There is a sticker under my hood it says 700rpm at sea level and then it says 650rpm at high altitude what do these mean like when the car is idling in drive what should it be 650 and in park 700?
Question about my car idle rpms
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Cars in lower altitudes have denser air which makes more power and can get better mpg and combust more air/higher rpm. Higher altitude will have thinner air so not as much of it will be going through your car resulting in less air/fuel less rpm needed to work.
What kind of car/engine is it? That sticker sounds like something that would be used for setting up the tuning on a carb engine. An engine with EFI has sensors to measure RPM and the ECU should command the minimum RPM necessary for stability by moving the idle air valve actuator.
So someone tuning the engine at, say, Denver or Albuquerque should aim to tune it to idle at a slightly lower idle RPM than someone at sea level. Such set points might not be optimal efficiency wise, but a car that is tuned for a low altitude might encounter problems if driven to a high altitude and would need more idle air to maintain an acceptable (ie: non-stall) idle. While if one is already at a high altitude, well, such is presumably already adjusted accordingly.
So someone tuning the engine at, say, Denver or Albuquerque should aim to tune it to idle at a slightly lower idle RPM than someone at sea level. Such set points might not be optimal efficiency wise, but a car that is tuned for a low altitude might encounter problems if driven to a high altitude and would need more idle air to maintain an acceptable (ie: non-stall) idle. While if one is already at a high altitude, well, such is presumably already adjusted accordingly.
1988 nissan 200sx 3.0 v6 and i live in chicago,IL weather now ranges from 50-80f
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Originally Posted By: Marco620
Cars in lower altitudes have denser air which makes more power and can get better mpg and combust more air/higher rpm. Higher altitude will have thinner air so not as much of it will be going through your car resulting in less air/fuel less rpm needed to work.
More power sure, but mpg? Denser air = more fuel needed per intake stroke to keep stoichiometric ratio
Or am I missing something?
Cars in lower altitudes have denser air which makes more power and can get better mpg and combust more air/higher rpm. Higher altitude will have thinner air so not as much of it will be going through your car resulting in less air/fuel less rpm needed to work.
More power sure, but mpg? Denser air = more fuel needed per intake stroke to keep stoichiometric ratio
Or am I missing something?
They'd have you set the idle in park unless specifically mentioned otherwise, as they don't want the car running over you in drive as you adjust it.
Back in the 80s the EPA had a separate "high altitude" certification that, IIRC, was for over 4000 feet.
The way the cams are tuned etc means your car will start making a lot less vacuum under the idle RPM. Less vacuum means your fuel metering thinks there's a load on the engine and starts putting more fuel in, so, counterintuitively, a higher (proper) idle will save gas and emissions. Way back in the day mechanics used to set idle to max vacuum.
Back in the 80s the EPA had a separate "high altitude" certification that, IIRC, was for over 4000 feet.
The way the cams are tuned etc means your car will start making a lot less vacuum under the idle RPM. Less vacuum means your fuel metering thinks there's a load on the engine and starts putting more fuel in, so, counterintuitively, a higher (proper) idle will save gas and emissions. Way back in the day mechanics used to set idle to max vacuum.
Back when we had to set idle manually as part of a tune up mechanics (including home mechanics) used to have the ubiquitous "dwell tach meter". These things still turn up at yard sales for a couple bucks. They were pretty accurate as far as tachs go.
Setting the points dwell, though, is something you kids get to skip.
Setting the points dwell, though, is something you kids get to skip.
Originally Posted By: slowdime
Originally Posted By: Marco620
Cars in lower altitudes have denser air which makes more power and can get better mpg and combust more air/higher rpm. Higher altitude will have thinner air so not as much of it will be going through your car resulting in less air/fuel less rpm needed to work.
More power sure, but mpg? Denser air = more fuel needed per intake stroke to keep stoichiometric ratio
Or am I missing something?
What you are both missing is that the vehicle doesn't have to push as much air, to move at the same speed. Normally aspirated engines make less peak power, but vehicles get better mileage at elevation. I live at 7200', and all my vehicles get about 10% more mileage than they do at sea level.
Originally Posted By: Marco620
Cars in lower altitudes have denser air which makes more power and can get better mpg and combust more air/higher rpm. Higher altitude will have thinner air so not as much of it will be going through your car resulting in less air/fuel less rpm needed to work.
More power sure, but mpg? Denser air = more fuel needed per intake stroke to keep stoichiometric ratio
Or am I missing something?
What you are both missing is that the vehicle doesn't have to push as much air, to move at the same speed. Normally aspirated engines make less peak power, but vehicles get better mileage at elevation. I live at 7200', and all my vehicles get about 10% more mileage than they do at sea level.
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