Not to beat a dead horse, but found this on a Ford website regarding the Ecoboost and the coking issue. Here are some excerpts if you are interested in what they have to say. Thought some of you might be interested in this info. Some of the language is clearly "marketing" oriented so FWIW . . .
Designed for long-life reliability, EcoBoost’s turbochargers feature water-cooled bearing jackets. This architecture is designed to prevent oil “coking” that could occur in previous-generation turbochargers. The new design means that EcoBoost drivers don’t need to observe special operating precautions, such as idling the engine before switching it off.
Turbochargers operate at high speed – up to 170,000 rpm – and under intense temperatures of up to 950 degrees Celsius (1,740 degrees Fahrenheit). Some previous-generation turbos were reputed to suffer from oil coking, in which they would bake their lubricating oil. Because oil coking can lead to premature turbocharger bearing failures, Ford’s advanced engine engineers specified the use of new, water-cooled turbochargers to combat this problem.
“During normal turbo operation, the turbo receives most of its bearing cooling through oil,” said Keith Plagens, turbo system engineer. “After shut down, the problems with turbos in the past were you would get coking in the center bearing. Oil would collect in the bearings, the heat soaks in and the oil would start to coke on the side and foul the bearing. Water cooling – used in the EcoBoost engine – eliminates that worry.”
The new EcoBoost V-6 uses two Honeywell GT15 water-cooled turbos.
“The EcoBoost engine uses passive thermal siphoning for water cooling,” Plagens explains. “During normal engine operation, the engine’s water pump cycles coolant through the center bearing. After engine shutdown renders the water pump inactive, the coolant flow reverses. Coolant heats up and flows away from the turbocharger water jacket, pulling fresh, cool coolant in behind. This highly effective coolant process is completely silent to the driver, continuing to protect the turbocharger.”
To validate their water-cooled turbo design choice, Ford engineers put EcoBoost through a special turbocharger test.
The test ran EcoBoost at maximum boost flat out for a 10-minute period. Then the engine and all cooling were abruptly shut down and the turbo was left to “bake” after this high-speed operation. If that sounds severe, imagine repeating this cycle 1,500 times without an oil change. That’s what EcoBoost’s turbos endured.
After 1,500 cycles, the turbos were cut open for detailed technical examination. The turbos passed the severe test with flying colors.
“We’ve attained things here the customer would never be able to do in their vehicle,” Plagens said. “Ten minutes of peak power (355 hp, 350 foot-pounds of torque) is something that’s probably only achievable in a vehicle for fractions of a minute, 10 seconds maybe in the extreme. We run it for 10 minutes many, many times over, and that’s far, far more harsh and severe than a vehicle test would be.”
EcoBoost also endured Ford’s standard engine durability test signoff. Back in the dynamometer lab, the 3.5-liter EcoBoost V-6 went back up to full revs – and maximum turbo boost – for a real endurance test. This time the duration was a bit longer – 362 hours at full throttle. That’s like running the 24 Hours of Daytona for more than 15 days straight.
Other tests subjected EcoBoost to a grueling range of operating temperatures.
“We run all of our durability testing at the maximum temperature,” Plagens said. “For the turbos, the test is 150 hours long. Every 10 minutes the test alternates between peak power at max exhaust temperature and completely cold motoring. The goal is to verify that the turbochargers can withstand extreme thermal cycling without affecting their performance. It’s pretty brutal and extreme but it’s important to prove out durability.”