“The boost gauge is seriously flawed. I’m here to kill the boost gauge”
“The true arbiter you should be talking about is not boost, it’s MAP, or manifold absolute pressure,” explains Banks. “If you want to talk about pressure in a supercharged engine, or a naturally aspirated engine, that’s what drives the charge into the cylinder. The boost gauge never includes the ambient pressure, but that is part of the charging of the cylinder. So, stop talking boost and start talking MAP.”
The starting point for Banks’ discussion was recognizing air density, which is a function of temperature, atmospheric pressure and humidity. He then referred to the SAE J-607 standard day formula, which is used as a correction factor for testing engines on the dyno, and calculated that 100 percent ambient air density (AAD) occurs anytime 1,000 cubic feet of air has a mass of 76.4 pounds.
“You mix pounds of air with pounds of fuel,” reminds Banks. “You don’t mix a boost number. It’s pounds of air with pounds of fuel, that gives you the air-fuel ratio, and the air-fuel ratio is pretty sacred. You don’t want to get too far off.”
There are any number of conditions that provide racers with 100 percent ambient air density, as demonstrated by the chart above. Whether it’s a change in temperature, altitude (which affects pressure) or humidity.
“When I say 100-percent day, the dyno correction factor corrects you back to that value,” says Banks. “The engine cares about the air mass, it really doesn’t care about those other numbers. Because without the air mass, you don’t burn the fuel mass.”
Further massaging of the formula reveals that 10 pounds of air per minute is required to make 100 horsepower with gasoline.
“Now, let’s tie this all together. Engines pump CFM,” says Banks. “The density of the CFM determines the horsepower.”
For the sake of clarity, Banks is referring to engine CFM, not the inductive CFM that measures air passing through a carburetor or throttle body.
“The throttle does not control the engine CFM, it controls the intake manifold air density,” adds Banks, noting that “density machines” include but are not limited to cold-air scoops, turbochargers, superchargers and intercoolers. “Density machines increase or decrease the manifold air density, or MAD, of the engine CFM being pumped. That’s why without a throttle your gas engine would never idle. The mass flow would be too high. Diesels are a different matter, as they throttle on fuel.”
According to Banks, MAD is extremely important because MAD multiplied by the engine CFM pumped determines the engine’s power output. He also says MAD can be displayed as a mass per unit volume or a percentage of the J-607 standard day. He prefers the percentage, and that’s the factor shown in all the accompanying illustrations.
The baseline engine used for the demonstartion is a Hilborn-injected small-block Chevy that Banks built in the early ’60s for land speed racing in a Studebaker. Like others, he followed standard tuning advice for jetting carbs or pilling the Hilborn, much of which relied on a log book of conditions at the track.
“But I wanted to know, what was the pressure and temperature in the intake manifold?” remembers Banks.
Using surplus military gauges and a sling psychrometer he could compute density in the intake manifold. On a 100-percent, or J-607 day, at sea level on a dyno in Long Beach, California, the engine recorded 580 horsepower. Under different conditions at El Mirage and Bonnevile but with the same temperature, you can see the power drops.
“Manifold air density and power are directly related,” stresses Banks. “I thought 580 horsepower would be enough to get into the 200 club at Bonneville. Here I am down to 86 percent on the density.”
As you can see by the first photo of the Studebaker, there was no hood scoop. The intake air was coming from under the hood. Banks then built a cold air box for the injector stacks that actually produced positive pressure.
“I picked up .6 pound of pressure over ambient at 190 mph. Now, I’m back to 522 horsepower,” says Banks, adding that the lesson learned was: “Never run a open element filter under the hood.”
A T-shirt with the now-familiar slogan “I’d rather be blown than injected” prompted Banks to investigate boost. He was also thinking about the challenges of Pikes Peak. Still using his SBC engine as a demonstration model, he factored in the altitudes at the bottom and top of the mountain.
“Anyone who runs naturally aspirated at Pikes Peak, this should be really depressing, especially if you dyno’d engine at Long Beach,” quips Banks.
Not all methods of boost are equal. Above are calculations using 20 pounds of boost with a Roots supercharger, screw supercharger, belt-driven centrifugal supercharger compared to the original baseline SBC.
Since the different types of boost affect manifold temperature, Banks reasons the next step is reducing manifold air temperature or MAT. And below you can see the difference with and without intercooling when using a turbo. Then below that you can see the difference between types of intercooling methods.
“Can you get the idea that the boost gauge doesn’t mean a damn thing,” says Banks. “All these examples are at 20 pounds of boost and power levels are all over the place.”
For added emphasis, Banks crunched the numbers to illustrate how much power his SBC would make while turbocharged and intercooled at various altitudes, including Pikes Peak.
Banks closed out his presentation with another formula: MAD – AAD = boost air density, or BAD
“BAD is the only true evaluation of a boosting system,” he sums up, noting that AAD + BAD = MAD. “MAD is the key indicator of an engine’s power potential and can be compared at any altitude, anywhere on the planet.”