This article originally published in Issue 53 of the Turbo Diesel Register.

Technical Topics

PART FOUR: FUELING BOX POWER ENHANCEMENTS FOR THE '03-'04 (305HP) ENGINES

In part three of this series (Issue 49, page 36) I reported achieving 500-plus horsepower at the rear wheels by using the TST PowerMax CR fueling enhancement and the ATS Aurora 5000 large single turbocharger.

This article continues to analyze power enhancement products by reviewing three new aftermarket fueling choices that deliver approximately 350 to 400 horsepower to the rear wheels. A different turbocharger choice, the ATS Aurora 2000 “small single,” which is appropriately sized for the 350 to 400 power levels, will be discussed. The turbocharger is central to any power enhancement project, as it ultimately determines the amount of air passing through the engine, as well as dictating tradeoffs between drivability and the power levels it can reliably support.

Throughout the article, I will present data comparing the Aurora 2000 with the Holset HY-35 as supplied on the stock truck. It is not my purpose to restate the shortcomings of the HY-35 in higher-than-stock horsepower applications, as this has already been thoroughly discussed throughout this series and in other
articles as well (see Joe Donnelly’s discussion in Issue 52, page 98). While some mention will be unavoidable, I will concentrate on
performance comparisons during actual driving conditions to show how theory correlates to the real world. The testing will show that the primary role of the turbocharger is to increase the air passing through the engine and, as in the previous articles, I will compare some of the important turbocharger performance parameters such as compressor output, temperature, and drive pressure.

I follow the general practice of describing the size of a turbocharger by referring to its compressor in terms of “small single” and “large single” which are used quite subjectively.

While the specific turbochargers tested here and in Issue 49 fit these generalizations, one should realize that there is much more to turbocharger performance than compressor size. Every design is a unique combination of individual components (turbine and compressor wheels, exhaust housings, compressor housings, bearings, wastegates, etc.) selected to fit a specific design goal. It is interesting to note that some turbocharger manufacturers build and/or market the entire product from the ground up, some achieve particular design goals by assembling purchased components, and still others build final products from a combination of purchased and proprietary (or even modified) components. large compressors. Generally, the small single/small compressor turbocharger will produce:

1. Faster spool-up, due to the smaller size of the compressor and exhaust housing.
   
   
2. Less (or zero) tendency to surge because a greater portion of actual driving conditions are within the compressor’s efficiency map.
   
   
3. Higher exhaust gas temperatures (at higher power and boost levels) due to less efficient turbines and compressors.

The ATS Aurora 2000 is designated as a small single because of the smaller size of its compressor. The picture shows the Aurora 2000 and, for comparison purposes, the Aurora 5000 large single turbocharger that was tested in Issue 49.

The ATS Aurora 2000 small single (right) shown next to the Aurora 5000
large single (left). Note the smaller compressor housing of the Aurora 2000,
used to achieve fast spool-up. The Aurora 2000 is useful for
power levels up to approximately 450 horsepower at the rear wheels.

As it turns out, the stock Holset HY-35 compressor is similar in size externally to the Aurora 2000 compressor, and it fits the mall single designation as well. However, this is where the resemblance ends. Test data confirm that the Aurora 2000 has a much more efficient turbine than that which is found in the stock turbocharger, and the Aurora 2000 is capable of substantially higher air flow. For comparison, from Issue 49, page 39, is a picture of the stock
turbocharger next to the Aurora 5000.

The very large ATS Aurora 5000 (left) shown next to the stock
turbocharger supplied for the '04.5-'05 trucks (center) and for the '03-'04 trucks (far right).
The Aurora 2000 turbine lowers drive pressure and increases turbine efficiency
compared to the stock Holset HY35 turbo (not shown) without sacrificing spool-up.

