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Old 13-11-2017, 01:16 PM   #2
Detailing at Double Speed
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Having done a fair bit of reading and research into this topic I'll try to summarise what I've learned.

The "power" of the pressure washer has to do with the pressure that is developed at the nozzle. You may be aware that pressure comes about due to resistance to flow. The pump in the PW provides the flow, the nozzle provides the resistance.

Too little resistance (ie a nozzle orifice that's too big) = not enough pressure, and a PW that doesn't seem very powerful.

Too much resistance (ie nozzle orifice too small) and the pump will struggle, it may stop and start intermittently if the machine detects too high a back pressure. No use either!

What you need is the right size of nozzle - erring slightly on the side of caution (better to lose a little pressure rather than put unneeded strain on the pump). Have a look at this (PDF link) for a bit more detail and a handy lookup table. According to the Karcher product page (and specifically the specifications on page 14 of this PDF), your K4 is rated for 6.3 litres per minute at 11MPa, or 110 bar.

Using the table linked above, on the 110 bar column the flow rate of 6.3 litres per minute would suggest a nozzle between a 0.25 and 0.3. Note that none of this has anything to do with the gun you are using - it's all about the size of the nozzle. It depends on the lance you're using but you might be able to change the nozzle size (they are frequently available separately, sometimes with a quick release and sometimes with a thread).

If you can post a picture of the new lance & gun, I can try and suggest the best way to update the nozzle. It would also be well worth going back to the place you bought it from and asking them what nozzle size it is. My guess is that you've got a 0.35 or 0.4 that is reducing the pressure and hence effective power of your machine.
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