INDUCTION–ALL HOT AIR??

A THERMAL FEEDBACK PRIMER

I LOVE what I do, and that is a blessing. Focusing on the plaguing mysteries of the technical world, I get to figure stuff out…stuff that other folks stumble upon…stuff that has no apparent explanation…the elusive. I DETEST band-aid thought processes that merely patch symptoms, and do not identify the cause.   Hence I move slowly sometimes, cause can be tricky to spot.  Occasionally, I put headstone captions on technical John Does of vehicle engineering.  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 to what I finally found with a paradigm shift, patience, a casual observation and a $15 gauge. This is a perfect example of a problem that required stepping “out of the box” before the elusive can come sharply into frame. Keep in mind, the principles in this article can be applied to ALL turbocharged vehicles, not just the D-max. 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.   

PERFORMANCE ANXIETY

It was 2004 when the LLY model Duramax replaced the LB7. Promises of more power, an advanced variable geometry turbo, among other announcements, were considered worth the wait. But it became clear right away, there was a problem. This vehicle didn’t really seem to live up to the promises. Economy was reduced, performance hindered, and many towing customers could not use it for the advertised load capability…some even overheated. Compared to the 1st generation LB7 Duramax, it seemed to be dragging a small “anchor” behind it.  

When I was approached about this, it sounded exactly like what makes me get out of bed, so I got involved. I really had no idea what I was getting into, or that solving this would take years. In fact, this became a “cold case” several times. While others continued to scour the cooling system for defects, I intuitively searched for a flaw that tied together performance loss and reduced thermal capability …a single process or deficiency that would explain both issues, perhaps even a domino effect. Indeed, as it turns out, that is exactly what I discovered.  

This mechanism has nothing to do with the cooling system, yet it elevates oil, transmission, and coolant temperatures. It is not related to power generation, yet it reduces dyno performance. This mechanism even feeds on itself, growing in destructive intensity under its own genesis. A mechanism so elusive, it didn’t have a name yet, so I gave it one. What I finally settled on was named for a cyclical power eroding paradox in the forced induction system…a death spiral I call “Thermal Feedback”.  

Thermal Feedback: 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, it needs to:  

1. source oxygen for combustion
2. keep that air clean
3. keep that air cool and dense, AND
4. not fight the turbo compressor with… “NEGATIVE BOOST”  

“Not fight the turbo compressor?” As odd as it sounds, every intake does this, and it is the (necessary) cost of running air through filtration and a 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, of any fluid.  Consider garden hose water flow. 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 huge 80 psi gusher. Without that 80 psi of pressure, there would be no flow. Now attach a 100 ft long, ½” diameter garden house, and that flow is now much less. This is because friction is eating away at the hose outlet pressure. At the water exit you have less pressure, say 30 psi, (yet still 80 psi back at the hose bib). 50 psi is given away to hose restriction (fight). It can even be calculated at 0.5psi per linear foot of hose. Now let’s upgrade that ½” hose to a ¾” hose. With the larger flow area, water velocity is reduced, so friction and drag (fight) is less. Flow rate even increases, and the hose end pressure is higher, say, 55 psi now.
Now we are only losing 0.25 psi per foot.  

The same applies to our induction air plumbing. The engineer can design for more or less restriction, and less is always better, though practical limits exist due to compact packaging constraints, the ever present requirement to fit a wraparound, snail-like power train into an ever-shrinking engine compartment.  

THERMO-FLUID CLIFF NOTES     

The more air you want to move, the bigger the “fight”. The smaller the conduit, the bigger the fight. The more turns or constrictions, the bigger the fight. With these restrictions, the turbo compressor, which is nothing more than a centrifugal pump for air, must work (exactly) that much harder to make the desired flow rate. If you have expectations of 20 psi of boost, and there is 5 psi of frictional fight, or negative boost, then the compressor has to work 5 psi harder, and output 25 psi total boost: so more compression is needed for the same end pressure requirement.      

In this compressor diagram, the discharge product is hot (red). That is because compression creates heat. Air may go in at 100 degrees (IAT), but, because of compression, it will be coming out at 360 F, and frequently much hotter. Some of this heating is unavoidable and is produced in proportion to required compression. When induction tract inefficiencies exist, more compression is required, so even hotter, less dense air will result. Engineers must design the induction tract size for the expected flow rates, minimizing losses, yet avoiding over sizing, as this can create spool up delays.      

CLEARING AWAY THE SMOKE     

One day last summer, I was looking for suitable intake locations to locate a water mist nozzle for pre-turbo water injection, an effective way to knock down this compression heat. It will be the topic for a future article. As I removed the entire intake, I came to the end, pulling off the compressor inlet, or “mouthpiece”, shown here. It is the middleman that connects the intake tube to the turbo compressor.      

After removing it, I looked at the inside of it, seen below, and my initial thoughts were a comedy of disbelief, shock, and …ummm, pleasure? It’s not obvious from this view. The internal shape and dimension was a tragic crucifixion of air flow efficiency; something I would expect to applaud when emerging from my son’s play dough factory.      

