How slowly can your engine idle stably ( w-o load / rocking ) ?

Question

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.

Mike Romain · Accepted Answer

Well, I for one like to use my heater and wipers and stereo at idle so I want my engine running 'at spec' or in my case 650 rpm to keep the alternator turned on so I don't kill my battery.

I think 'pulling power' at low RPM tells how healthy the engine is. My carb engine will walk up sand pit walls as low as 400 rpm without stalling.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > I think this minimum idle speed indicates [i] engine's low end

Steve · Answer

I also like my cam lobes to stay pointy rather than rounding off, which is possible with flat-tappet cams due to collapse of the oil film if the cam is turning too slowly. Another good reason to stick with specified idle RPM.

Roller cam engines could safely idle at cranking speed, I suppose. I've read that the Chrysler 5.7 Hemi factory idle speed is rather low (~5xx RPM) for efficiency. But I'm sure the alternator is selected to have sufficient output at that speed, the oiling system still delivers adequate volume, the water pump circulates enough coolant, etc. etc. etc.

Mike Roma> Well, I for one like to use my heater and wipers and stereo at idle so I

Steve W. · Answer

Well I have a carbed engine in the shop that idles at about 60 RPM. It's a 35 HP Fairbanks-Morse. It weighs about twice what that entire Honda weighs though.....

lugnut · Answer

Yep!  And, it is WFO at less than the Honda should idle.Lugnut

Steve B. · Answer

You must have owned some carb cars that were in pretty poor shape. Both of my old girls *can* idle just fine in the low 400s (and on plain ol' unleaded). I would never leave them there because you don't get enough oil pressure for comfort and the alternator cuts out.

Last time I cleaned the throttle body on the Cadi ('94 Deville) I adjusted the manual idle l and it had no trouble idling at 450rpm either.

Steve B.

How slowly can your engine idle stably ( w-o load / rocking ) ?

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