A motorised bicycle pump or a real 4WD compressor ?

Here’s a few tips on selecting your next compressor.

I’m pumped. No, I didn’t fall uncomfortably on my air compressor, instead I’ve been psyching myself up to talk about lots of hot air and how to tell the difference between a motorised bicycle pump and a real 4WDers compressor. Insert manly grunting noises now.

For most manly blokes an air compressor isn’t just a little device for pumping up flat tyres, it’s an awkward extension of their blokey-ness, a representation of the big thumping, heart-throbbing V16 engine they’d much rather have under the bonnet of their 4WD. We get out of our 4WDs at the end of beaut day beach driving, and drag out our huge double-pumping, triple-finned chrome-tipped compressor and sneakily glance along the convoy to check on how big the other bloke’s is…..

But seriously, does it really matter if yours has two pistons or one, a shiny paint job or a V-twin configuration suitable for inflating the Goodyear blimp. No.

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

One of the most popular accessories to have in your 4WD has been bastardized to death. There is so much junk on the market now its getting near impossible to tell the rubbish from the reliable. So here’s a few tips on selecting your next compressor and what to avoid.

Number one look at the flow rate (litres per minute) at different pressure levels, not just at Free Air Delivery, FAD. Many compressors are around 72lpm or 85lpm, 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 in your tyres. If the flow rate drops significantly under increasing pressure, this is an indication of a poorly matched motor, or 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 drop down to 20lpm, which is quite ridiculous. 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 running it gets drained so voltage drops, the motor pulls more current and gets hotter…… you get the picture. It’s a literal downward spiral.



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

In conjunction with this flow rate is the term duty cycle. This is confusing or not well understood for many people. Here’s the best description I can provide that is hopefully easy to grasp.


The Duty Cycle is normally described as a percentage figure, and maybe 30% or 70% duty cycle as an example. The higher the ‘number’ the better the performance of the compressor, that’s the key point. But 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.

Lastly, and again tying in with the above two points, 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 element of your potential new compressor, as you may not have the capacity in your system to handle a large 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 discharged battery.

Now without getting into electrical technicalities and formulae, it is generally observed that as the load increases on the compressor via the pump working harder against an increased tyre pressure, it gets hot (see duty cycle) and slows and will pull more current from the battery. Plus if you are using lengthy cables between the battery and compressor, this will cause further voltage drop issues and exaggerate the problem. It is vital to ensure you have adequate circuit protection in place to avoid over-heating and fires.

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, packaging and the “we’ll throw in a Heavy Duty compressor” deals, and get something that really works straight out of the box.

Stay in touch and see you Outback

Chris Blakemore


Chris Blakemore is owner of FNB4WD in Mt Barker South Australia.
He’s passionate about delivering quality products with excellent personal service.

FNB4WD achieved a Top 25 place in the SA Fast Movers Awards 2013.
Staff continually research new products and develop their industry knowledge, in an effort to provide the best possible service. All work performed at FNB 4WD Supplies is performed by qualified technicians and experts in their own fields.


Posted in General and tagged , , , , , .