Mouth agape with tingly passion, I quickly measured it, and then I had pencil on paper to determine what the internal air velocity was under typical max power. Within moments I had enough to conclude that I’d have to dust off the fluid mechanics and thermodynamic texts to verify the surging feeling that I may have FINALLY found the most elusive of defects…and a new headstone caption.      

      

MAKING OF AN AIRFLOW MODEL      

FLOW SCIENCE     

In the following flow model, air is flowing around zero-radius miter corners, a similar model to our LLY mouthpiece. A narrowing of the conduit occurs initially, followed by enlargement. This flow model is an example of poor flow. The LLY mouthpiece has regrettable similarities.      

Poor conduit design      

There are several things at work here conspiring to increase pressure drop, that evil restrictive fight. First, note the immediate flow separation at the inside of the turn on the first corner. The main flow stream, subjected to huge centrifugal forces (would you believe, 9000 G’s?), then runs toward the outside of the turn. If you look at the blue lines of flow, you will see that as the main flow stream progresses, the lines get closer together, squeezed together toward the outside wall. This is a pseudo-compression of sorts, and a result of this virtual restriction. In this regime, a large low pressure area is created on the inside of the turn, the lighter colors depict these forces. The low pressure is pulling on the main flow, kinda sorta like a strong magnet, and equilibrium is struck, albeit at great cost to flow.      

With more of the air forced to the outside of the turn, close to the wall, it experiences an increase in boundary layer friction: the resistive counter force that results from fluid flowing across a stationary boundary, also known as skin friction. The formations of stagnation pools, shown in the outside corners, add to momentum losses. Air comes and goes from these pools, analogous to vortex “hydraulics” in the rapids of a river, places where river rafting accident victims are often recovered due to the circular current that traps them.      

YES, SIZE MATTERS     

Here, the throat size is increased for this regime. Inside flow detachment still exists, but the flow progresses with less compression, the low pressure area is MUCH smaller and the darker colors signifies less pressure degradation. The lines of the main flow stream are further apart, so there is less layered viscous friction. Downstream, reattachment to the inside wall occurs quickly. The resultant downstream flow pressure is increased, going from green in the restricted regime, to blue with the throat sizing improvement.      

Improved design      

This difference can amount to several PSI. This shows the importance that plumbing size alone can make in undersized applications. Size matters. The flow separation occurs in both cases, and this is due to radius issues. Air cannot stay attached to a mitered corner with any significant velocity, so the unrounded inside corner is very offending. The higher the expected velocity, the greater the radius is required to avoid detachment.      

Experimental core (left)      

      

As a general rule, the elbow radius should be at least twice the conduit diameter. Manufacturers routinely violate this rule in deference to packaging constraints, as in the case of our tightly constrained induction plumbing. Let’s face it, within practical constraints, there is nothing we can do about this space limitation, but we can make the most of it. If the best we can do is delay the detachment, this will reduce the low pressure zone size and magnitude. This is where optimization kicks in. Provide the largest inside radius possible, with simultaneous increase in velocity-reducing plumbing size. Reworking these 2 concepts alone can effect an 80% reduction of the offending pressure drop in this atrocity of air flow.      

FANTASTIC VOYAGE     

Following combustion air from the airbox it travels through a 4” tube, finally to the mouthpiece. On measurement of the stock LLY mouthpiece, flow area gradually narrows to 2.4”, and airflow, at full throttle, accelerates to 250 mph! At this point the flow is forced to turn 100 degrees on a sharp bend like the illustration above. One pound of air every second is forced into an instantaneous right turn at 250 mph! It is a staggering energy conversion that results in enormous restriction, reduced pressure, and added heat (converted from turbulence). If an aircraft wing were subject to this same force, it would snap before the pilot could even black out from the G-force acceleration. So my curiosity kicked in and I calculated the centrifugal force experienced by air in this turn:      

A pilot flying at 250 mph at the air races, turning on a 100 ft radius experiences a 9 G force, enough to black out if he’s not real careful. The 250 mph air stream in this tight radius mouthpiece experiences a thrilling “E-ticket” 9000 (9 THOUSAND) G’s!      

One pound of air every second, getting a 9000 G pull takes requires gobs of energy. Where does all that energy come from? Oddly enough, it comes from the exhaust. You’ll see that it results in higher EGT. This will be very important later.      

      

Using a $15 Magnehelic pressure differential gauge, purchased on Ebay, I tapped some sensors into this Zytel Nylon train wreck and went to work. With stock level airflow, total intake resistance was 4.8 psi, a staggering 136 inches water column. Of that amount, this one piece alone is responsible for 84% or 4.1 psi. The pie chart shows the relative deficiency of each intake component.      

      

Note how much is a result of element resistance: the smallest slice. It is the same element that gets replaced with supposed “better flowing” elements on promises of better performance. Laughable.      

This was a perfect opportunity to test a new KN drop in element. This 3% number dropped to 2%, a 1% improvement to total intake resistance. Calculations suggest that this 1% improvement will result in an estimated 0.15% increase to airflow. Despite creative marketing attempts to the contrary, this is what you should get, assuming the element is new. In my real world (no flow bench) physical testing, I found no measurable improvement to flow, never mind what happens when it gets dirty. Off it came.      

