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So you were passing a semi truck the other day and noticed something odd: a huge silver barrel hanging just in front of the driver’s door on its cab. No, this isn’t some chromed-covered R2D2 helping the trucker navigate the highways of America. It is a massive external air filter engineered to help older semi trucks’ engines breathe easier. Most modern semi trucks have the air filter mounted internally to improve aerodynamics.

But why are these filters so big? Because big engines need air, a ton of it. Each cylinder’s intake valve opens as its piston is dropping and the vacuum created sucks fresh fuel/air mixture into the combustion chamber. The more power the engine must use to pull this mixture into the combustion chamber, the less power it puts to the wheels.

Diesel semi trucks use a turbocharger (an air pump powered by the exhaust gas) which compresses the fuel air mixture to fit even more into the cylinder. A larger air filter allows more airflow and makes the turbocharger and engine’s job easier. There’s only one problem: a turbocharged diesel V8 takes up all the room beneath a classic semi truck’s hood. So where do you put the air filter?

Semi truck companies such as Peterbilt and Kenworth used to install huge stainless steel barrel-shaped filter housings/air intakes on the side of their trucks. These were built by companies such as Vortox and Donaldson. You can see the various types of air semi truck air filters in the video below:

Like many other vehicles, semi trucks have grown steadily larger over the years. Engineeers have also been able to more efficiently package their components to improve aerodynamics. That big trash-can-sized air filter housing on the outside of the truck isn’t great for aerodynamics. So over the years, the air intake/filter housing migrated beneath the hood of semi trucks.

And external air filter by Vortox or Donaldson is an iconic component of a classic semi truck. It is an easy way to differentiate it from a newer model. For this reason, the drivers who keep older semi trucks on the road often decorate these filter housings, or even customize them by placing lights inside.

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Staff Writer

Henry Cesari

Henry Cesari joined MotorBiscuit in 2021 and brings his deep interest in vintage cars, trucks, and motorcycles to the site. Having restored his first classic car at just 16 years old, Henry has wrenched on everything from overland campers to Japanese motorcycles and even pre-war Bugattis. Henry is also an avid attendee at local car shows and genuinely enjoys connecting with fellow auto enthusiasts.

Henry earned a Bachelor’s in Anthropology and English from the University of Vermont. He has since incorporated his love of the written word with his admiration of classic cars. While Henry is interested in vintage cars, he believes the golden age of cars is yet to come. He is confident journalists will play an important role in this revolution by exposing the industry as it is now and envisioning its possible future.