Whether we’re talking about a street car, a racecar or even a tow vehicle, one of the easiest-to-install and most cost-effective performance parts we can add to it is a cold-air intake. They have become so ubiquitous because they work well enough that we don’t even ask why or how.
We wanted to know more, so we asked a lot of how and why questions, and what we learned was eye-opening. We spoke with engineers from K&N and Corsa Performance, two industry leaders in the air intake business, and we learned a lot. Read on and you will, too.
Cold air intakes have been around a long time. Think muscle cars with hood scoops. A lot has changed since then, specifically emissions regulations and more precise fuel-delivery systems, but the best intake systems are based on the same principles, and the math and science behind them is as fascinating as it is instructional.
The whole point of a cold air intake is to get colder air, which is denser, and holds more oxygen for a given volume. The colder, the better. But there is no formula or rule of thumb that will tell us how much of a power increase to expect with a given decrease in temperature. There are many variables.
“There’s not one standard scale that we use that will show that if you have a decrease in temperature by X amount, you’ll gain Y amount in horsepower,” said Jonathan Fiello, vice president of product development and engineering for K&N Engineering in Riverside, Calif. “You’re going to see significant gains down low and then you’re going to see significant losses up high in terms of temperature, but the displacement of the engine has a big impact on it, and also the fuel being burned, the type of induction, whether it’s forced induction or naturally aspirated.”
Power gains are mapped by a dynamic curve, which is true about nearly everything in engine and intake arithmetic. It’s not a linear scale. The math is a bit involved, but it helps explain a lot of the weird science behind systems we sometimes take for granted.
According to Daniel Marty, engineering manager for TMG Performance Products, parent company for Corsa Performance and Volant Performance in Berea, Ohio, to begin to calculate a prospective power gain, you take the initial temperature and divide by the colder temperature, then take the square root of that.
However, you must do it in the Rankine scale, not Fahrenheit, so you add 460 degrees to the Fahrenheit temperatures to get Rankine. Say, for example, you’re talking about the difference between 100-degree air and 70-degree air, once you did all the math, you get a 2.8 percent increase in power for that 30-degree difference. Easy, right? Not exactly.
“Not every 30-degree temperature change produces 2.8 percent, because it’s the ratio of the absolute temperatures. So, if you were to slide up the scale and say the difference between 250 and 220 degrees (Fahrenheit), it’s going to be a smaller improvement, because you’re dividing that same difference by a larger number,” Marty said. “And then square rooting that, so the colder you get it, the more that temperature matters. So, it’s not based on temperature difference. It’s based on temperature ratio and the square root of that ratio. And that’s just not very intuitive. The simplest thing, of course, is to think colder is better.”
If you consider the environment of a racecar on a track, the temperatures are not likely to be what you would consider “cold,” and they might not be that far apart, diminishing expected performance gains as outlined in the examples above.
Say the under-hood temperature of a racecar at speed is 180 degrees. Air from the outside is considerably cooler, but we race in the summer, when ambient temperatures are still pretty hot. Say it’s 90 degrees. So, the ambient temperatures are still hot enough that the incoming air isn’t cold at all. Eighty degrees is better than 180 degrees, but after you do the math, the power increase is roughly 4 percent, best case scenario. Not a lot, but you’ll take it.
But there is another critical element to cold air intakes for making power, and that comes in the form of increased and improved air flow.
A cold-air intake that restricts air flow compared with a stock system will nullify all the benefits of decreased air intake temperatures. For a system to be effective, a system must ensure the volume of cold air coming in is equal to or better than the volume of air you were getting with the stock system. Fiello and Marty point out that is becoming more challenging as OEM systems improve. In fact, many OEM systems already are cold air intake systems.
The key to gaining power from air flow is to minimize disruptions and obstructions to achieve laminar air flow, that is, any airstream that is flowing in a largely parallel path, according to Marty, and to increase the volume of air going into the engine, which will require more fuel to make additional power. Laminar air flow is the most effective in the subsonic world.
There is a limit to what a stock engine control system will accommodate, particularly when you are talking about newer cars. Fiello said that as fuel economy and emissions standards tighten, factory fuel trim windows have narrowed. Kits that produce significantly greater air flow often require a new tune, something you’d get from companies like HPTuners, HyperTech, Cobb, etc.
The math for air flow is much simpler and the power gains greater, Marty said. “If, for instance, there’s a restriction in your intake that’s causing, let’s say even just a 5 percent increase in pressure drop through that intake system, and you could get that down to 1 percent pressure drop, that pressure drop, you’re not talking about a square root function now. You’re talking about a ratio. So, you get more power proportionally from a relatively small change in restriction, than you get from a relatively small change in temperature.”
If you drive your street car in HPDE, most of the cold air intakes you’ll find have been engineered for you, top to bottom. They have been tested to take in colder air, reduce restriction and meet emissions standards. If you’re designing a system on your own for racing, the science behind air intake systems provide a few rules to live by.
Fiello said the simplest way to pick up a little more power is to us a freer-flowing air filter, something K&N has specialized in for decades. Off-road racing trucks and dirt-track cars need significant air filtration media, but a road racing car that — presumably — stays on track doesn’t need as much. So instead of having four layers of oiled cotton gauze, you might only need two layers of cotton, with no oil on it, which would result in freer air flow.
When routing your intake tubing, use larger tubing than stock, minimize bends — think increased radius — and have it pick up its air from as fresh and unobstructed a place as possible. The air ducts that take the place of the left headlight on the BMW E36 and E46 are great examples. Fender wells might have cooler air than under the hood, but there are lot of pitfalls there. Brakes emit heat and dust, and there’s a lot of turbulence in a fender well.
“So, number one, don’t overthink filtration,” Fiello said. “And number two, make it as simple and as least restrictive as possible.”
Also remember to keep the number of joints to a minimum. Each time you have a joint in intake tubing, you have a potential disruption point in that sacred laminar air flow.
“We tend to make up for that in the aftermarket by paying more attention to making sure wherever we have a joint, that it’s a minimal disruption. So, if it’s a silicone hose over a duct, in really good system you’ll see the silicone hose has a recess for the duct to fit into,” Marty said. “But it’s not something you will see in a cheap Chinese thing, by the way.”
Do an air intake system well enough, and you might need a tune because it’s outside the fuel parameters of what a stock tune will flow. That’s because OEM engineers have a lot more restrictions placed on them. Emissions and fuel economy are big ones, for sure, but so are things like packaging and intake noise, which some customers don’t want to hear.
“The advantage of a cold air intake system is that cold air, but it’s also the reduction of restriction,” Marty said. “The reduction of restriction though, mostly matters when your foot’s on the floor, because that’s when the system’s wide open and it wants the most air and most of the pressure drop through the intake system is through the parts that we’re thinking about.”