For additional comparison purposes the Aurora 2000 turbine (right) is shown
next to the Aurora 5000 turbine. The Aurora 2000 turbine lowers drive pressure and increases turbine
efficiency compared to the stock Holset HY35 turbo (not shown) without sacrificing spool-up.

The Aurora 2000 is a hybrid design built around a combination of purchased internal parts (wheels and bearings), and proprietary castings manufactured by ATS. The internal components have been selected for extreme durability, high rpm and heat tolerance, as evidenced by the three-year, 150,000 mile warranty. The shaft bearings, normally stationary in conventional designs (including the stock turbocharger), are themselves allowed to spin independently of the shaft they support, creating dual-concentric oil film surfaces to bear the load. This greatly increases durability and allows for very fast wheel acceleration. The dual-concentric bearing design can also endure extremely high shaft rpm because the oil film directly supporting the shaft itself (inner bearing surface) sees only about half of the actual shaft rpm.

Figure 2 shows the simple bearing structure of the stock HY-35 turbocharger. The shaft itself (not shown) extends from one bearing, through the spring, and into the opposite bearing. The purpose of the spring is to spread the bearings apart, forcing them against the machined surface of the cartridge.

Figure 2: Simple, low-cost bearing structure of the stock turbocharger.
The bearings are held in place by the spring (forcing them outward
against the cartridge) and see the full rpm of the shaft.

Figure 3 shows the more robust bearing structure of the Aurora 2000. Not only are the bearings themselves larger, each bearing is kept in place by two snap rings instead of a separating spring. This precision placement allows them to fit into an oil-fed, machined race so that they can rotate with respect to the cartridge housing.

Figure 3: More robust bearing structure of the Aurora 2000.
The bearings are held in place with snap rings which allows
precision placement in the cartridge. The bearings rotate with
respect to the cartridge and independently from the shaft, and
see only about half of the actual shaft rpm.

The castings manufactured by ATS contain proprietary alloy formulas which are optimized for heat tolerance and resistance to cracking. The in-house foundry allows them to manufacture custom geometries to obtain bolt-on compatibility with the stock exhaust system. This means that the various aftermarket exhaust brakes (which take the place of the stock cast elbow) are bolt-on compatible with the Aurora 2000.

During this evaluation two other components were installed on the test truck: The ATS two-piece exhaust manifold and the Scotty 3 air intake system from Scotty Systems. The exhaust manifold was recommended by ATS as a more durable, heat tolerant component. It is virtually immune to cracking, because of the interference fit that expands and contracts as needed, and also because of the particular alloy formulations used in its manufacture.

I found the Scotty 3 air system (Issue 52, page 95) to be an effective companion to the Aurora 2000 turbocharger. Because of its enclosed design, the Scotty 3 is effective at quieting down an otherwise noisy turbocharger. In addition, the dual-inlet design brings in air from the front grille as well as the stock location (right front fender), and its two-stage, oiled foam filter is an excellent combination of filtration efficiency and flow. In my tests of similar filter media using engine lube oil analysis, I found silica levels (a typical indicator of dirt) to be as low as I have ever tested.

With these parts installed on the test truck, the first thing I did was to run the Aurora 2000 at the stock fueling level. I wanted to see how responsive the turbocharger was and how closely its performance worked with the OEM fueling curves. Were it not for the increased whine from the compressor, largely muffled by the Scotty 3, it was difficult to tell that I had installed an aftermarket turbocharger. Spool-up was very nearly the same, and the turbocharger easily sailed into the 26 psi (boost pressure) region, which for the ‘03-’04 trucks is higher than the stock ECM allows. Because the Aurora 2000 contains no wastegate and is therefore capable of higher boost levels than stock, I found that the ECM de-fueled fairly aggressively to control maximum boost levels. This was reflected on the dyno, where the horsepower curve showed a particular wavy behavior as the ECM corrected for what it thought was too high of a boost level (see page 18 for the horsepower graphs). Thus, it is obvious that the Aurora 2000 should be used with a power module that utilizes “boost fooling” logic to the ECM.

