Air temperature effect on boost pressure?

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    Air temperature effect on boost pressure?

    How does ambient air temperature effect boost pressure? Does cold air make boost pressure go up or go down? So far, I am of the opinion that cold air makes boost pressure go down because cold air, although more dense, has less pressure.

    For example, if you have two cans with pressure gauges on them. Fill both cans with the same mass of air. Now heat one can on a stove, and cool the other can in a freezer. The heated can will show a higher pressure reading on its gauge than the cooled can. This means that cold air has less pressure than hot air. Therefore, since cold air has less pressure than hot air, you should see less boost in cold weather than you would in warm weather on a compressor that has a fixed rpm that it runs at (like a belt driven supercharger).

    Although you should see less boost in the cold weather, the power of the engine increases because the air is more dense and therefore there is more mass of air in the engine's cylinders.

    The bottom line here is that it appears that a supercharger with a fixed compressor speed will produce less boost pressure in cold weather than it will in warm weather simply because cold air, although more dense, has less pressure. However, the fact that the cold air is more dense, even though it has less pressure, means that an engine will produce more power in cold air.

    Anyone have any opinions on this? What effect does cold ambient air or hot ambient air have on boost pressure from a compressor that runs at a fixed speed such as a belt driven supercharger?
    Last edited by KevinB6876; 11-15-2011 at 02:51 PM.

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    SupraForums Member T6Rocket's Avatar
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    All I know is, on a car with a smallish wastegate where it is borderline for boost creep, the colder the temperature, the more the boost creeps as RPM rises.

    On a car with a properly sized wastegate, the boost controller should keep the boost consistent regardless of temps, so who cares.

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    Basic physics tells us any container of a gas when heated will increase in pressure.

    However, as T6 was mentioning, it's likely to have a wastegate creep a bit during cold weather.

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    I don't see any difference in boost pressure as air temp changes. The boost controller will control the wastegate to produce the same boost level, regardless of ambient air temperature. It's not like your example of a can with fixed volume. We're controlling to a target pressure and the volume changes with temperature. But I agree that since more air is entering the motor when air is colder, along with more fuel, it makes more power at colder air temps/same boost.
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    Nick, there really isn't anything to say against what I stated. I stated fact, and what happens to many people. Good for you that there is no creeping. I mean, it's not supposed to, but a lot of people see a change, even on modern stock turbo vehicles.

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    Quote Originally Posted by T6Rocket View Post
    All I know is, on a car with a smallish wastegate where it is borderline for boost creep, the colder the temperature, the more the boost creeps as RPM rises.

    On a car with a properly sized wastegate, the boost controller should keep the boost consistent regardless of temps, so who cares.

    Al
    I want to take a compressor speed that is controlled by a wastegate out of the equation. Turbos vary compressor speed with a wastegate that changes turbine speed.

    It's easier to imagine this with a FIXED compressor speed, such as a belt driven centrifugal compressor. On a compressor which has its speed fixed such as a belt driven centrifugal compressor, the question becomes is there an effect on the amount of pressure produced by that compressor with changes in ambient temperature?

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    Kevin, it depends on the atmospheric conditions. As air cools, it becomes more dense, heavier. The atmosphere presses down at sea level raising barometric pressure. But the cold air by it's nature taking up less volume at times is balanced by increased pressure. So again, you literally have to look at your local weather conditions. Because some times pressure won't increase due to more dense air. But again it increases because of the weight of the air pressing down more at sea level generally.

    If the compressor wheel is static at say 50,000 rpm. And the barometric pressure is lower, you'll have less boost. If it's higher you'll less boost. Since the turbo works by multiplying inlet pressure.

    If atmospheric pressures are the SAME, but you have a higher temp, you'll likely have HIGHER boost pressures, because of ideal gas laws.

    PV = nRT
    Last edited by Beau; 11-15-2011 at 04:13 PM.

