This Duramax made a strong case for looking past boost pressure and watching air density instead. With the twin turbos feeding a screw supercharger, we saw the odd result: less blower speed made more power. The likely reason is parasitic loss. A screw blower compresses air internally, and when it has to work on air the turbos have already packed in, the drive requirement can climb hard without delivering a matching horsepower gain. That showed up clearly on the dyno. Backing the supercharger down dropped airflow only slightly, but horsepower jumped and brake specific fuel consumption improved dramatically. In other words, the engine got more efficient because less power was being spent driving the blower. From there, adding fuel carried the combination past 1,200 horsepower, but exhaust temperature pushed beyond the target limit for comfort. The real takeaway is simple: these chargers are density machines, not just boost machines. If the system makes big pressure but wastes power doing it, the combination is working the wrong way.
Gale Banks opens in Dyno Cell No. 1 with an unexpected result from the supercharged, twin-turbo Duramax program: every time blower speed is reduced, horsepower increases. The team had already made four key runs. The first produced 820 horsepower. After backing down the blower slightly, output jumped to 956 horsepower. Reducing blower speed again raised power to 1,017 horsepower, all while using essentially the same fuel and maintaining a nominal 20:1 air-fuel ratio on the first three runs. On the fourth run, blower speed was left alone and more fuel was added, producing 1,172 horsepower at roughly 15:1 air-fuel ratio.
That trend suggested the turbochargers were responding favorably as blower speed came down. Exhaust gas temperature was beginning to approach the edge of the safe range, but Banks still wanted to reach 1,200 horsepower. The immediate question was not whether the engine could make the number, but why reducing supercharger speed was improving the combination instead of hurting it.
Banks frames the entire test around manifold air density rather than boost pressure. His target for 1,200 horsepower is a manifold air density of 250 pounds per 1,000 cubic feet or more. In his view, manifold air density is the primary indicator of an engine's power potential, whether the engine is naturally aspirated, turbocharged, supercharged, or, as in this case, using twin turbos, a supercharger, and two charge-air coolers.
He emphasizes that turbochargers, the supercharger, and both charge coolers are all density machines. Their purpose is not simply to create pressure, but to increase the mass of air entering the intake manifold. At 250 pounds per 1,000 cubic feet, every four cubic feet of intake air weighs one pound, or one quarter-pound per cubic foot. On the iDash DataMonster, the top reading is manifold air density, the second is manifold air pressure, and the third is manifold air temperature. Banks treats boost as only one input to the density calculation, not the final answer. For this run, alarms were set at 1,850 degrees EGT and 17.5:1 air-fuel ratio.
During the live pull, the engine was brought in around 2,300 rpm and loaded upward while the team monitored density, coolant heat exchanger operation, and coolant flow, which was reported at 47 gallons per minute. As the run progressed, power climbed past 800 horsepower and then beyond 1,100 horsepower. Banks initially saw 1,205 horsepower on the screen, but later data review showed the run actually reached 1,218 horsepower.
The pull also triggered concern over turbine-related limits, prompting an immediate review of the logged data. Banks notes that the SD card contains hundreds of runs, but he selected five representative tests to explain the development path: supercharger only, the first twin-turbo-plus-supercharger run, a reduced-blower run, a safe-limit run near 1,200 horsepower, and the final run that exceeded the target.
The first baseline used the Whipple supercharger alone with Zoomie headers. This engine began life as a monster truck program and evolved into what Banks calls Mad Max, so the original goal was sharp throttle response. The blower was tested by itself first to verify that response before adding the twin turbos.
In this configuration, the 7.0-liter Duramax made 642 horsepower at 5,500 rpm with 20.9 psi of boost. Banks considered that low boost for a diesel, and he says the engine did not develop the swirl rpm associated with higher boost pressure, leading to inefficient combustion. The supercharger was driven at 100 percent over crank speed, so at 5,500 engine rpm the blower turned 11,000 rpm. Air mass flow was 118.2 lb/min, requiring 1,624 cubic feet per minute of ambient air. To achieve an 18:1 air-fuel ratio, the engine consumed 6.5 lb/min of DF2 diesel fuel.
Exhaust gas temperature was a comfortable 1,296 degrees, helped by the complete absence of backpressure or turbine drive pressure. However, the camshaft had been designed for an engine with drive pressure. Without it, valve overlap allowed air to blow through the combustion chamber, diluting the measured air-fuel ratio with extra oxygen. That meant the indicated 18:1 ratio might not have reflected true combustion conditions. The result was a very inefficient engine, confirmed by a brake specific fuel consumption of 0.61 lb/hp-hr. Banks says that is poor, and that a more desirable BSFC would be in the 0.400 to 0.500 range.
He also explains why the blower was not spun harder. A graph of boost and compressor discharge temperature showed that around 9,500 supercharger rpm, boost began to flatten, and by 10,000 rpm it had leveled off at 20.9 psi. Temperature, however, kept rising. At 11,000 rpm, discharge temperature had reached 338 degrees and was climbing rapidly toward a 350-degree limit. That temperature ceiling led him to leave blower speed where it was.
The next step was to add the twin turbos ahead of the supercharger, blowing through an intercooler, into the Whipple, through another massive intercooler, and then into the engine. On the first compound run, output rose to 820 horsepower. Although that was 178 horsepower more than the blower-only baseline, Banks considered the gain far too small for what the data suggested should have happened.
