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Old 05-10-2020, 05:54 PM   #1
1crosscut
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Default engine rpm at idle

Had someone ask me what the rpm should be at idle. I Thought I would ask as I don't know. Always set it by ear and and low enough to shift easy.
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Old 05-10-2020, 06:44 PM   #2
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Default Re: engine rpm at idle

5 to 7 miles per hour at idle in low gear. Five works fine for me.
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Old 05-10-2020, 08:06 PM   #3
Purdy Swoft
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Default Re: engine rpm at idle

I believe that the listed RPM is 700 at idle . I haven't tested mine with a tachometer.

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Old 05-10-2020, 08:06 PM   #4
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Default Re: engine rpm at idle

Thank you.
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Old 05-10-2020, 08:13 PM   #5
Purdy Swoft
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Default Re: engine rpm at idle

You are very welcome , Dave .
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Old 05-10-2020, 10:35 PM   #6
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Mine idles real nice at 550 rpm. Any thing slower and It gets a hiccup.
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Old 05-11-2020, 03:49 AM   #7
Werner
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Default Re: engine rpm at idle

Guten Tag Dave,


if the engine is warmed up, the lowest sure idle RPM is 450 RPM. Iridium sparks, ignition timing + 5 .
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Old 05-11-2020, 04:31 AM   #8
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Default Re: engine rpm at idle

Yes I reckon idling should not be over 500 rpm and usually aim get 420 ish. But it does vary from car to car. I believe the record is that set during the Scandinavian Granstreffen Sow Running Challenge - an amazing steady beat of 180 rpm!
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Old 05-11-2020, 06:08 AM   #9
Dean Lemoine
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Default Re: engine rpm at idle

Is there a point where low idle RPM could affect oil pressure?
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Old 05-11-2020, 09:52 AM   #10
Bob Bidonde
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Default Re: engine rpm at idle

Dean,

The oil pump is designed for volume of flow, not pressure. The pump needs to supply enough volume at any engine speed to keep the dams in the valve chamber overflowing with oil. So considering the pump's output volume at a very slow idle speed, I have not yet found any Ford literature that addresses this concern. You have raised an interesting question.

I would be concerned idling an engine at speeds where the fan's revolutions can be counted visually. For example, an idle speed of 200 RPM makes the pump shaft speed half of the engine's speed or 100 RPM. Is the oil pump's output volume sufficient in this case? Keep in mind that, in addition to supplying oil to the crankshaft and camshaft bearings, the pump also supplies the oil that fills the connecting rod troughs.

I will do some more research. Perhaps someone on the Barn has played with the oil pump at low revs, and could enlighten us as the to the pumps output volume.
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Old 05-11-2020, 11:24 AM   #11
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Default Re: engine rpm at idle

Pm sent

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Old 05-11-2020, 11:30 AM   #12
Purdy Swoft
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Default Re: engine rpm at idle

If mine is idling too slow it can effect shifting . I'm more interested in drivability than a really slow idle .
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Old 05-11-2020, 11:38 AM   #13
Bob Bidonde
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Default Re: engine rpm at idle

Hi Purdy,

When I was running a stock 6V electrical system, I set the engine's idle speed with the hand throttle lever to zero the ammeter. That resulted in a fast idle, I would was greater than 600 RPM.

Idle speed definitely has an effect on gear shifting with higher idle speeds making it more likely to grind the gears. High idle speed is especially annoying to get the car into 1st gear without grinding!
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Old 05-11-2020, 12:05 PM   #14
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Default Re: engine rpm at idle

I agree that too high idle speed also can cause problems . I get a gearing down effect when letting off the gas when idle speed is too low and it can effect gear shifts. . I run performance grind cams in some of our model As . Some of ours idle at around maybe 900 RPM . Again , I have never installed a tachometer on any of our model As . I have also used the throttle lever to increase idle at times on these cars .
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Old 05-11-2020, 12:27 PM   #15
Tom Endy
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Default Re: engine rpm at idle

See attached article about Model A RPM.

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Attached Files
File Type: pdf 4-Tachometer.pdf (220.5 KB, 88 views)
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Old 05-11-2020, 12:49 PM   #16
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Default Re: engine rpm at idle

Pm Sent

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Old 05-12-2020, 06:19 AM   #17
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Default Re: engine rpm at idle

The bearing surfaces are separated by hydrodynamic pressure generated by the rotating journal pulling oil round the bearing from the feed wells. This oil pressure is far higher than what the pump delivers. If too low, due to an idle speed insufficient to keep the surfaces separated, boundary lubrication occurs and wear starts. That is why "90% of engine wear occurs at start up" is often quoted.
So how slow is too slow for hydrodynamic pressure to keep bearing surfaces apart?
I often wonder about this. Has any one seen specific research for the Model A engine? I am sure lots of people have opinions. But I would like to hear facts.
I see people chugging up to meetings with spark fully retarded doing very low revs in first gear to show how well tuned their engines are, and think to myself "wow that could be boundary lubrication" but I dont really know.
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Old 05-12-2020, 08:58 AM   #18
barkleydave
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Default Re: engine rpm at idle

The Puck-Puck idle when full retard sounds cool but thats about it.
Normal advance of 1/2 to 3/4 I like an idle 500-600 rpms. It is happy and does not stall on hot days in traffic. Too low heat builds up and can become unstable. Just my own experience with my A
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Old 05-12-2020, 09:11 AM   #19
Bob Bidonde
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Default Re: engine rpm at idle

Hi Tom,
In your article about the tachometer, I copied these 3rd gear RPMs & road speeds. In red are the data from Dyke's Encyclopedia.

Speeds calibrated by GPS. In high gear: 20 MPH - 800 857 RPM 25 MPH - 1000 1071 RPM 30 MPH - 1300 1285 RPM 35 MPH - 1500 1500 RPM 40 MPH - 1800 1714 RPM 45 MPH - 2000 1928 RPM 50 MPH - 2200 2142 RPM 55 MPH - 2400 2357 RPM

Correlation is reasonably good.
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Old 05-12-2020, 09:32 AM   #20
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Default Re: engine rpm at idle

Hi SAJ,
Theoretically, hydrodynamic pressure in bearings varies with load, speed, fluid viscosity,
bearing design features and other things. I have not seen or know of any tests or studies done specifically about the Model "A". I would love to read the engine crankshaft designer's notebook, but I doubt it exists.
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