cylinder bores will last much longer if they only have to cut aluminum rather than aluminum and an iron coating. Moreover, the latest machining techniques are said to achieve 10-percent great adhesion on the 5.2-liter than it did on the 5.8-liter.
Of course, if you are new to learning about this technology, you might be concerned about its durability. You can quit worrying. Odds are your moving parts will wear out long before the coating will show signs of wear.
“The durability is not a problem. We have fleet tested these things beyond where we would normally fleet test. Because this was a new technology, we took them out and ran them in severe-duty applications. Some logged over 300,000 miles and the bores were still fit for purpose. The wear was almost as low as we could measure,” Chris said. “We have really designed the system to work together; the piston, the coating, the rings, it’s all a package. We really balance the materials with the design and the process so that the system can just function as it’s intended for a long, long time. We put over 1,000,000 miles of fleet durability and this goes back to before the decision to put it in the Shelby the first time on the 5.4. So you’ve got all that history and all the Shelbys that have run and raced. Now you have the GT350; all the racing and testing we’ve done on those. We have always passed with flying colors. The material is very durable and robust.”
Ford is starting to use the technology in a four-cylinder diesel in Europe. The combustion pressures are obviously much greater there, so durability is not a concern. If you do ever have a failure because you added too much boost, timing or rpm to a stock engine, it’s likely other parts will fail before the coating does.
So, the Shelby GT350 benefits from this lightweight, durable PTWA coating in its cylinder bores, which not only allows for more displacement, but the larger bores unshroud the big valves in the Voodoo cylinder heads for improved breathing. Better yet, a PTWA-prepped block offers improved cooling at the block-to-head mating surface, which allows for a more aggressive calibration and improved performance.
Obviously there’s a lot of technology in the latest Shelby GT350. While the Flat-Plane Crank gets all the headlines, you can now understand how the PTWA liners quietly enable much of the performance you feel.