There's a concept in fluid mechanics called "critical pressure ratio". It's the ratio of downstream/upstream absolute pressure of a flow path with a restricting orifice, and a carburetor at the idle position of running engine is an example.
Standard sea level atmospheric pressure is 29.92" Hg, but lets just call it 30". Typical L-79 idle vacuum is 14" @ 750, so absolute manifold pressure is 30-14 = 16" and the pressure ratio is 16/30 = .533. A base engine is about 18" @ 500, so absolute manifold pressure is 12", and the pressure ratio is 0.400.
The magic number for air - the critical pressure ratio is 0.528. If the pressure ratio is below this value, flow velocity past the orifice is sonic and the flow is "choked". No matter how much further we reduce the pressure ratio below 0.528, mass flow will not increase. It is constant at pressure ratios of 0.528 and below. This constant flow rate is why base engines idle smooth.
The L-79 is just above critical and SB mechanical lifter cams that idle at less than 14" are higher. Flow velocity past the throttle valve is subsonic and can vary due to cyclic variation factors such as varying flow to each cylinder and variations in exhaust gas dilution. This is what causes "idle lope", and it can also be seen as some variation in manifold vacuum around a median reading. On a base engine the vacuum gage will usually be dead steady.
At 750 a L-79 is just above critical, so it exhibits just a little lope. Actually, the pressure ratio is at the ragged edge, and it probably wanders between just above and below critical. If you increase idle speed by about 100 revs it will drop below critical and smooth out. Since "idle lope" is a sign of a "big cam", guys usually like it, but some guys let their engines idle so low with such a high pressure ratio that they barely run. Once idle vacuum achieves about 15", the pressure ratio will be well enough below critical for the engine to idle smooth.
This leads to the subject of idle speed. Chevrolet service manuals and AMA specs for vintage Corvettes usually don't call out a specific idle speed, but there is an implied idle speed with the initial timing specification. For the 30-30 cam engines the initial timing spec is called out at 700 RPM, which is the speed that the centrifugal spark advance is about to begin, but I don't think many would be satisfied with the idle quality of a 30-30 cam at 700, and it might be unstable, which can lead to the engine stalling.
My idle speed recommendation has always been "as low as possible commensurate with acceptable idle quality". This is subjective and up to each individual owner. Some might like a lot of lope on SHP engines and others prefer just a little lope. My recommendation for mechanical lifter engines is 900, which is a good starting point, and owners can go up or down from there depending on what they think is "acceptable idle quality".
Air conditioned cars require some work to achieve acceptable idle qualilty because there is no "idle up" function (or automatic idle control system like on modern cars) when the compressor is engaged, so if your car has A/C and the compressor engages at a stop light, idle speed may fall off to the point of poor idle quality or even stall, so A/C car idle speed should be set with the compressor engaged - idling in neutral with a manual transmission and idling in Drive with an automatic, and you may also want to add some electrical load like turning on the headlights.
Variations in local air density can cause variation in idle behavior. It's usually not severe and may go unnoticed, but if you live at sea level and drive up to the mountains you may experience poor idle quality or stalling due to the lower air density, so it's a good idea to bring a screw driver with you to make idle speed adjustments as necessary if you are going to spend some time at higher altitude than your normal domicile.
Back when I rode my Honda CB1100F up into the mountains north of LA, we would usually stop at a lookout point on Angeles Crest Highway that was at about 7200 feet. Fortunately the bike had a thumb wheel idle speed adjustment under the carbs, so before we stopped I would reach down under and give it about half a turn. If I forgot, the engine would stall when I came to a stop.
Speaking of "manifold vacuum" or "manifold pressure" can be confusing, and hopefully this discussion makes it a little more clear. An example is large supercharged/turbocharged reciprocating aircraft engines. Typical takeoff power ratings are stated at a specific RPM and manifold pressure, say 60" Hg. This is absolute manifold pressure, so boost is about 30" or one atmosphere. Since these engines' induction pressure is always higher than atmospheric under any condition other than idle, absolute manifold pressure is a surrogate for load. Most Pratt and Whitney R-2800s are takeoff rated at 2750/60", and economical cruise, which is usually about 65 percent of max takeoff power is probably about 2000/45".