      

On the left – The restriction of the stock intake
On the right – The restriction of the prototype mouthpiece      

As the pie chart shows, the mouthpiece creates 5 times more resistance to flow than all the remaining parts of the intake COMBINED! The VGT needs to make more boost to compensate for these losses. This means the compressor must work to create an additional 4 psi, spinning considerably faster to do this, and that is a bad thing, not a good thing. I also charted total intake resistance, varying airflow.      

5 lb/minute is what you would get at idle.      

15-20 lb/min represents typical unloaded highway driving.      

35 lb/min comes when pulling a trailer on level ground.      

50 lb/min is typical of 280 HP full throttle stock tuning.      

60 lb/min is typical of a +100 HP boost augmented tune.      

75 lb/min is typical of a +200 HP race tune.      

      

Every PSI is equivalent to 28 inches water column, IWC. At 54.5 lb/min there is 4.8 psi (136 iwc) of restriction, lots of negative boost, or fight. I bring you this chart to illustrate an important point: the more airflow there is, the more potential for improvement exists. When we fix airflow flaws like this, we are removing a fixed percentage of the problem. Clearly, if we only ever used 15 lb/min (grocery getter vehicle), then we would have little to discuss because losses are well under 1/2 psi. But if we want to examine race day air flow of 65 lb/minute, there is 175 iwc (over 6 psi) to address. If I can easily remove 80% of that amount, this will be huge.      

GETTING IN SHAPE     

On a mild temperature day, stock tuned, Mass Air Flow (MAF) rate is about 45-50 lb/min, depending on elevation, or roughly 700 cfm. I realize most folks don’t attribute a weight, or mass, to air. To give an idea, the air in an 8’ x 10’ room is heavy, it weighs 50 lbs. We don’t acknowledge that because our bodies are tuned to resist it, but it is pressing down on us all. Every day our lungs inhale and exhale 30 lbs of air. One pound of air runs through the mouthpiece each second at max load. It takes us 2 days to inhale as much air as the duramax workhorse does in one minute, foot on the floor. That entire roomful is inhaled in one minute at stock intake rates.       

When the induction system is restricted we can observe a relationship predicted in loss equations. The restriction is a function of the square of velocity in the conduit and bends. If airflow is doubled, then the loss/restriction will increase fourfold. Best to start out with low restriction, since 4 times a really small number is still somewhat small. 4 times a big number… is a BIG problem. That can be seen approximated in the chart above. Compare the 20 lb/min and 40 lb/min (double) restriction. At 40 lb/min, the 60 iwc loss is fourfold the 20lb/min loss of 15 iwc. Look at 75 lb/min of airflow. It is not possible because the losses are so great, around 250 IWC, or about 8 psi. Attempting to spin the compressor fast enough to do this, puts the compressor into choke, supersonic airspeeds, for which overspeed mechanical failure is the foregone conclusion. It is our turbochargers equivalent of… a heart attack.      

Why? How did GM unintentionally handicap the LLY Duramax?      

THE SHORT HISTORY     

Back in 03, GM was on a deadline to certify a stiffer Clean Air Act and CARB standard for emissions improvement, a mandated diesel emission requirement that would apply in 06. The company, in its effort to be diligent, certified these guidelines early, in 04, the LLY emissions package. They were happy to advertise it in press releases also. Among the changes introduced, an emissions friendly Variable Area Turbine Nozzle (VATN) turbocharger, aka VGT, aka VVT. Also new was an EGR tube positioned in real estate previously occupied by induction intake components. The compressor mouthpiece was redesigned and squeezed in and as a result, aerodynamically compromised to fit the smaller space. That is where that big 4 psi pie slice comes from: poor planning, poor design. As OEM design mistakes go, this is an 8 or 9 out of 10. 10’s are reserved for things like Space Shuttle o-rings and Chevy Vega aluminum cylinder sleeve design. ☺ I’m not throwing GM under the bus here, they are all human. Humans err.      

THOU SHALT NOT RESTRICT THE COMPRESSOR!     

The 11th Commandment. 4 psi of restriction just didn’t seem like anything that should be so fatal. Here are some factoids I came up with:      

Enough wind to create a 4 psi load on your body would impart a total force of 8000 lbs, considered enough to create skin pealing injuries. 4 psi is the weight of the entire atmosphere to 2 miles above the surface. A 1 gallon gas can experiencing a differential of 4 psi, is subjected to a crushing 1200 lbs of total force (shown). The strongest tornado ever recorded contained less than a 4 psi wind…though its wind load would impart a force of 120,000 lbs to the side of your house, that’s equivalent to the max thrust of an 8 engine B-52G Bomber…more than enough to mulch it. 4 psi is 40 times the amount of air pressure required to force the eye blink reflex. A typical passenger aircraft wing loading is 1.0 psi, so 4 psi is 4 times more pressure than is required under the wings to keep a 350,000lb jumbo jet aloft. And finally, to hold back the walls of the Red Sea requires a miraculous force of 3.98 psi. Moses rocks!      

If the element got dirty enough to become a 4 psi restriction, it would see a force equivalent to standing 3 grown men on it. The element is considered worthless when enough dirt accumulates to a 0.4 psi restriction, so a 4 psi intake restriction would be equivalent to stacking 10 clogged filter elements on top of each other.      

