With the ability to vary the flow of exhaust gasses across the turbine wheel, variable geometry turbochargers (VGT’s) get us as close as possible to having the best of both worlds. At low rpm, restriction in the exhaust side (i.e. drive side) of the turbo is increased, effectively making the VGT act like a much smaller turbo. At high rpm, all restriction is removed and you get the impression that a much larger turbo is feeding the engine. Throughout the rpm band and no matter the vehicle’s speed, response is quick and crisp. Simply put, a VGT provides the ultimate in drivability—and it all but revolutionized the out-of-the-box performance of today’s diesel pickups.
VGT, Brought to the Masses
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By no means did the diesel truck segment introduce the world to variable geometry turbo (VGT) technology, but when it debuted in 2003 aboard the best-selling Ford Super Duty, it was brought to the masses. Despite surrendering 1.3 liters of displacement to its predecessor, the 6.0L Power Stroke came with a responsiveness that was light-years ahead of the 7.3L. And when combined with the new, 32-valve cylinder heads and higher pressure HEUI injection system, the 6.0L also produced 35 more lb-ft of torque (560 lb-ft) than the highest rated version of the 7.3L ever did (525 lb-ft).
Everyone’s Doing It
Ford’s employment of the VGT was also a sign of things to come, as GM followed suit for the ’04.5 model year by introducing a variable geometry turbo on its LLY Duramax (the 6.6L V8 that succeeded the LB7). Then, with the unveiling of the 6.7L Cummins for the ’07.5 model year, Ram also made the switch to variable geometry technology. At this point, the general public’s view of diesels as being slow, laggy Neanderthals that perpetually clog up traffic was beginning to change.
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- Why VGT Turbo’s Fail—And How You Can Prevent It. November 23, 2017; Story By Mike McGlothlin; With the ability to vary the flow of exhaust gasses across the turbine wheel, variable geometry turbochargers (VGT’s) get us as close as possible to having the best of both worlds.
The Dark Side of Progress
However, as with any whiz-bang technology, there are bound to be downsides—and variable geometry turbo’s have several of them. While instant throttle response, improved fuel efficiency and cleaner emissions are among a VGT’s strong suits, longevity, cost of replacement and all-out performance potential are not. As it turns out, many turbos are particularly failure prone due to their design not allowing them to survive the types of conditions that exist in the exit side of a diesel engine’s turbocharger. Read on as we explain the ins and outs of VGT failure, how you can avoid it, the best way to fix it and why fixed geometry turbochargers are making a comeback.
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Great on Paper…
VGT’s are great for instant throttle response at any engine speed. It has the ability to help meet stringent emissions standards and the capability of doubling as an exhaust brake, but they have several key weaknesses. Stuck vanes are one of the biggest problems experienced with VGT turbochargers. When soot, carbon, rust and other forms of corrosion build up in the turbine housing, it can cause the vanes that direct exhaust gasses across the turbine wheel to seize up. Depending on which position the vanes get stuck in, you’ll either have great response down low but no top-end power, or vice-versa. Light-throttle and easy, steady-state driving typically causes vanes to stick, but any higher mileage turbo (100,000-plus miles) is at risk of vane seizure. Believe it or not, some turbos can come out of it if driven hard, but more often than not the turbo either needs to be pulled and cleaned, or completely replaced.
Problem: Stuck vanes
Symptom: No response at low rpm or extreme response at low rpm (depending on what position the vanes become bound up in)
Reason: Lack of exercise (lack of WOT or spirited driving), steady-state driving and/or extensive idling (conducive to excessive soot/carbon buildup), rust and corrosion accumulation, or foreign debris damage
Fix: Clean exhaust side of turbo, replace turbo, or switch to a fixed geometry unit
This is one of the single most problematic VGT turbos you will ever encounter. It’s the GT3782VA Garrett VGT off of an ‘04 6.0L Power Stroke suffering from a very common problem: a failed unison ring. The unison ring controls the movement of the exhaust vanes (and remember, the vanes are what direct exhaust gasses across the turbine wheel).
The Infamous Unison Ring
With rust buildup and soot and carbon present, it causes the unison ring to bind, which in effect means the vanes can no longer route exhaust gasses appropriately. A bound up unison ring usually trips a P0299 under boost code on ’03-’07 Fords. Replacement of the unison ring and a thorough cleaning of the turbo is typically all that’s needed to rectify the situation, but in some instances turbo replacement is necessary. Better yet, a 6.0L turbo cleaning is fairly easy for a trained mechanic and usually takes no more than three hours’ time.
Mechanically Actuated, Electronically Controlled
While vacuum actuation is one method of variable geometry functionality, all turbochargers used in the pickup segment are electronically controlled, mechanically actuated units. This also means that both the electrical side and the mechanical side can fail. Mechanically, the actuator is susceptible to soot and carbon buildup hindering its performance and this type of issue is more common on engines equipped with exhaust gas recirculation (EGR). Although it isn’t common for the electronic side of an actuator to burn up on Power Stroke or Duramax-powered trucks, the VGT solenoids do fail occasionally—and, luckily, are a relatively cheap (and easy) fix. However, outright actuator failure is more common on the 6.7L Cummins (shown above).