Since normally aspirated engines' manifold pressure can never be greater than atmospheric (and are, in fact, always at least slightly less), manifold vacuum is a good surrogate for load.
Duke
Standard sea level atmospheric pressure is 29.92" Hg, but lets just call it 30". Typical L-79 idle vacuum is 14" @ 750, so absolute manifold pressure is 30-14 = 16" and the pressure ratio is 16/30 = .533. A base engine is about 18" @ 500, so absolute manifold pressure is 12", and the pressure ratio is 0.400.
The magic number for air - the critical pressure ratio is 0.528. If the pressure ratio is below this value, flow velocity past the orifice is sonic and the flow is "choked". No matter how much further we reduce the pressure ratio below 0.528, mass flow will not increase. It is constant at pressure ratios of 0.528 and below. This constant flow rate is why base engines idle smooth.
The L-79 is just above critical and SB mechanical lifter cams that idle at less than 14" are higher. Flow velocity past the throttle valve is subsonic and can vary due to cyclic variation factors such as varying flow to each cylinder and variations in exhaust gas dilution. This is what causes "idle lope", and it can also be seen as some variation in manifold vacuum around a median reading. On a base engine the vacuum gage will usually be dead steady.
At 750 a L-79 is just above critical, so it exhibits just a little lope. Actually, the pressure ratio is at the ragged edge, and it probably wanders between just above and below critical. If you increase idle speed by about 100 revs it will drop below critical and smooth out. Since "idle lope" is a sign of a "big cam", guys usually like it, but some guys let their engines idle so low with such a high pressure ratio that they barely run. Once idle vacuum achieves about 15", the pressure ratio will be well enough below critical for the engine to idle smooth.
This leads to the subject of idle speed. Chevrolet service manuals and AMA specs for vintage Corvettes usually don't call out a specific idle speed, but there is an implied idle speed with the initial timing specification. For the 30-30 cam engines the initial timing spec is called out at 700 RPM, which is the speed that the centrifugal spark advance is about to begin, but I don't think many would be satisfied with the idle quality of a 30-30 cam at 700, and it might be unstable, which can lead to the engine stalling.
My idle speed recommendation has always been "as low as possible commensurate with acceptable idle quality". This is subjective and up to each individual owner. Some might like a lot of lope on SHP engines and others prefer just a little lope. My recommendation for mechanical lifter engines is 900, which is a good starting point, and owners can go up or down from there depending on what they think is "acceptable idle quality".
Air conditioned cars require some work to achieve acceptable idle qualilty because there is no "idle up" function (or automatic idle control system like on modern cars) when the compressor is engaged, so if your car has A/C and the compressor engages at a stop light, idle speed may fall off to the point of poor idle quality or even stall, so A/C car idle speed should be set with the compressor engaged - idling in neutral with a manual transmission and idling in Drive with an automatic, and you may also want to add some electrical load like turning on the headlights.
Variations in local air density can cause variation in idle behavior. It's usually not severe and may go unnoticed, but if you live at sea level and drive up to the mountains you may experience poor idle quality or stalling due to the lower air density, so it's a good idea to bring a screw driver with you to make idle speed adjustments as necessary if you are going to spend some time at higher altitude than your normal domicile.
Back when I rode my Honda CB1100F up into the mountains north of LA, we would usually stop at a lookout point on Angeles Crest Highway that was at about 7200 feet. Fortunately the bike had a thumb wheel idle speed adjustment under the carbs, so before we stopped I would reach down under and give it about half a turn. If I forgot, the engine would stall when I came to a stop.
Speaking of "manifold vacuum" or "manifold pressure" can be confusing, and hopefully this discussion makes it a little more clear. An example is large supercharged/turbocharged reciprocating aircraft engines. Typical takeoff power ratings are stated at a specific RPM and manifold pressure, say 60" Hg. This is absolute manifold pressure, so boost is about 30" or one atmosphere. Since these engines' induction pressure is always higher than atmospheric under any condition other than idle, absolute manifold pressure is a surrogate for load. Most Pratt and Whitney R-2800s are takeoff rated at 2750/60", and economical cruise, which is usually about 65 percent of max takeoff power is probably about 2000/45".
Since normally aspirated engines' manifold pressure can never be greater than atmospheric (and are, in fact, always at least slightly less), manifold vacuum is a good surrogate for load.
Duke
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