I think this minimum idle speed indicates [i] engine's low end torque [ii] how healthy / efficient an engine is ( @ least during low rpm ) [iii] how good the petrol is ( lousy petrols give less torque ). In this humid air @27-33ºC, my honda F20A
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originally ( with cheapo 4-1 exhaust*manifold, & carbon core spark cables ) could idle @ 680 rpm & no lower, but after I [i] disabled its internal EGR ( worse than useless in cooling combustion / cylinder head temperature ) [ii] corrected a few ( electrical ) design flaws [iii] replaced
* with a 4-2-1 *, & spark cables with tri-core 0.2 ohm cables [iv] cleaned injectors using detergent, this F20A ( with just mineral oil w-o fullerene ) can idle @ 580 rpm ( per tachometer ) using BP ron97 ( has the best v-f-m in m'sia ; Vpower costs 11+% more ) : mpg is @ an all time high. I'm sure engines can idle slower in dry air, none with carburetors that I've owned can idle this slowly I presume those with vtec / supercharger, or 1 coil for each s-plug ( distributorless ), can idle slower than a F20A can.