Boost had increased by 108 percent, more than doubling, yet the power increase was only 28 percent. The wastegates never opened, turbo speed was a safe 128,000 rpm against a 148,000 rpm maximum for the Precision turbos, and the supercharger remained at 100 percent overdrive, still turning 11,000 rpm at 5,500 engine rpm. Air mass flow climbed to 165.4 lb/min, a 40 percent increase, and ambient airflow demand rose to 2,272 cfm. Fuel flow increased from 6.5 to 8.5 lb/min, up 31 percent, while air-fuel ratio actually leaned slightly to 19.5:1.
EGT rose 17.6 percent to 1,524 degrees, and BSFC worsened to 0.620. Even with the leaner mixture, the engine became 1.6 percent less efficient. To Banks, every major indicator said the engine should have been making much more power than it was. Something in the compound arrangement was consuming energy or failing to convert airflow into useful output.
Banks then focused on the supercharger itself. Because a screw blower compresses air internally in the rotor set, unlike a Roots blower, he suspected the Whipple was being asked to compress air that was already too dense from the turbochargers. His theory was that feeding high-density air into the screw blower was dramatically increasing parasitic drive load. If that was true, slowing the blower might actually free up horsepower.
That is exactly what happened. Supercharger overdrive was reduced from 100 percent to 33 percent, dropping rotor speed from 11,000 rpm to 7,315 rpm, a 33.5 percent reduction. Boost fell 9 percent to 39.6 psi, and turbine drive pressure dropped 8 percent from 55.8 to 51.4 psi. The wastegates still never opened, and turbo speed actually decreased by 2,000 rpm. Air mass flow slipped only 3.9 percent, from 165.5 to 159 lb/min, with ambient airflow at 2,184 cfm. Fuel flow also dropped, from 8.5 to 8.1 lb/min, nearly a 5 percent reduction.
Despite less blower speed, less airflow, and less fuel, horsepower jumped by nearly 200. Air-fuel ratio became slightly leaner, EGT fell from 1,524 to 1,492 degrees, and BSFC improved dramatically from 0.620 to 0.480. That represented a 26 percent increase in engine efficiency. For Banks, this was the clearest evidence yet that forcing very dense turbocharged air into the screw blower was creating a major parasitic penalty.
With EGT and turbo shaft speed still appearing manageable, Banks decided the next run should define the safe limit of the combination. For short-duration operation on these Precision turbos, he considered 1,750 degrees EGT to be the limit. The goal was therefore simple: add enough fuel to reach about 1,750 degrees and see what power the engine would make.
That fourth run produced 1,172 horsepower. Combined boost rose 6.6 percent to 42.2 psi. Turbine drive pressure, however, spiked from 51.4 to 60.7 psi, an 18.1 percent increase. Even so, the wastegates still did not open, and turbo shaft speed remained unchanged at 126,000 rpm. Supercharger overdrive stayed at 33 percent, with blower speed still 7,315 rpm. Air mass flow increased only 2.6 percent, but fuel flow rose 32.1 percent in order to reach the target EGT. Air-fuel ratio richened substantially to 15.2:1, which Banks describes as fat.
There was likely some smoke at that point, and EGT reached 1,789 degrees, slightly above the intended red line. BSFC worsened again to 0.550, making the engine 14.6 percent less efficient than the previous reduced-blower run. Even so, Banks judged the setup safe enough for short-duration events and close enough to the target to justify one final push.
To gain the remaining power needed for 1,200 horsepower, the team added more fuel. The target was only another 28 horsepower, but the engine overshot and reached 1,218 horsepower in the data. Boost increased another 6.6 percent, drive pressure rose 4.4 percent, and turbo shaft speed climbed to 132,000 rpm. Supercharger overdrive and blower rpm remained unchanged. Air mass flow rose to 167.9 lb/min, up 2.9 percent, and ambient airflow reached 2,306 cfm. Fuel flow increased 3.7 percent, while air-fuel ratio changed only slightly from 15.2:1 to 15.1:1.
The problem was temperature. EGT jumped from 1,789 to 1,876 degrees, 7.2 percent beyond the temperature deadline. Banks notes that the turbochargers still looked fine afterward and the engine survived the run, but he does not consider that acceptable operating territory. Efficiency remained poor at 0.550 BSFC, confirming that the engine had reached the power goal by adding fuel rather than by improving the effectiveness of the air system.
Banks closes by arguing that the data still do not explain why the compound system is so inefficient or why it fails to make the power he believes the airflow and boost should support. In a series-compounded arrangement, the boost numbers are impressively large, but he says the result does not pencil out. The turbos alone are capable of producing roughly the same 45 psi seen on the final run, so blowing into the supercharger should have multiplied the effect rather than merely matching it.
He wants to break the system down by component and determine which devices are actually producing boost, which are producing density, and how much horsepower each contributes. That includes the ambient air density, both turbochargers, the supercharger, and the intercoolers. His view is that these are not boost machines but density machines, and only by assigning density gain and horsepower contribution to each stage can the team identify what is helping and what is hurting.
Although the engine achieved 1,218 horsepower, Banks is not satisfied. His conclusion is that the current order of the compound system is likely backwards. He suspects the arrangement of turbos first and blower second is fundamentally wrong, and he intends to prove it with the data in the next episode.