And then, in a fleeting moment of self honoring pleasure, as if to make all these analogies irrelevant, it then occurred to me, …and perhaps you have connected the dots also…4 psi is the price of admission for… a 9000 G plumbing design flaw. Whoa!      

Obviously I found something bad. But how bad? How do we quantify the damage done by such an oversight in engineering? By looking at some alternatives to improve our induction system, we can evaluate the importance of intake restriction, upstream of the compressor and then compare it to intercooler restriction, downstream of the compressor.      

GET OUT YOUR PENCILS     

PROBLEM 1: Rex Racer, who lives near sea level, just got his tax refund. He is given a choice of 2 modifications to make to his inefficient induction system. Each involves a 3 psi reduction in restriction. The vehicle in question is our MAP sensor governed, variable geometry, turbocharged LLY Duramax. In each case, intake plenum pressure is governed to the same 34 psia (absolute) of MAP, manifold absolute pressure, or 20 psig of boost. Air flow was experimentally determined, and ranges from 45-49 lb/minute.      

Option A-INTAKE this involves redesigned pre-compressor intake plumbing, resulting in a 3 psi reduction of restriction upstream of the compressor, the intake side. It involves an “intelligently” designed intake with new pre-compressor plumbing. The cost is $500.      

Option B-CAC this involves reducing the losses downstream of the compressor. The improvement is also a 3 psi reduction. It involves an optimized CAC, new boost tubes, and improved intake plenum riser, at a cost of $2000.      

1. Which option should Rex go with?      

2. Which option has the best performance improvement?      

In other words, which type of restriction is more critical to avoid, restriction near the compressor inlet, or restriction near the compressor outlet? Or, is there even any difference?      

In each case, the boost at the intake plenum is identical. That is how the VGT works: it works as hard as necessary to satisfy the boost sensor located in the intake plenum. So, if the boost is the same, the only thing we can accomplish is to make that 20 psi of air easier to produce: make the compressor easier to turn, and less parasitic to the engine. We also know that the lower the air temperature, the better for air density and power production. So reducing the heating of the air serves several positives. In fact, and as we’ll see later, there are 5 major benefits when reducing compression heat this way.      

Also, if turbo rpm can be reduced while delivering the same boost, then it requires less power to drive it. That amounts to HP given back to the wheels, also examined later.      

To solve this problem, thermodynamic compressor performance must be examined for heat impact. A compressor map is the place to start evaluating. It is used by designers to mate the turbo to a specific application. For example, if I needed an air supply of 80 lb/minute, this compressor would not work. This shows a 68 lb/min maximum design air flow, the right edge of the map curves.      

This map also represents the thermodynamic performance characteristics of the compressor for various conditions. It is most useful for predicting thermal efficiency, or how hot it will make the compressed air. Efficiency is depicted by concentric “islands”. The limits of intended operation are bounded on all sides by curves. You don’t want to find yourself unintentionally operating near a boundary.      

We can plot the full throttle operating point, shown as a red dot, on this chart when we know the conditions. The vertical axis is pressure ratio, PR. PR is simply the Compressor Output Pressure, divided by the Compressor Input Pressure, at the compressor mouth, or PR=COP/CIP.      

      

At sea level, ambient pressure is 14.7 psi. If there were no resistance losses at all, a perfect system, then:      

PR=COP/CIP=34/14.7=2.31. (Perfect World)      

With reasonably low resistance, it would be around 2.6, but this is not the case. In our problem there is 3 psi on both sides of the compressor that are individually considered. So initially:      

PR=(34+3)/(14.7-3)=37/11.7=3.2 (Our Initial Condition)      

Note that 3 psi restriction is added on the outlet or work side, and subtracted on the inlet side. If you don’t see why, it will be explained soon.      

Plot this PR with the airflow measured at 45 lb/min in the map shown above. We might sense something is wrong already. Turbochargers are usually sized so that the o.p. is usually located somewhere near the middle, in the more efficient islands, buffered from the problematic boundaries. This o.p. is dangerously close to the upper rpm limit of operation.      

We need to compare this o.p. with the o.p. of options A and B. But first, it is important to understand the impact of removing restriction, and how that changes CIP or COP.      

So often the assumption is made that Turbo Compressor Inlet Pressure, CIP, is the same as atmospheric pressure. This can be far from the truth, as in this case.      

       

       

       

On the low pressure side, the CIP side, Option A removes restriction, increasing CIP. The Option B impact on COP is similar. Each is like going to a wider garden hose. Basically, if 20 psi is demanded at the plenum, and if there is 3 psi of restriction between the compressor and the plenum, then 23 psi must exist at COP, since 3 psi will be lost in between. If the restriction is removed, then COP is beneficially reduced, to 20 psi. Summarizing, in each case there is 3 psi less compression. At first glance, you might draw the conclusion that they are equivalent scenarios. They are not.      

In option A, CIP is increased by 3 psi,      

PR=COP/CIP      

PR=(34+3)/14.7=2.52.      

In option B, COP is decreased by 3 psi      

PR=COP/CIP      

PR=34/(14.7-3)=2.91.      

