More Power With Less Boost: Banks 1,200-HP Tri-Charged Duramax

We’ve said it before, and we’ll say it again: when Gale Banks speaks, we listen. A while back we brought you the 10th installment of Banks’ Diesel Monster Truck Engine and you loved it. Now, Banks has modified that configuration into what he now calls “Mad Max” which has pushed the testing beyond what was ever originally intended.

Gale Banks doesn’t do anything half-assed, and he certainly doesn’t conduct his testing in an unorganized, haphazard manner. In fact, this 2.5-year-long process has been to ensure that not only do all the goals for the project get achieved, but they do so with a minimum (read: none) of high-speed dyno-cell redecoration.

Recently, he achieved his goal of making 1,200 horsepower with the setup, and while for most people, that would be the entirety of their video, Banks — ever a scholar — wants to not only dissect his findings on the dyno, but explain them all to the audience. In a world where data is hoarded and protected like the gold in Fort Knox, Banks is freely sharing not only his findings, but explaining the findings as well.

This engine is quite a feat of engineering. Two 68mm turbochargers feeding into a 5.0-liter Whipple twin-screw supercharger is pretty uncharted territory.

If all you want is to see the big-number horsepower runs, the first five minutes of the video has you covered. Banks starts out by recapping the development work done to the Mad Max 7.0-liter Duramax L5P-based engine. The current configuration consists of a par of Precision Turbo 6870 ball-bearing turbochargers feeding into a Whipple 5.0-liter twin-screw supercharger, in an impressive twin-charging (or would that be triple-charged?) setup.

Using his preferred metric of Manifold Air Density, Banks breaks down the amount of air he needs to reach his 1,200-horsepower goal, and how, exactly, it will be measured on the dyno. “Manifold Air Density is the best indication of the engine’s power potential. You can forget about boost pressure, because it’s part of the MAD calculation. Manifold Air Density is the bottom line,” says Banks.

However, during testing, Banks found that if he “turned down” the supercharger, he started making more power — which goes against common logic. If you’re actually interested in the data and Banks’ findings, you need to watch the entire video and read past this portion of the article.

In order to make it easier for viewers to understand the various configurations of the engine and stages of testing, Banks has broken it down into five sections, which he thoroughly explains in the video, and we’ll summarize here.

Banks is quite fond of using Manifold Air Density to measure power potential instead of boost pressure or MAF counts alone.

“Just” a 5.0-Liter Whipple Supercharger

“Originally, this was a Monster Truck build, and we wanted snappy throttle response,” says Banks of the first iteration of the engine, with just displacement and a 5.0-liter Whipple twin-screw supercharger on it (which was tested in several variations, if you read our previous article on the subject). “We only twisted the supercharger hard enough to make about half of the power goal, since we knew we’d be adding the twin-turbos.”

On No. 2 diesel fuel (DF2), the supercharger only configuration made 642 horsepower at 20.9 psi from the Whipple. That was with a 100-percent overdrive (so, spinning the blower at twice crank speed), which provided a peak supercharger speed of 11,000 rpm. “At about 9,500 supercharger RPM, the boost started flattening out, and spinning it harder was rapidly increasing temperatures,” says Banks, which is why they settled on the 20.9 psi max boost for this test.

That much horsepower ingested 1,624 cubic feet per minute of air. 6.5 lb/min of fuel provided an 18.1:1 Air/Fuel ratiom which Banks was incredible comfortable with, and provided extremely safe 1,296°F exhaust gas temperatures.

However, Banks noted that the camshaft in that application was designed for the additional turbocharged boost and had significant overlap, which without the restrictions of a turbine in the exhaust allowed air to flow directly through the open intake and exhaust ports. This configuration was incredibly inefficient, as indicated by the Brake Specific Fuel Consumption of this test, in the 0.610 range. “Part of the reason [this combination] is inefficient, is that we effectively have no swirl RPM and at 5,500 rpm, there’s not a lot of time for that diesel combustion event to take place,” Banks explains.


Here, you can see why they decided on an 11,000-rpm, 20.9-psi peak supercharger output. At 9,500 blower-rpm boost starts to level off, and it stays the same between 10,000-11,000 rpm with charge temps increasing.

Adding A Pair of Hairdryers

The with the first results from the dyno after the turbochargers were added being lower than expected — only generating 820 horsepower — Banks started poring over the data. The new power number was only up 28-percent, while the boost level had more than doubled at 43.5 psi (+108%) and exhaust backpressure — or “turbine drive pressure” as Banks more aptly puts it — exceeded the boost pressure at 55.8 psi.

