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What To Look For In An Air Compressor

Adding Onboard Air To Your 4×4

Photography by Harry Wagner

Onboard air is one of the most useful accessories you can add to your 4×4. You can buy a new Jeep or truck with locking differentials and all-terrain tires, but no vehicle currently comes from the factory with onboard air. That’s ok though because the aftermarket has plenty of options to help you air your tires back up at the end of the trail. You are airing down off-road, right? We have advocated the benefits of airing down in the past, whether you are rockcrawling or traveling over miles of washboard roads. Onboard air has plenty of other uses though as well, from engaging air lockers to airing up your mattress or blowing dust out of your rig. Larger tasks like reseating a tire bead or running air tools won’t be possible with a cheap compressor you plug into your cigarette lighter. There are several different specifications to consider when purchasing an air compressor.

Cubic Feet Per Minute (CFM)

CFM is a measure of a compressor’s flow rate, or the amount of air that a compressor can produce at a given pressure. CFM is often expressed at a given pressure. Compressors boasting big CFM numbers at low PSI are good for things like air mattresses and soccer balls, but they slow down significantly at the pressure required for filling a tire. Flow rate is also important for things like running air tools, which typically require 3-5 CFM to operate. You might not want to bring your Snap-On air gun on the trail, and we can’t say we blame you, but efficiency is one of the biggest benefits to high end air tools over their cheaper counterparts.

Duty Cycle

Duty cycle is the amount of time a compressor can be running versus the amount of time it needs to cool down to prevent overheating, typically for a given hour. For example, a system with a 100% duty cycle can run continuously at a specified pressure and temperature, where a compressor with a 50% duty cycle will operate only half of that time under the same conditions. A lower duty cycle isn’t always a bad thing though, as many compressors with lower duty cycles flow a higher CFM. Think of it like a sprinter versus a marathon runner. Small tires can be filled in a sprint, but if you are running 37s or 40s, duty cycle becomes more of a concern. Quality compressors, such as those from ARB and Viair, are equipped with a thermal overload switch that will automatically shut the compressor off until it cools down to prevent damage.

Portable or Hard Mount?

Another factor is what compressor to purchase is whether you want a unit that is portable or hard mounted to your vehicle. Both have their advantages, depending on your specific needs. If you have multiple vehicles, or don’t want to deal with the complexity of hard wiring a compressor, a portable unit is likely a better choice for you. A permanently mounted compressor can be placed under the hood though where it doesn’t take up valuable storage space, and hard wiring provides a better connection to the charging system for the high amperage demand of air compressors. Hard mounting the compressor also allows you to add a storage tank, which provides a reserve that useful when running air lockers, reseating a bead, or loosening hardware with an air gun.

Construction

Of course, even the most powerful air compressor won’t do you much good if it cannot withstand the rigors of off-road use. Look for a compressor constructed with high-quality components and the ability to be serviced in the field, if necessary. Features to seek out include thermal protection, as noted above, and vibration resistance, such as the use of rubber mounting feet. You also want a compressor that is sealed from dust and water resistant, preferably with an Ingress Protection (IP) rating, such as IP55. The first number refers to the intrusion of things like dust and dirt, while the second number refers to liquids. The higher the numbers, the better sealed the compressor is.

What Else You Need

Before you can air up your tires you will need to add a gauge and hose. Analog gauges tend to be less expensive than digital gauges, and we prefer gauges that have a locking chuck that stays on the valve stem without having to hold it the entire time. Many kits come with coiled hoses, but if our experience a flat hose actually takes up less storage space when it is not being used since it packs flat. For drivers looking for a complete on-board air solution, ARB is offering air compressor kits with everything you need under one part-number. They are available with a single-motor or twin-motor compressor, and include an air up gauge, hose, manifold, all wiring, and universal mounting bracket in addition to the compressor. Of course, these parts can be sourced individually as well if you say, don’t need the manifold to engage your air lockers, or want to use a vehicle-specific bracket for the compressor.   

Constructed of lightweight, high-strength engineering grade materials, ARB’s Twin Air Compressor boasts an output of 6.16 CFM and a 100% duty cycle to air up a 37-inch tire from 10 psi to 30 psi in under two minutes. Each of the two water-sealed motors feature hard-anodized cylinders and Teflon impregnated carbon fiber piston seals, real roller bearings and ball bearings (rather than bushings), and a linear brush preload system for extended life.

Don’t want to mess around with sourcing a bunch of different parts? ARB’s All-In-One compressor kits are available with either their single or twin compressors and include everything you could ever need to mount, wire, and plumb your air system for running Air Lockers and filling your tires. They also come with a high-quality inflation gauge, hose, and every little bolt and fitting to complete the installation.

Designed to withstand the harshest environments, each ARB air compressor is individually leak tested, current draw tested and flow tested under heavy load at the factory before packing. For total peace of mind, all ARB compressors come with a comprehensive two-year warranty. To further increase longevity, ARB compressors are rebuildable, with spare parts readily available from 4WP stores or online here on our website.
The twin motors incorporate internal thermal protection against extreme temperature damage while a heavy duty in-line maxi fuse for each motor provides professional circuit protection. Each unit has a thermal cutout protection switch, and a pressure cutoff regulates tank pressure between 135 psi (on) and 150 psi (off). There is also a 180-psi over-pressure safety valve in the case of switch failure or the operator connecting it to an external high-pressure source.

Our Wrangler Rubicon came from the factory with locking differentials, so we don’t need the ARB compressor in order to activate our lockers, but it is still a great upgrade for inflating tires and a myriad of other tasks. That being said, if you’re building a stock TJ that will end up being a trail machine, this is a sound upgrade as you’ll no doubt be stepping up to a set of ARB Air Lockers. While space is tight under the hood, the Twin Air Compressor fit on top of our 4.0L engine next to the brake booster. While it is susceptible to heat soaking in this location, there are few other options under the hood where we wanted to mount the compressor to prevent taking up any valuable storage space inside the Jeep.

The ARB digital tire inflator makes checking and adjusting your vehicle’s tire pressure a simple operation thanks to its big easy-to-read digital display, large ergonomic thumb-operated inflator button which can be used even with gloves on, and the bleed button on the side if you inflate your tires a little too much and need to equalize them. And the 20-foot air hose in ARB’s Pump Up Kit allows us to reach all four tires with the digital inflator and once finished, coils up tight and is easy to store when not in use.

ARB’s portable air compressor comes in a rugged case completely sealed from the elements and is easy to lash down and store. It is available with the same 100% duty cycle compressors used for their hard mount systems, along with a one-gallon air tank to assist with tasks such as reseating a tire bead, when a quick blast of air is needed.

Smittybilt’s 12-volt air compressor is a great option for those on a budget. It has a low purchase price, is portable, and features a 1/3 horsepower oil-less motor. It flows 2.5 CFM and has a 60% duty cycle and comes with heavy duty easy clip battery connectors. It also comes with a quality coiled hose that can easily reach around your vehicle to hit all four tires as well as a carrying case at no extra cost. Packed with features such as a thermal cutoff switch, a washable air filter and an integrated 150 PSI air pressure gauge, it really is a no-brainer.

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