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    Quote Originally Posted by KevinB6876 View Post
    For example, if you have two cans with pressure gauges on them. Fill both cans with the same mass of air. Now heat one can on a stove, and cool the other can in a freezer. The heated can will show a higher pressure reading on its gauge than the cooled can. This means that cold air has less pressure than hot air. Therefore, since cold air has less pressure than hot air, you should see less boost in cold weather than you would in warm weather on a compressor that has a fixed rpm that it runs at (like a belt driven supercharger).
    The analogy you have used is correct, but the conditions which surround it do not apply when considering air going through a turbo (or super) charged system. The analogy you have used is a closed system where Pressure, Volume and Temperature are all directly related and if one changes then at least one of the other variables has to change, so you are correct that if the temperature goes up so does the pressure because the volume has stayed the same. However, our cars run in an open system where the three variables mentioned above are free to change and don't necessarily have to relate to one another. Eg. air heated in an open system is free to expand because it is not contained by the boundaries of the system (or a can).

    Colder weather = air that is more dense which means for each liter of air pulled through the engine contains more air than if the ambient temperature was higher. So you are pressurizing air that is already more dense, so you should see higher boost levels. Yes, there will also be changes in atmospheric pressure, but as long as you are at the same elevation I don't believe the change in atmospheric pressure is enough to make any kind of difference.

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    Guys, take the wastegate out of the picture. A wastegate confuses the issue because it adds another variable into the equation since the wastegate changes the turbine speed and therefore changes the compressor speed. I do not want responses like "the wastegate will maintain the desired pressure." I'm fully aware of what the wastegate does. The fact that it compensates by maintaining pressure just makes figuring this out even more complex.

    Picture a system without a wastegate, where compressor speed is fixed. A belt driven centrifugal supercharger is a good example. The question becomes, will a belt driven centrifugal supercharger, which has a fixed compressor speed, have a boost pressure change from hot to cold weather? And if so, does its boost pressure increase or decrease?

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    I have already considered this as well. Compressing air that is more dense, and therefore has more molecules, will result in a higher pressure after it's compressed. However, this is offset by the fact that colder air naturally has less pressure. So it seems the two variables, density and temperature, offset one another to some extent. Maybe one offsets one more than the other. It is possible, that for example, increased density could exceed decreased pressure and therefore result in a pressure rise when that increased density of air is compressed on top of it. Hard to say exactly. It is also possible, that for example, decreased pressure could exceed increased density and result in a pressure loss when that increased density of air is compressed on top of it.

    That is why I'm asking on these forums. I am not able to come up with a conclusive answer to this question lol. I was hoping someone on here would be able to answer this conclusively.

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    How confused do you want to get?

    http://en.wikipedia.org/wiki/Density_of_air

    http://en.wikipedia.org/wiki/Atmospheric_pressure

    http://en.wikipedia.org/wiki/Atmospheric_tides

    Ok, I know it's wikipedia, but still. One of the articles says that the daily flux in pressure is only a few bars of mercury. At 1 atm mercury measures 760mm, so a change of a couple of bars in pressure is only about 0.3%. Let's say a the daily swing in temperature is 10 deg celsius, so the change in air density is about 3.6%, so the change in density out weighs the change in pressure. Assuming the relationship between the two is proportional and linear then the change in density has a greater impact that the change in atmospheric pressure.....did I actually just type this out???? I'm such a #%&in' nerd!

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    Quote Originally Posted by Beau View Post
    Nick, there really isn't anything to say against what I stated...
    Beau,
    Who said there was? Don't take what I said above as directed toward what you said since when I wrote my reply your post wasn't even up yet...neither was T6 Al's. I was probably distracted in work for those ten minutes before I hit the reply button.

    I know cold air can cause creep because the boost controller is still targeting the same boost pressure, but with colder inlet temp more air is pushing through the motor and out the exhaust and this can overwhelm a marginally sized wastegate. The stock twins setup is a good example of that. My present system doesn't creep, probably because I have plenty of wastegate capacity (two TiALs) and how they're plumbed into my twin headers. So I was replying to Kevin's original post. Otherwise, personally I don't like the closed container analogy for the originally stated question and I really don't have anything to add to what's been said.