The turbo speeds were in the safe range at 128,000 rpm and produced a total 165.4 lb/min mass airflow into the engine, which was a 40-percent increase over the airflow of the supercharger alone for 2,272 cfm. Fuel flow was increased 31-percent to 8.5 lb/min, and the AFR leaned out by almost a point and a half at 19.5:1. EGTs increased to 1,524°F, while the calculated BSFC for the pull also increased to 0.620, meaning the engine became more inefficient. But why?

“[With these numbers] everything indicates we should be making a hell of a lot more power than we are making,” says Banks. “While scratching my head I got to thinking that with a screw blower, it’s actually compressing the air in the rotor set. That means we’re asking the supercharger to compress a fluid that is far more dense than it was originally designed for, and that might be whacking out the blower’s parasitic drive requirements.”

That thought led Banks to consider reducing the speed of the supercharger in the quest to make more horsepower. A generally backwards way of making horsepower, but when you consider that Banks is blazing an uncharted trail here, who is to say what is backwards.

Banks’ theory is that by forcing the twin-screw supercharger to compress and already heavily compressed intake charge, the parasitic efficiency drops significantly.

Backing Down the Blower

With his idea set, Banks reduced the drive pulley size to go from a 100-percent overdrive to only a 33-percent overdrive, resulting in a new peak blower speed of 7,315 rpm. If the last set of results was confusing, this set was damn-near baffling. With less blower speed, the total boost pressure was down to 39.6 psi (a 9-percent reduction) but horsepower was up 25-percent, with a peak power number of 1,017 horsepower.

On that pull, there was a turbo backpressure reductio of about 8-percent and a related reduction of 2,000 rpm of turbo shaft speed. There was a 4-percent reduction in mass airflow and a 4.7-percent reduction in fuel flow, now moving 8.1 lb/hr. The AFR on that pull went up slightly and EGTs dropped slightly to 1,492°F.

“What’s really cool is that BSFC went from a 0.620 all the way down to 0.480,” Banks says. That’s an engine efficiency increase of 26-percent. A-ha!” From these numbers, Banks surmises that there is an issue with shoving heavily compressed air into a screw-type supercharger. “I bet no one saw that one coming, because I sure as hell didn’t” laughs Banks.

Finding The Safe Limit

With a new hypothesis somewhat proven out, it was now time for Banks to implement his usual testing protocol to see where the safe maximum of the combination was. “The EGT limit for shorter-duration runs on these Precision turbos is 1,750 degrees. So the goal on the fourth test was to add enough fuel to get to 1,750 degrees,” explains Banks.

After making the necessary changes, the dyno rewarded Banks with a reading of 1,172 horsepower. The additional fuel (flow was increased 32.1-percent) brought the AFR down to 15.2 in an area Banks refers to as “fat city.” The boost number increased to 42.2 psi with a huge increase in turbo backpressure, up 18.1-percent to 60.7 psi, but with no change in turbo shaft speed from the previous test.

EGTs peaked at 1,789°F, right at if not slightly above the safe limit for the turbochargers. As the increased backpressure might suggest, overall engine efficiency dropped on that run, with the BSFC increasing to 0.550 lb/HP/hr, which Banks is ok with in short durations.

Setting It On Kill

With the “safe limit” bringing Banks only 28 horsepower from his goal of 1,200 horsepower, it was time to get a little risky. By adding a little more fuel, 11.1 lb/min, Banks was rewarded with an additional 2.8 psi of boost (45.0 psi total) and a horsepower result of 1,218. That extra boost added some extra backpressure in the exhaust, but also netted another 4,000 rpm of turbo shaft speed, for a total of 132,000 rpm.

The AFR richened up a bit at 15.1:1 but the BSFC stayed the same at 0.550. Unfortunately, that run pushed the EGTs over 100 degrees past the safe limit, peaking at 1,876°F. While no lasting damage was seen on the Precision turbos, Banks isn’t comfortable staying there for any period of time.

“I’m looking at that last run’s boost pressure and thinking that the turbos alone can make 45 psi. Blowing into the supercharger should have multiplied the boost numbers,” says Banks. “This just doesn’t pencil out.” To that end, he now has a new hypothesis: he thinks that the supercharger and the turbochargers are just in the wrong order.

“You’d think I’d be happy. We just made 1,218 horsepower. But, I think we’re doing this all wrong. I think this ‘turbos first and blower after’ setup is backwards. And I think I can prove it with all the data in the next episode,” says Banks, confirming that he’s not done with this project yet.

Here you can see all of the test results next to one another with Banks' thoughts on the right.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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