I’ve been told once or twice before I’m full of hot air, especially when talking compressors.

For many people, a big thumping, multi-piston compressor is what they’d much rather have under the bonnet of their 4WD. But seriously, it doesn’t matter if yours has two pistons or one, a shiny paint job or can inflate the Hindenburg in under a minute. No.

What really matters is flow rate, duty cycle and amperage.

This essential item for your 4WD has been flogged to death in the market. There is so much junk around now it’s near impossible to tell the rubbish from the reliable. So here are a few tips on selecting your next compressor and what to avoid.

First look at the flow rate (lpm) at different pressure levels, not just at 0psi or Free Air Delivery. Many compressors are around 65-72lpm, personally I run a 160lpm compressor at 0psi. But FAD rating is as good as blowing hot air out your…….hose, because that is what you’re doing. Real flow rate performance of a good quality compressor is measured at increasing pressure levels, such as when inflating your tyres. If the compressor flow rate drops significantly under increasing pressure, this is an indication of a poorly matched motor, or just poor quality motor driving the pump. As the pressure builds the workload of the 12 volt electric motor will increase and how it performs under this increasing load will determine how long you are left standing at the side of the track re-inflating your tyres.

Some compressors can drop down to 20lpm, which is quite silly. Where an average tyre holds roughly 100litres of air at 40psi, and although it calculates on paper this will take five minutes, it is invariably longer as the motor gets hotter, the battery supplying power gets drained, voltage drops, the motor pulls more current and gets hotter…… you get the picture. It’s a literal downward spiral.

Sure, first sign of a good compressor is a good starting high flow rate at FAD, but a consistently high flow rate at 40psi is crucial. This means your time standing in the sun waiting for tyres to inflate is greatly reduced and you can get back on the road soone

Next is the term duty cycle. This is confusing or not well understood for many people, so here’s the best description I can provide that is hopefully easy to grasp.

The Duty Cycle is normally described as a percentage, maybe 30% or 80% duty cycle as an example. The higher the ‘duty cycle’ means better performance of the compressor. If you already own a compressor this may help understand how to best operate that particular model.

The Duty Cycle of an Air Compressor is calculated by dividing the total run time by the amount of time the compressor can run before resting. For example:

D = R / T with ‘D’ being Duty Cycle, ‘R’ being run time before resting and ‘T’ being the total time.

30% Duty Cycle = 3 minutes run time / 10 minutes total – So an Air Compressor that can run for three minutes before needing to rest for 7 minutes has a 30% Duty Cycle.

Conversely, by knowing the Duty Cycle the run time can be determined by multiplying the Duty Cycle by the total time (usually 10 minutes). For example:

R = D x T

3 minute run time = 30%(.3) Duty Cycle x 10 minutes total – So an Air Compressor that has a Duty Cycle of 30% can run for 3 minutes before needing to cool for 7 minutes.

If the compressor you are looking at buying doesn’t offer this information, then maybe avoid it, as you might find out the hard way why it wasn’t clearly noted on the packaging.

Finally there is amperage as a limiting factor on the choice of compressor. This is the current drawn from your battery by the compressor. High performance compressors may draw up to 45amps or more. Check this information of your new compressor. It is recommended to run your engine when using a compressor as this will help keep a little bit of charge going into your battery, and will hopefully avoid you having to try and restart your 4WD on a flat battery.

When selecting a compressor, consider the current demand of the unit and if this will be suitable for your vehicle and set up. There is little use in having a thumping big compressor if your wiring or battery dies in the first minute.

Hopefully this information gives you a few tips on what to look for in a new compressor. Importantly, look beyond the glossy advertising and the “we’ll throw in a heavy duty strap” type deals, and get something that really works straight out of the box.

See you On Track
Chris