Changing to higher fueling levels, I found the Aurora 2000 turbocharger to be quite happy in the 400 horsepower region, producing boost pressures into the mid 30s at 2,700 feet above sea level. This turbocharger is well suited for maximum boost pressures in the upper 20s to low 30s as well as for sustained operation at 350 to 400 horsepower. Its durability suggests that it could easily accommodate bursts into the mid 30s and 450 horsepower. Presenting absolutely no surge behavior, this turbocharger appears to be well suited for towing.

Next up I will evaluate the Aurora 2000 against the stock turbo under driving conditions. Consistent with Issue 49, I collected data on the compressor and turbine sides individually (via compressor output temperature and drive pressure, respectively). For this article, I was able to collect meaningful exhaust gas temperature data as well. Typically exhaust temperature studies suffer from repeatability and accuracy problems as weather and driving conditions vary. But by collecting a substantial amount of data (some 700 points) over the same test loop with consistent daytime temperatures, I was able to generate a compelling set of results. I used slow and deliberate acceleration and then captured data points simultaneously using a SPA digital gauge’s peak detect functions.

The areas of study will be as follows:

• Compressor output temperature
• Drive pressure
• Exhaust Gas Temperature
• Turbine Differential Temperature

I measured compressor output temperatures by inserting a low-temperature thermocouple directly into the compressor discharge. I also measured the intake air temperature on the turbocharger side of the air filter, and subtracted this figure from the compressor output temperature. As the goal of the turbocharger is to increase air passing through the engine without too much heat build-up, any reduction in the air temperature exiting the turbocharger is welcome. Figure 4 is a graph comparing the compressor output temperature of the Aurora 2000 with that of the stock HY-35.

Figure 4: Scattergram of normalized compressor output temperature
versus boost, for both the stock turbocharger and the ATS Aurora 2000
small single under highway driving conditions. At 2000 rpm, the air
delivered to the intercooler by the Aurora is approximately
10-20 degrees cooler than that produced by the stock turbo.

The pressure of exhaust gasses in the manifold, referred to as drive pressure, tells you how efficiently the turbocharger evacuates exhaust gasses through the turbocharger exhaust housing, past the turbine wheel, and into the exhaust downpipe. The small exhaust housing of the stock turbocharger serves to accelerate exhaust gasses as they approach the turbine wheel. This design yields very fast spool-up and supports moderate power levels, but presents a pressure restriction as the exhaust gasses exit the manifold. Such a restriction manifests itself in the form of drive pressures that are higher than incoming boost pressures and results in high exhaust gas temperatures.

Figure 5 shows the drive pressure behavior of the Aurora 2000 turbine at cruising speeds. The straight line in the graph represents the condition where drive pressure equals boost pressure. Note that for the Aurora 2000, drive pressure is typically lower than boost pressure. By comparison, the stock turbocharger runs with a drive pressure that is always higher than boost pressure. Comparing the two turbochargers against each other reveals that the Aurora 2000 runs approximately 5 psi lower in drive pressure during moderate to high boost pressures.

Figure 5: Scattergram of turbocharger drive pressure, comparing the
Aurora 2000 with the stock HY-35 at 2000 rpm. The Aurora runs with
drive pressure approximately equal to or less than boost pressure.

The Aurora 2000 was specifically designed to address high exhaust gas temperatures during heavy loads. With its responsive spool-up behavior, I wondered how much improvement I would see compared to the stock turbo. Figure 6 shows actual exhaust gas temperatures measured using the Aurora 2000 and the stock turbo. The temperatures were measured pre-turbo (in the exhaust manifold) under the same driving conditions over the same stretch
of highway. Two striking features emerge from the data:

1. The Aurora 2000 actually ran a tiny bit hotter than the stock turbocharger at low to moderate boost pressures. Under these conditions both turbochargers performed very well in this region, producing exhaust gas temperatures well below 1000°.