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    It doesn't matter, the answer is in the thread 3 times, but the OP will keep asking until everyone gives up.

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    Not so boring anymore tissimo's Avatar
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    Yes its why we have intercoolers, to not run to much boost.

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    ^^^ wut

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    I see a 1 psi increase on stock twins with the "bleeder t mod" at night in colder weather.
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    The only reason temperature and or boost pressure is important is due to the effect they have on air density and mass. Pump efficiency (turbos and superchargers) is measured in cubic feet per minute (cfm) or cubic meters per minute at a fixed pressure. Boost pressure is regulated by wastegate spring pressure and boost controler setting. Typically the settings on electronic boost controllers are for gain and max boost. Turbos will pump more mass of air if the atmospheric pressure is high and/or temperature is low. As you can see from the combined gas law (Charles's law, Boyle's law, and Gay-Lussac's law) basically changes in either variable (volume, temperature or pressure) will result in an equal offsetting change in one or both of the other variables. The formula is:
    pV/T = K

    and, when comparing two different conditions is as follows:
    p1xT1/V1 = p2xT2/V2

    where:

    p is the pressure
    V is the volume
    T is the temperature measured in kelvins
    k is a constant (with units of energy divided by temperature)

    Standard atmospheric pressure at sea level is 29.92 in of mercury with a normal range of 28-31 inches so ~10% variation high to low (+4% and -6.5%). There can be extremes (tornados, hurricanes. etc). Weather patterns have the biggest effect on barometric pressure not temperature. Since the effect of temperature is linear on pressure a 20 deg C rise in temperature (10 deg C to 30 deg C or -263 to -243 deg kelvin) will result in an 8% reduction in pressure. This assumes that the temperature increase of the air column is linear, but since most of the impact of temperature rise is near the earths surface the effect is not linear throughout the air column. As you can see there are a lot of variables. Suffice it to say that colder air is denser, contains more oxygen molecules and thus will produce more hp in either a turbo or super charged engine (that's why dynos have correction curves for temperature). Typically you will also see more boost pressure at lower tempertures (for the same gain setting on the boost controller) because the pump (turbo or compressor) is more efficient at pumping denser fluids. If you go to the extreme, a turbo (the pump) will produce zero pressure in a vacume but much higher pressure with dense fluids.
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    "Since the effect of temperature is linear on pressure a 20 deg C rise in temperature (10 deg C to 30 deg C or -263 to -243 deg kelvin) will result in an 8% reduction in pressure."

    You're going to confuse him with this. You're talking about an atmospheric condition.
    If you have a fixed mass of air in a intake runner for instance, an increase in temp will result in an increase in pressure. But obviously would produce less power.

    Make sure you're explaining to op as I did when describing what happens. Ie more dense air in the atmosphere due to cooler temps literally cause more weight, and thus more pressure.

    But the fact it, air heats up in a lot of different ways independant of atmosphere.

    So the question was. Does pressure go up as things get hotter, and the answer is yes, as far as the car is concerned. However all the car cares about is the pressure, so if the atmospheric pressure drops, then of course boost will drop as well (with a fixed rpm compressor wheel)