Problem: Failed actuator/VGT controller
Symptom: Excessive smoke, high EGT, loss of power at low rpm, turbo or boost-related CEL (example: P2262 code on ’07.5+ Rams)
Reason: Excessive soot/carbon buildup
Fix: Cleaning or replacement of actuator (if possible) or replacement turbo
While stuck vane scenarios are more common on the turbos aboard Power Stroke and Duramax engines, the Holset HE351VE found on ’07.5-and newer 6.7L Cummins mills is notorious for actuator failure (the big, honkin’ attachment to the left of the compressor housing). We’re told Cummins no longer offers the actuator for individual sale, which means you’re stuck with replacing the entire turbo if the failure-prone actuator checks out. A brand new turbo can cost you upward of $2,500, although remanufactured versions exist.
Actuator Cleaning Intervals
Believe it or not, the ’07.5+ Cummins owner’s manual specifies a cleaning interval for the turbo’s actuator. Unfortunately, just like the EGR valve cleaning interval, it is usually ignored or unbeknownst to the truck’s owner. The accumulation of soot and carbon can drastically effect the actuator’s mechanical performance, especially when a leaking or sticking EGR valve contributes additional soot buildup into the actuator housing.
The Magical Side of the Holset HE351VE
The ability to vary the turbine housing A/R is what makes a variable geometry turbo so drivable. The Holset HE351VE shown above can act as small as a charger with a tight, 9cm turbine housing for bottom end response, or as big as a charger with a loose, 26cm turbine housing for great top-end flow. It all depends on throttle input.
Use Your Exhaust Brake
The best thing you can do to avoid soot, carbon and rust building up on the exhaust side of the turbo (especially on the 6.7L Cummins’ HE351VE) is to exercise the exhaust brake regularly. Activating the exhaust brake keeps the surface of the sliding nozzle ring clean.
It's In Their Nature
By nature, variable geometry turbos create more drive pressure (exhaust side) than a comparably sized fixed geometry unit. Instant drive pressure gets loads up and moving quickly, but the charger has to be driven harder. This means more drive pressure and (many times) more shaft speed is present throughout the rpm range. And once added fueling is introduced (by way of aftermarket programmers and/or injectors), considerably more stress is placed on the VGT—a component that’s already living near the edge of its capabilities.
Problem: Failed bearings in the center section (thrust bearings or ball bearings)
Symptom: Compressor wheel making contact with inducer bore, excessive in-and-out or side-to-side play in the shaft
Reason: Excessive shaft speed and/or drive pressure
Fix: Turbo rebuild (if possible) or full turbo replacement
Power Adders Can Kill Them
In the diesel truck segment, added fueling is both easy to come by and the simplest way to add considerable power. The only problem is that while an extra 100, 200, even 300-rwhp can be added without upgrading the factory turbocharger(s), it is often pushed way out of its map. The added fueling provides naturally aspirated gasoline-like responsiveness at low rpm, but thanks to extreme shaft speed and excessive drive pressure on the exhaust side a VGT can become a ticking time bomb.
High Drive Pressures Contribute to Blown Head Gaskets
While excessive boost levels are the primary contributor to head gasket failure, extreme drive pressure can play a supporting role. This is especially true in the 6.7L Cummins’ case. While the Holset HE351VE turbo may only be seeing 35 psi of boost on the intake side, more than twice that number can be present on the exhaust side (i.e. drive pressure). Not only is this obviously very hard on the turbo’s thrust bearing assembly, but it contributes to more cylinder pressure being present in the engine.
When Safety Provisions Fail…
Even though the Garrett GT32 SST found on the ’11-’14 6.7L Ford Power Stroke features an internal wastegate, the drive pressure created via aggressive tuning can overwhelm it (as the wastegate can’t bleed off the excess drive pressure quick enough). When the wastegate can’t rid itself of extreme drive pressure fast enough, that energy is used to spin the turbine wheel (and the corresponding shaft) even quicker. So even though the GT32 SST is equipped with a wastegate, it is often the victim of overspeed failure.
Fixed Geometry: Ol’ Reliable
On all Big Three applications (Cummins, Duramax and Power Stroke), enthusiasts and owners fed up with VGT failure often make the switch to a fixed geometry turbocharger. The fixed geometry platform is known for its longevity and in the modern era a fixed geometry turbo can be spec’d to achieve adequate all-around performance. Dozens of aftermarket variable-to-fixed geometry turbo conversion kits exist, as well as dozens of turbo sizing options. In the photo above, the popular BorgWarner S467.7 sits in place of a failed factory Holset HE351VE on a 6.7L Cummins. Adding the S400 frame BorgWarner turbo was a bolt-on process thanks to BD Diesel’s new Rumble B Turbo Kit, which comes with everything needed to perform the swap (exhaust manifold, downpipe, air intake and all hardware).
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