2. The real story is in the shape of the exhaust gas temperature curves at high boost pressures. Here the Aurora 2000 clearly outperforms the stock turbocharger. At boost pressures above the low 20s, the graph shows how fast exhaust gas temperatures rise for the stock turbocharger approaching approximately 1300° at 25psi of boost. The Aurora delivered a 300° exhaust gas temperature advantage at this power level.

Figure 6: Graph of actual exhaust gas temperatures under equivalent driving conditions,
comparing the Aurora 2000 with the stock turbocharger. When boost pressures are in the low 20s to low 30s,
the stock turbocharger does not operate efficiently while the Aurora 2000 is quite happy.

The Figure 7 scattergram shows the difference between pre-turbo and post-turbo exhaust temperatures, or turbine differential temperature. The results are quite revealing. I installed two identical SPA exhaust gas temperature gauges in the test truck (one before the turbocharger and one after) I was able to meaningfully compare the performance of both the stock and the Aurora 2000 turbines under the same conditions. As an aside, the Aurora 2000 itself provided an additional convenience: because it bolts up to the same exhaust cast elbow utilized by the stock turbocharger, I was able to use the same thermocouple install locations for both turbochargers.

Do you notice the similarity in shape of figure 6 to figure 7? The data shows why the Aurora 2000 yields such impressive exhaust gas temperature performance with boost pressure well into the mid 30s. As boost approaches the mid 20s, the stock turbine doesn’t efficiently utilize the energy in the exiting exhaust gasses, causing turbine differential temperature to rise significantly. Over this same region the turbine differential temperature of the Aurora 2000 remains relatively constant, revealing a more efficient design.

Figure 7: Scattergram of turbine differential temperature, comparing the stock HY-35 to the Aurora 2000.
The similarity in shape between this graph and figure 6 shows that the superior exhaust gas temperature
behavior of the Aurora 2000 is largely due to turbine efficiency.

This result also shows why attempts to increase boost by disabling or modifying the wastegate of the stock turbocharger have not met with resounding success. The limitation is in the entire turbine assembly: the turbine wheel and exhaust housing, not just the wastegate set point. The data shows that when boost is high, the energy trapped in the turbine itself (in the form of heat) rises significantly. This means any additional boost created by modifying the wastegate is useless because the turbine cannot evacuate the additional air. The result is additional stress on the turbocharger bearings due to increased shaft rpm and higher (but useless) boost pressures.

Recall my earlier statement that the primary role of the turbocharger is to increase air passing through the engine. While we tend to emphasize the air intake side, compressor efficiency and the importance of supplying cool air to the engine, it is just as important to consider the exhaust side, turbine efficiency and the importance of evacuating hot air from the engine. The previous two graphs show that the compressor side of the Aurora 2000 is more efficient than that of the stock turbocharger at the test point of 400 horsepower. And you should note that the exhaust gas temperature advantage of the Aurora 2000 at high boost pressures is most heavily influenced by its turbine.

• The obvious: The stock HY35 turbocharger is a good match for the stock 305 horsepower or 325 horsepower rated HPCR engines.

• Modern small single compressor designs, with extended compressor wheels and more efficient housings, can deliver a greater charge density and increase power output when compared to the stock turbocharger. For example, the Aurora 2000 with a four-inch exhaust and Scotty 3 air system installed produced 27 additional horsepower in the tests presented here.

• At high boost pressures the turbine of the stock turbocharger shows a sharp increase in turbine differential temperature, trapping energy in the turbine and preventing the evacuation of exhaust gasses. An aftermarket design, such as the Aurora 2000, can dramatically improve exhaust gas temperature behavior and increase usable power.

• By making efficiency improvements to both the turbine and compressor sides, an aftermarket small single turbocharger can be a good replacement for the stock unit supporting moderate power increases while maintaining stock-like drivability.