Plot these 2 solutions on the compressor map (on the right).      

AND THE WINNER IS…      

Option A! It has the best combination of highest compressor efficiency and lowest PR results in the lowest heat production, and lowest compressor RPM. Remember each option is producing the same boost level, yet, because of efficiency difference, the airflow amount will differ.      

The turbo is easier to drive, since less work is required to drive the lower rpm, 15,000 RPM less on the chart! This means lower exhaust back pressure and lower EGT, all with about 5% more compressor efficiency (less heat). Oddly, the above benefits result from the elimination of a mere 3 psi restriction which is accomplished with the efficient mouthpiece on the left, below. Even if the price tags were reversed, option A is still, by a large margin, the preferred mod, and will yield the best performance results all around.      

The more efficient configuration puts less energy into heating the air and more into compressing. The combustion air is cooler and more dense when restriction is removed. That is the key to making more power.      

      

Note also the new charted o.p.. Option A resides in a heavily buffered location on the map, no longer in danger of running off the map if conditions change. For example, as in the case of an elevation increase which lowers CIP, increasing PR.      

There is a bonus that is not immediately apparent. I mentioned it already but it is worth repeating. Air flow increases 8-10% with Option A. This is very important, that is 10% more air, same boost, and the same turbo. It is just acting like a bigger turbo, with less heat and turbo lag, best of all worlds without the $2000 replacement cost. In other words, restriction elimination decreases the heat producing negative consequences while increasing airflow and power…and not just marginally…but dramatically!      

As a side note, this is similar to what twin turbochargers do to obtain very high efficiency. The COP of the 1st compressor increases the CIP of the 2nd, larger compressor.      

“HP FOR SALE…CHEAP”     

Let’s break in that new Dremel set you got last Christmas, you can do this while the holiday turkey is smoking on the grill. If you are handy, it won’t cost you 1 cent. Take off as much material as pos      

      

sible, rounding as you go. I circled the area that is most offending, but there is a lot of room to be made inside this piece by going through the entire inside. Being thorough it takes 2 hours to completely machine it out.      

WARNING: DON’T OPERATE your engine with your induction tract dismantled-serious injury can result!

      

Before and After      

Use a respirator, this material is a potential fiber hazard, and you don’t want to breathe the dust you create. Dupont acknowledges it is “injurious to the respiratory tract”. I also supply one you can purchase if this is not for you. By doing this mod you can shave 1 psi from stock airflow. If you run higher boost levels, and 4000 rpm tuning, it will reduce your loss by 2-3 psi. You also get FREE air (5%), FREE power (2-4%), and added turbo longevity.      

While inspecting a number of these mouthpieces, on various vehicles, I stumbled upon an unusual observation. I found one (out of 20) that already had manual trimming done from the factory, in a vehicle belonging to Todd Greene. That leaves more questions than answers, but it makes me question whether this issue was already understood by the good folks who brought you the Duramax.      

I have kept one eye open to find an obvious disparity in equipment that would explain why some LLY’s seem to suffer heat issues more than others. This internal round over on some vehicles could perhaps be the source of this modest disparity.      

PERFORMANCE TUNING-BOOST CONTROLLERS     

Boost enhancement has become a very popular (cheap) performance add-on. This shows the compressor operating point should you decide to augment the boost of the LLY, with a common 6 psi boost enhancer.      

PR = (34+3+6)/ (14.1-3) =3.87 (assumes 1100 ft)      

This is a compressor overspeed EVERY time you punch it. They don’t tell you that, mainly because the inventors never considered what you are seeing here. After all, proving that your 50 pounds of molten metal exploded because of irresponsible innovation is an uphill battle. In reality, lifespan reduces rapidly in proportion to the distance the O.P. is placed from the map boundary. Boost enhancement has created numerous problems and ruined turbochargers prematurely and this is why. The turbo manufacturer will not give any life expectancy for this kind of use. I caution, if you tow with an LLY truck above 6000 ft with one of these devices, you will soon be replacing the turbocharger.      

Define irony: Justifying the use of boost increase to reduce turbine damaging EGT, only to realize that the compressor then failed due to the resulting induced overspeed and mechanical stress.      

One of the biggest pitches for these devices is reduced EGT. It is well known that high EGT will shorten turbine blade life. But some people have become consumed with this operating parameter as THE all-telling longevity indicator, and this is a big big mistake. One way to reduce EGT is to provide surplus air dilution, that is, more air than can be combusted with the provided fuel amount.      

As we now know, air dilution in this manner comes with disadvantages affecting longevity, dangerously increased turbo rpm. Blades exceed Mach speed, and enter choke region operation. Centrifugal blade creep mechanically stresses the metal blade, threatening wall contact. Easily, 50% of the turbo life can be eroded in a few seconds of thrill seeking. In extreme tuning cases, like “stacking” multiple tuners, turbo lifespan can be reduced to time served. It is clearly the most destructive way to reduce EGT. Do not use these devices for towing or other extended duty cycle work. Still skeptical?      