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    Beau, I believe his question was more about does boost pressure increase as ambient temperature increases. The turbo does not add heat, it increases pressure by packing more air into a fixed volume (the piping, intercooler, intake plenum and cylinders). Heat increases as a result of thermodynamics (Boyle's law). In a closed container with a standard volume of air if you increase temperature you will increase pressure. However, the situation with a turbo is not a fixed container it is dynamic. The pump keeps forcing air until either it is bled off (BOV) or the input energy is reduced (wastegate). The pressure and temperature in the intake plenum or anywhere downstream of the turbo will be proportional to the temperature and pressure of air on the intake side + the impact of any input energy (i.e. turbo compression). As you compress the air in the turbo the temperature naturally rises in accordance with Boyle's law. If you pump in colder denser air it will also be colder and denser than warm less dense intake air, when it reaches the intake plenum. The pressure will be either be the same or higher depending upon how you are managing boost pressure and the efficiency of the turbo (theoretically it will rise proportionatly to the ambient pressure but turbo efficiency is a variable). As you said, colder intake air should result in more weight for a given volume of air (absolute pressure), thus more mass and more oxygen. However, we don't measure absolute, we measure gauge which is relative to atmospheric. If the barometric conditions are low (ie. say a low pressure front at 29 inches Hg) then 18psi of boost would still be 18 psi gauge but the absolute pressure would be (29/29.8x14.7) + 18=32.3 psi or about 0.4 psi less than if atmospheric conditions were standard sea level. Ambient air temperature is not a good indicator of barometric pressure. For example; the temperature today in Brownsville, Texas is 90 deg F and barometric pressure is 1011 millibar while the temperature in Seattle, WA is 43 deg F and barometric pressure is 1012 millibar. That's a 47 deg temperature difference and essentially the same barometric pressure.
    Quote Originally Posted by Beau View Post
    "Since the effect of temperature is linear on pressure a 20 deg C rise in temperature (10 deg C to 30 deg C or -263 to -243 deg kelvin) will result in an 8% reduction in pressure."

    You're going to confuse him with this. You're talking about an atmospheric condition.
    If you have a fixed mass of air in a intake runner for instance, an increase in temp will result in an increase in pressure. But obviously would produce less power.

    Make sure you're explaining to op as I did when describing what happens. Ie more dense air in the atmosphere due to cooler temps literally cause more weight, and thus more pressure.

    But the fact it, air heats up in a lot of different ways independant of atmosphere.

    So the question was. Does pressure go up as things get hotter, and the answer is yes, as far as the car is concerned. However all the car cares about is the pressure, so if the atmospheric pressure drops, then of course boost will drop as well (with a fixed rpm compressor wheel)
    Last edited by normore; 11-16-2011 at 03:44 PM.

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    Thanks for all the responses guys.

    To clear up any confusion as to what my question was. My question was as follows:

    Does pressure, measured at either the compressor outlet, or at the intake manifold, change when there is a reduction in ambient air temperature, and if so, by how much?

    To make my question more clear to understand, let me give an example. Let's take a car with a fixed rpm compressor, such as a belt driven centrifugal supercharger. This compressor always turns at 50,000 rpm, and always produces 10psi of pressure in 100F ambient temperature at that 50,000 rpm.

    Now, let's move this car into a cold weather environment, where ambient temperature is 20F.

    Would there be a pressure change on the boost gauge of this car from its normal 10psi pressure? Would the boost pressure, measured at either the compressor outlet or the intake manifold, go up or would it go down or would it stay the same? Also, if it would go up or down, how much would it go up or down by?

    That is the question. I hope that clears things up.
    Last edited by KevinB6876; 11-16-2011 at 07:35 PM.

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    Well it appears I was not able to get consistent answers to this question. Some say pressure goes up, others say pressure goes down. It's hard to say who's right or who's wrong.

    I just wanted to know if there would be any change in pressure when going from hot ambient temps to cold ambient temps. But it appears the answer is more complicated than previously thought, hence all the complicated and conflicting responses.
    Last edited by KevinB6876; 11-18-2011 at 07:07 PM.

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    The answer has been provided, either you are incapable of understanding or don't want to. Turbos are more efficient at pushing dense cold air so for the same amount of energy input, pressure will be higher in a turbo system. If you introduce some outside hea source it will increase pressure but that is not how turbos or superchargers work. But really pressure is irrelevant. Engine power is related to air mass not pressure. Pressure is relevant only as it pertains to mass.
    Quote Originally Posted by KevinB6876 View Post
    Well it appears I was not able to get consistent answers to this question. Some say pressure goes up, others say pressure goes down. It's hard to say who's right or who's wrong.

    I just wanted to know if there would be any change in pressure when going from hot ambient temps to cold ambient temps. But it appears the answer is more complicated than previously thought, hence all the complicated and conflicting responses.

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    Last edited by Beau; 11-18-2011 at 11:06 PM.

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