Duramax LLY Overheating and Thermal Feedback Primer

Volume 1 Issue 4 - Diesel Articles

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Duramax LLY Overheating and Thermal Feedback Primer
LLY Mouthpiece Air Flow
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I love what I do, and that is a blessing. Focusing on the troublesome mysteries of the technical world, I get to figure things out: things that have no apparent explanation... the elusive.

What follows is one such mystery, wrapped in a clever disguise and hiding in plain sight for years. It remained undetected for so long because there is not a single vehicle sensor, diagnostic, or gauge that is set up to alert us to what I finally found with patience, a casual observation, a $15 gauge and a paradigm shift.

As you read, keep in mind that the principles in this article can be applied to all turbocharged vehicles, not just the Duramax. You may well find inspiration to look at other unsolved mysteries by the time we are done. If you do, I would love to hear about it.

Duramax LLY Disappointment

It was 2004 when the Duramax LLY model replaced the LB7. Promises of more power, an advanced variable geometry turbo (VGT), among other announcements, were considered worth the wait. Unfortunately, it became clear right away that there was a problem. The engine did not seem to live up to the promises. Economy was reduced, performance was hindered and many towing customers could not use it for the advertised load capacity. The vehicles even overheated. Compared to the first generation LB7 Duramax, the LLY seemed to be dragging an anchor behind it.

Stock Duramax LLY Mouthpiece

(Click any image for more information or to enlarge.)

When I was approached about these issues, it sounded exactly like what makes me get out of bed, so I started digging. I really had no idea what I was getting into, or that finding the root cause would take years of part-time pursuit. In fact, this became a cold case for me several times. While others continued to scour the cooling system for defects, something within me searched for a flaw that tied performance loss and reduced thermal capability together: a single process or deficiency that would explain both issues, perhaps even a domino effect. As it turns out, that is exactly what I would discover.

A Cooling System Riddle

The defective mechanism is not a part of the cooling system, yet it results in elevated oil, transmission and coolant temperatures. It is not related to power generation, yet it results in reduced dyno performance. This mechanism even feeds on its own detrimental performance, growing in destructive intensity; a process so elusive, it didn’t have a name yet. What I finally settled on was named for a cyclical, power-eroding process in the forced induction system: a death spiral I call Thermal Feedback.

Thermal Feedback is the gradual loss of performance under sustained workload, which results from the byproducts of the load conditions themselves.

This story begins with a look at the purpose of the intake. For maximum effectiveness, the intake needs to:

  • Source air (oxygen) for combustion
  • Keep that air clean
  • Keep that air cool and dense and
  • Not fight the turbo compressor with negative boost.

The Turbo Fight

“Not fight the turbo compressor?” As odd as it sounds, every intake does this, and it is the necessary cost of pulling air through filtration and the intake conduit. This restrictive fight always acts on the air in the direction opposite to the direction of flow. It can be said that this restriction is the true cost of air transport, or of any fluid.

As an example of this restrictive fight, consider the water flow in a garden hose. If you have an 80-PSI water supply to the house and you turn on the hose bib with no hose attached, you will get a strong, 80-PSI gusher. Without that pressure, there would be no flow. Now attach a 100-foot long, half-inch diameter garden house and that flow is much less. This is because friction is eating away at the hose outlet pressure. At the end of the hose you have less pressure, say 30 PSI (still 80 PSI back at the hose bib). Fifty PSI is lost in the fight (hose restriction). It can even be calculated at 0.5 PSI per foot of hose. Now, let’s upgrade to a three-quarter-inch hose. With the larger flow area, water velocity is reduced, so friction and drag is reduced. Flow rate increases, and the hose end pressure is higher now, say 55 PSI. Now we are only losing 0.25 PSI per foot. The intensity of the fight has been reduced and we get more water out of the end of the hose.

The same principles at work in this garden hose example apply to induction air plumbing. Engineers can design for more or less restriction – less is always better – but there are practical limits to limiting restriction caused by compact packaging constraints: the ever-present demand to fit a wraparound, snail-like power train into a seemingly shrinking engine compartment.

Thermo-Fluid Cliff Notes

  • The fight involved in moving air is affected by three factors:
  • The amount of air being moved,
  • The diameter of the conduit through which the air is being moved and
  • The number of turns in the conduit and the sharpness of those turns.

The turbo compressor, which is nothing more than a centrifugal pump for air, must work exactly that much harder to overcome these restrictions in order to deliver the desired flow rate or boost. If you have expectations of 20 PSI of boost and there is five PSI of frictional fight, or negative boost, then the compressor must work five PSI harder and output 25 PSI total boost. More compression is needed for the same end pressure requirement.

It is the job of the compressor section of the turbo to compress the air it receives from the induction system. One undesirable but unavoidable by-product of this compression it that the air heats up. In Compressor Section of a Turbo diagram, the discharge product is hot (red). Air may go in at 100ºF inlet ambient temperature (IAT), but because of compression, that same air will be coming out at 360ºF. When induction tract inefficiencies exist, the air is heated from its ambient temperature before it ever reaches the turbo: even more compression is required resulting in hotter, less dense air leaving the turbo on its way, eventually, to the combustion chamber.

Clearing Away The Smoke

One day last summer, I was looking for suitable intake areas to locate a water mist nozzle for pre-turbo water injection, an effective way to knock down this compression heat. As I removed the entire intake, I came to the end, pulling off the compressor inlet, or mouthpiece. It is the middleman that connects the intake tube to the turbo compressor. After removing it, I looked inside and my initial thoughts were a thunderstorm of disbelief, shock, and, strangely enough, pleasure. The internal shape and dimension were a tragic crucifixion of air flow efficiency, something I would applaud only if it were emerging from my son’s play dough factory.

Mouth agape, I quickly measured it and then I had pencil on paper to determine what the internal air velocity would be under typical max power. In those few moments that changed my perspective on this issue, I had enough to conclude that I may have finally found that most elusive of defects.

Air Flow Models

Two of the factors mentioned above that contribute to air flow restriction are the diameter of the conduit and the sharpness of those turns. Now we will look at two air flow models to discover how these factors negatively affect airflow and how improved design allows the air to flow more efficiently.


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