Want to get a debate going at your local car meetup? Ask people if they believe cold air intakes actually “work.” If your local car get togethers are anything like mine, you’ll get a lot of interesting opinions and contradictory facts. Some will argue that factory air intakes are restrictive, some will say that factory air boxes suck up a lot of heat from the engine compartment, and others will say that automakers would have already installed these parts if they worked as well as companies like K&N say they do.
Here’s the deal: there’s a definitive answer to the question “Do cold air intake kits really work?” Once you understand the engineering and science behind air intakes, you’ll know the answer as well as anyone (and you’ll be able to explain why, too). Here’s how it breaks down.
Let’s Start With the Factory Intake
An internal combustion engine is basically a pump – you put air and fuel in one end, and you get exhaust gases, heat, and a spinning crankshaft at the other. Basic chemistry tells us that an engine needs about 14 times as much air as fuel – at least when you’re comparing a unit mass of gasoline (with MTBE) to a unit mass of air.
Chemistry aside, this means that your engine needs a LOT of air. A 5.0L V8 operating at relatively high volumetric efficiency (85%) will require as much as 500 cubic feet of air per minute (500 cubic feet per minute, or CFM) at wide open throttle. To put 500 CFM in terms we can understand, human lung capacity is about 0.2 cubic feet. If you can imagine taking 40 deep breaths in one second, than you can understand just how much air a 5.0L V8 is sucking up at wide open throttle.
For the purposes of determining if cold air intakes actually work, we need to remember two things about all this air coming into your engine:
- The density of air varies with temperature. Higher temperatures mean lower densities.
- 500CFM of air flowing into your engine makes noise.
Why Almost All Factory Intakes Are Restrictive
When Toyota, Honda, Ford, GM, etc. design an air intake, they’re primarily focused on building something that’s cost effective, quiet, and efficient. To their credit, engineers at these automakers do a great job of building air intakes that hit all these notes. However, no amount of clever engineering can make up for the following fact:
You can have free, unrestricted air flow, or you can have quiet air flow…but you can’t have both.
Nearly all factory air intakes are designed with some sound baffling, and this sound baffling is the primary reason that aftermarket air intakes all manage to improve performance to some degree. When you install an aftermarket air intake, you’re trading increased engine noise for an increase in power.
Most enthusiasts couldn’t care less about the extra engine noise (most of us like it, in fact), so the trade-off is a good one. Still, there is a trade-off being made. Provided the aftermarket intake you’re installing is designed carefully and correctly, there’s going to be some sort of power gain.
What About “Cold Air” Intake Kits?
Recall that air density varies with temperature. If, for example, we put 1/10th of one pound air into a balloon at 70 degrees Farenheit, and then we take this balloon outside where the temperature is 100 degrees Farenheit, the balloon will expand about 5%. If we take the balloon to the Mojave desert and get the temperature up to 130 degrees Farenheit, the balloon will expand more than 10%.
Sidebar: The differing densities of air at different temperatures is precisely what makes hot air balloons rise. If you fill a simple balloon with air that’s hotter than the ambient air, it will be considerably less dense and therefore float upwards.
The point? As intake air temperatures increase, your engine’s power decreases. Since your engine’s cylinders can only hold so much air, lower air densities lead to less oxygen in the combustion chamber. Reduced oxygen means reduced fuel use, and reduced fuel use means less power.
Experienced drag racers are careful to measure ambient air temperatures whenever they’re tuning, because air temps can dramatically impact quarter mile times.
A cold air intake, like these K&N cold air intake systems, draw in air from outside the engine compartment. This air is “cold” compared to the air inside the engine compartment, which is where most factory air intakes source their air. Colder air is more dense, and therefore cold air intakes improve engine power.
Aftermarket Cold Air Intakes Are Only As Good as Their Engineering
While it’s true that aftermarket air intake kits can improve performance by a) removing restrictions and b) reducing air intake temps, the truth is that these improvements are meant to be part of a system. You won’t notice a massive difference if this is the only mod you make, but it’s a quick and simple trick you can use to get another 5-15 horsepower on average. And if you’re new to tuning, you’ll recognize that as alot.
If you buy a cut-rate air intake system that isn’t carefully designed for your application, you might not get the results you’re looking for. I’ve tested kits over the years that added absolutely no power whatsoever, and generally these kits were cheap. If you buy a cheap kit – or a kit that isn’t backed by a brand name – you might be very disappointed. You might even decide to swear off all aftermarket air intakes as “gimmicks” or “junk.”
If you want to find a good air intake kit for your vehicle:
- Look for direct fit applications. Universal kits are hit or miss.
- Make sure the kit is from a good brand. Companies like K&N, aFe, Banks, AEM, Injen (and a half a dozen more) have actual engineers designing and testing intake systems.
- Buy a plastic kit whenever possible. Plastic has a much lower heat capacity than metal, which means plastic air intake kits don’t soak up heat like metallic kits. Some companies are using plastic designed to look like chrome or metal, a good option for anyone looking for some engine dress-up without sacrificing power.
- Dyno testing results matter, especially for brands you might not know about. K&N does a fantastic job of providing dyno testing data for most of their kits, as do some other manufacturers. If you’re not sure about a kit, ask for some dyno testing data.
- Get the best filter you can. An air intake is only as good as the filter you put on it. Every K&N air intake system comes with an excellent cone filter.
The Bottom Line
Factory air intakes are great, but like most of the parts on your vehicle, they represent a compromise between power, cost, and noise. By replacing your factory air intake with something like a K&N air intake, you will see small, but substantial gains in horsepower as well as a louder engine.
What’s more, people who install aftermarket air intakes typically notice faster throttle response and some loud (but good) engine sounds at wide open throttle.
Check out our K&N intakes and let your engine breathe.
A self-described “car nerd,” Jason is a automotive columnist who has written for the eBay Motors blog, Motor Car Digest, as well as his own sites TundraHeadquarters.com and AccurateAutoAdvice. With an engineering degree, a full-time job in the automotive parts industry, and a decade of experience working in auto dealerships, Jason brings an interesting perspective on all things automotive.