PROBLEM 2: What is the boost augmented pressure ratio if you are in Denver CO, assume stock intake? You can rerun the above numbers using atmospheric pressure of 11.5 psia (Denver) to replace 14.1 psia, and replacing the 34 psia of MAP with the augmented 40 psia. Because of the added airflow, the restrictions are up to 6 psia on the intake (cold) and 4.5 psia on the outlet (hot) side. (Solution at the bottom of the page.)      

Here is a lineup of 3 generations of mouthpieces: 01-04 LB7, 04-05 LLY, and 06-07 LBZ. From this angle, it may appear that the top LB7 piece has issues also, but it has no sharp turns…and that’s important at 300 mph.      

Remember the 9000 G’s that exists for stock flow in the LLY piece (middle)? The LBZ piece below comes in at 2000 G’s, with its lower velocity, and larger turn radius. The LB7 piece above it, with its narrow ducting, yet large radius sweep, comes in at under 1500 G’s.      

The job of the mouthpiece is to present the intake air to the compressor with minimal energy loss. (Avoiding the small garden hose) My subjective design grade for accomplishing this falls in line with the G-load disruption:      

LB7…………..1500 G’s………….A-      

LLY………….9000 G’s……….…F-      

LBZ/LMM…..2000 G’s………….B-      

Looks are not deceiving; the LBZ did away with 60% of the problem, returning the beloved Duramax to the people. In the last MaxxTorque issue, Joel Paynton’s effort to prove this does an exemplary job of demonstrating the changes in vehicle behavior by replacing the “defective” piece. The vehicle becomes thermally stable, stronger, more responsive, more economical…all things that CAN NOT be addressed with cooling system add-ons and band aids that have garnered hyped popularity.      

This particular flaw has at least 5 major performance symptoms, the damaging domino effect stemming from its own genesis.. If you want to repair all the symptoms this way then you will need 5 expensive band aids, yet there is still one that cannot be fixed any other way: parasitic power loss. It is so much cheaper, and more effective to fix the source. More on parasitic power loss, the promised thermodynamics lesson.      

GEEK SPEAK     

If you are not a geek, you can skip this part, it is not important enough to matter, as long as you believe the rest of this article. By way of theoretical proof, this whole article can be summarized thermodynamically. The turbine (right) and compressor (left) are connected by a shaft, a fact that simplifies this problem enormously.      

       

The turbine uses exhaust gases expanding across it to spin the whole assembly. This process, governed thermodynamically, cools and slows the exhaust, extracting work energy in the process. The compressor does the opposite: it uses the energy provided by the turbine, to compress (and heat) combustion charge air. 

      

What happens in the compressor section (boost) determines how much work the turbine must deliver… across the shaft, by the turbine. Simple enough.  

     

If the compressor makes more heat energy (for any reason), then it requires additional energy from the exhaust, via the turbine.   

    

Remember the statement I made earlier about the energy needed for 9000 G’s coming from the exhaust? Well here we are, full circle. That 4 psi intake restriction, which makes the compressor less efficient, requires the turbine to provide more shaft power, in turn requiring more energy from the exhaust tract which drives it. Hope I haven’t lost you, it is simply, the more the compressor has to work because of inefficiencies, the more exhaust energy must be converted to work in the turbine.    

The energy equation sometimes gets pretty complicated, but not here. The shaft connection makes this very easy to solve. Restating, and mercifully skipping the long derivation, the work energy delivered to the shaft by the turbine, W(t), MUST exactly equal the total heat energy being produced in the compressor.    

   

W(t) = Q(c), or     

  

 

W(t) ==== M*Cp*(COT-CIT)      

The extra equals, “=” symbols symbolize the turbo shaft that connects turbine and compressor.      

M is the mass air flow of the charge air, or MAF      

Cp is the heat capacity of air,      

COT=Compressor Outlet air Temp, CIT=Compressor Inlet air Temp.      

THIS IS THE EQUATION THAT UNLOCKS THE COMPRESSION REALITY. ALL THERMAL FEEDBACK ISSUES OR RESTRICTIONS OR WHATEVER, ON THE COMPRESSION SIDE OF THIS OPERATION (RIGHT SIDE), MUST BE SERVICED BY THE TURBINE (LEFT SIDE).      

Now we can calculate exactly how much turbine effort, W(t), is required. Since COT is goes down with restriction removal and compressor efficiency increase, Q(c) is reduced, and thus, turbine work W(t) is reduced. In other words, less HP goes into the turbine to drive the turbo. How much less? Well believe it or not, that skinny little shaft is seeing 80-110 HP at stock air flow rates. That was a big double take for me. Probing further, elevation makes it worse.      

In Denver, it is 120-130 HP unless you add a boost enhancing device, and then it will be over 220 HP! YIKES! Do you think this may have something to do with upper elevation turbo failures? If those numbers seem unfathomable, recall that it spins at 110,000- 190,000 rpm in these examples, so the torque on the shaft is relatively low. 220 HP on a 1” shaft is impressive still and it all comes from the exhaust. At somewhere around 300 HP, with the overspeed this represents, the turbocharger quickly self-destructs with a sheared shaft and materially fatigued bearing. I am very impressed that a 50 lb air pump can withstand these stresses for ANY length of time: hats off to Garrett! I do wonder if they were ever involved in how GM integrated the first VGT.      

FREE LUNCH SNACK     

The magic of the turbocharger is (suppose to be) that most of this turbine drive energy is not coming from the engines power production, but rather waste thermal energy in the exhaust. There is debated opinion on just how much turbine drive is parasitic and how much is free. I have seen claims range from 5% parasitic to 50% parasitic. I could not determine this exactly for this specific application, but I did determine that it is somewhat variable. For simplicity, I finally settled on 25% for evaluation purposes.       

       

Since easily 1/3rd or more of the diesel fuel combusted ends up expended as waste heat in the exhaust, we tap this energy to spin the shaft connected compressor. A reality, however is that there are some mechanical losses due to backpressure created by the additional exhaust restriction of the turbine. There is a parameter that can be monitored that gives insight into how parasitic the turbine is being. It is called drive pressure, measured at the turbine inlet. Drive Pressure Ratio is the ratio of this drive pressure to the boost developed by the compressor. A ratio of around 1.0-2.0 is usually a healthy system. That means 20 psi of boost would be seen with typically 20 psi of backpressure, for example. Not in this case, however.  

      

In reality, testing has revealed actual values of 2.0-2.5. Our 20 psi of boost is accompanied by 40 psi of drive pressure. This suggests that there is too much restriction somewhere. If we make a compressor side change that removes 2 psi of restriction, that should remove 6 psi (3X) of drive pressure, and that is what happens, it drops to 34, a 15% reduction. This, in effect, makes the turbo less parasitic, and makes boost production more of the “free lunch” we see advertised. Now the drive ratio is 34/20=1.7, a vast improvement.  

     

Let’s go back to Denver and 220 turbo shaft HP. If we use our 25% parasitic assumption then we will see 55 HP lost (.25 *220) at the wheels to additional backpressure in the above over-boost condition. By fixing the mouthpiece, that original 220 shaft HP is reduced to 160-170 HP, a 30% reduction (while simultaneously experiencing a 10% increase in compressor MAF). Now the parasitic loss to backpressure is .25*160=40 HP. That difference of +15 HP (55-40) can be measured at the wheels.   

    

It doesn’t end there. Coming out of the CAC, there is a 90 degree air temperature reduction in this improvement, from 330 F to 240 F. Power depends on air density, and air density is a function of both pressure and temperature. The rule of thumb says you reap a 1% power gain for every 10 degrees of air temp improvement. For this 90 degrees, that’s 9%, or another 24 HP gain. Added to the 15 HP parasitic savings, that’s 39 HP total. A 10% power gain can be felt, and this clearly qualifies as thrust you can feel. Assuming this added power can be used for faster speed, cooling airflow would be better. Also EGT has gone down, a very nice incidental. Some have reported over 200 F reductions.    

To sum up what happens with excess restriction, more of the engines power gets converted to waste heat in the induction system, power that is deprived of the wheels. That waste heat is manifested as increased EGT and CAC heat saturation. There are several performance debilitating affects of restriction, and removing it is essential.    

   

  

 

LLY to LB7: HEAT RELIEF     

OK, now let’s ask an obvious question:      

“Why doesn’t my old waste gated LB7 suffer these same issues?”      

Glad you asked, and it is true. 01-04 LB7: As exhaust meets the restriction of the turbine, it proceeds through the turbine OR is routed around the turbine in bypass, via the waste gate. In turbocharged vehicles, the turbine is the exhausts biggest restriction. As such, when the turbine is pushing back (backpressure) hard enough, the gate opens and exhaust bypasses the turbine, limiting shaft HP. In other words, the turbine will only be pushed so hard, limited by the waste gate. It is immune to failure and a little like trying to do pushups in wet cement. Push too hard and you sink right in. When the exhaust pushes too hard, that added pressure forces exhaust to go around the turbine, preventing added turbine and shaft stress. It’s perfect.      

Now back to the shaft connection, and the compressor. That spin force is turbine (and shaft) Horsepower. Since the turbine torque on the shaft is waste gate limited, then the compressors job is also limited. From the math above, we also know that the turbine power exactly equals compression heat. …..Can you see it now?      

The heat of compression is limited by the turbines waste gate…conversely the waste gate is set to limit the amount of heat/compression that the compressor is pre-certified to produce…being that they are shaft connected. THE WASTE GATE IS PERFECT INDUCTION OVERHEAT CONTROL… erfect Thermal Feedback containment. If we are to completely address proper induction design, the absence of the waste gate must be addressed and compensated for.      

04 and on LLY: Since the VGT in the LLY has no waste gate…there is no hardware safeguard against “thermal feedback”. The LB7 shaft may be limited to 100 HP, regardless of how much thermal feedback exists, and hence 100 HP worth of compressor heat. With no waste gate, the LLY can run well beyond 200 HP, so induction heat can double with thermal feedback influence…and does.      

Now if anyone tells you that a waste gate limits boost, you can politely disagree and claim that it does not. As its core purpose, it limits boost related heat production. As described earlier, Thermal Feedback cannot be eliminated completely without a work limiting (waste gate) device. What can be done? Nothing is as simple and reliable, but tuning software can control boost such that the VGT attempts to maintain a constant pressure ratio with changing ambient pressure conditions. This has challenges, and will be the topic of a future article that will introduce a new tuning tool, EFILive.      

COLLATERAL DAMAGE     

This added heat hurts in many less obvious ways. Ironically, the heat overload has been known to melt MAP (boost) sensors, typically damaging them in a way that would make them read low. You guessed it, that creates more boost, and more meltdown.      

Also, if you own a medium duty truck with black zytel nylon intercooler end tanks that are leaking and oil stained, they have melted and deformed…now you know why… “wink, wink”. Another not-so-smart idea. Nylon and aluminum have dissimilar heat expansion properties.      

A NEW INDUCTION OVERHAUL KIT IS NOW
AVAILABLE TO REMEDY THIS ISSUE.     

      

In addition to the machined LLY mouthpiece mentioned earlier, this affordable 6 piece Induction Overhaul Kit ends these performance restrictions and puts the LLY in a new class of performance and towing capability. Material tested to 350 F. It does not require any tuning changes, does not require MAF rescaling, does not require dipstick relocation, and has a very clean, oem appearance. It is far and away the smartest change you will ever make, guaranteed with full refund. Certainly do this before playing with tuning or boost.      

WHAT YOU WILL GET:      

15%-45% reduction in turbo pressure ratio, 3-6% more compressor efficiency, 7-9% at higher elevation. The main results of this are:      

1. HP-increased by 30-45 HP on grade (typical) and as much as 60 HP fan off.      

2. EGT-lowered by 100-200 F (typical) up to 250 F on highly boosted applications.      

3. Turbo Longevity-increased with lowered shaft RPM, approx 20,000-40,000 rpm less.      

4. Economy-increased with lowered parasitic activity, 1.0-2.5 mpg in field reports. This soon pays for itself.      

5. Steep reduction of noisy fan activity on hot days or heavy loads, 40-80% reduction.      

6. Coolant temperature benefits-removes THE cause of LLY load induced overheating, a heat-soaked CAC sitting right in front of the radiator. This also factually lowers transmission and engine oil temperatures.      

Other Lesser Advantages:      

Spool up time-reduced      

Compressor stall/ Turbo Bark-eliminated      

Lowered under hood temperatures      

Performance Garage Notes:      

It is widely accepted that 32 PSI boost represents peak capabilities for power production on the stock LLY. At this level, it has been shown that there is 8 psi of inlet restriction. If 6 psi (75%) of this is removed, then you can use that 6 psi to increase power. It also suggests that the turbo failure threshold is also 6 psi higher. That is a bold increase in airflow capability, with lower charge air temperature. OR conversely;      

If you typically use 32 psi now, and do this change, you should reduce boost to around 26 psi to maintain the same MAF and power level. This will add to the heat reduction benefits.      

The lower EGT in each case is considerably safer as well. Skeptical? I hope so. It is disappointing to see how many people just believe everything that they are told. The result of fixing it is more airflow, less heat, and significantly better performance. Ironically, this is what the intake companies want you to believe that they are doing for you. Ironically not one of these companies includes a better flowing mouthpiece. And ironically, 4 years after the debut of the LLY, not even one of these companies with their “Million dollar flow bench” builds an intake to address a real issue where real improvement can be had. Hey, I am just a guy with a $15 gauge and persistence.      

You may have seen efforts to address the unconventionally high exhaust drive pressure ratios, 2/1 and higher, up to 3/1, by looking to improve exhaust side turbine flow, with debatable progress. After all, the turbine is shaft connected to an anchor, the compressor. All we did here was trim the weight off the “anchor”, EGT and drive pressure dropped, a thermodynamic consequence of making the shaft connected compressor more efficient.      

WRAP UP     

Waste heat processes, made worse by plumbing inefficiencies, rob you of fuel economy, performance, longevity and load carrying capability. There are no band-aid cures, you must remove the cause, or you will be left with all the other symptoms.      

The LLY suffers from the introduction of undersized plumbing, the demise of the waste gate, and the absence of a suitable work limiting substitute. This limits the workload capability of all LLY vehicles, and disables the cooling system radiator with added ambient heat generated by the CAC, made worse by the lower ambient pressure found at higher elevation (see solution below). This is explored in a more comprehensive work which more thoroughly explains the concept of induction “Thermal Feedback”, and it’s culpability to overheat issues in turbocharged work vehicles.      

So there you have it, induction is “All Hot Air”, as the title asks. But it need not be.      

Problem 2 Solution:      

With boost augmented at altitude, there is 6 psi of inlet restriction, and 3.5 psi of outlet restriction. PR=(40+3.5)/(12-5)=6.2. It can’t be plotted because the page isn’t big enough! If you were unlucky enough to be reading this after you have already replaced your turbo, now you have an idea why it died. Use added boost only to define the power you want. DO NOT use boost devices exclusively to lower EGT. Instead, remove the cause of high EGT. In the process, you will solve several other problems. When you increase boost just to lower EGT, you create additional problems. Instead, first eradicate the source of performance eroding restriction, you will be impressed how this approach transforms a vehicles performance. It also increases MAF, Longevity AND Performance while reducing EGT with no harmful consequences;. If you try to use 32 psi of boost at this altitude, the PR calculates to over 12.0 with the added air flow and restriction.