Air temp and HP relationship
10-14-2005, 03:39 AM
#1
Race Director
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Air temp and HP relationship
I have read several times (forum sources) that a 10 degree drop in temp will increase power by 3-5%. I need to either get a more scientific source for this, or a large general concensus. Cheers guys! 
10-14-2005, 09:06 AM
#2
Three Wheelin'
I don't have any specific figures, but there are physical laws that apply to the Pressure, Volume, and Temperature of gasses.
When you cool a gas, its temperature goes down (duh) and the volume it occupies goes down which is reflected as an increase in density, or molecules/unit of volume.
From the standpoint of an internal combustion engine, a cooler air charge inducted at a given volumetric flow rate (cubic feet/minute) will have more oxygen molecules (O2), than a warmer air charge.
Because of the differences in their densities, if fuel flow is held constant, these two air charges will tend to produce different amounts of power.
To compensate for this behavior, Bosch adds a temperature sensor to the inlet of the air flow meter. Inlet air temperature temperature is used by the DME to calculate incoming air density so that fuel flow can be "matched" to maintain a proper fuel/air ratio.
When you cool a gas, its temperature goes down (duh) and the volume it occupies goes down which is reflected as an increase in density, or molecules/unit of volume.
From the standpoint of an internal combustion engine, a cooler air charge inducted at a given volumetric flow rate (cubic feet/minute) will have more oxygen molecules (O2), than a warmer air charge.
Because of the differences in their densities, if fuel flow is held constant, these two air charges will tend to produce different amounts of power.
To compensate for this behavior, Bosch adds a temperature sensor to the inlet of the air flow meter. Inlet air temperature temperature is used by the DME to calculate incoming air density so that fuel flow can be "matched" to maintain a proper fuel/air ratio.
10-14-2005, 04:01 PM
#3
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10 degrees F is going to be a bit less than 2% more air at a given temperature and pressure according to the ideal gas law(PV=nRT).
Additionally, you should gain some knock resistance as the air cools. That may give you another few percent, assuming you have a car that takes advantage of the cooler air to run more timing or boost.
So 3 or 5% sounds reasonable. Turbocharged cars will probably gain more, since they tend to be more knock limited than flow limited in terms of power production.
Additionally, you should gain some knock resistance as the air cools. That may give you another few percent, assuming you have a car that takes advantage of the cooler air to run more timing or boost.
So 3 or 5% sounds reasonable. Turbocharged cars will probably gain more, since they tend to be more knock limited than flow limited in terms of power production.
10-14-2005, 04:28 PM
#4
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I know every 15 degree drop in intake temps can run 1 more psi without making any changes.
And everyone is familar with the "auto" boost increase in the colder months due to cooler charge air, IC and turbo not getting as hot either.
And everyone is familar with the "auto" boost increase in the colder months due to cooler charge air, IC and turbo not getting as hot either.
10-14-2005, 04:31 PM
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I have seen tests that showed cooler air made more hp then more air. I.e. 300 CFM at 40 degrees makes more hp then 500 CFM at 70 degrees.
It was a test of cold air intakes and the ones that got cooler air in the motor made more hp then the ones that just got more air into the motor.
It was a test of cold air intakes and the ones that got cooler air in the motor made more hp then the ones that just got more air into the motor.
10-14-2005, 06:48 PM
#6
Ok, here it is once again...
(273+T2)/(273+T1)
Lets say you lower the temp from 40 degrees C to 30 degrees C.
(273+40)/(273+30)=3,3% moore densier air and its also about the same in hp gain
(273+T2)/(273+T1)
Lets say you lower the temp from 40 degrees C to 30 degrees C.
(273+40)/(273+30)=3,3% moore densier air and its also about the same in hp gain
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10-14-2005, 10:11 PM
#9
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As many of you know, there is an SAE correction factor for temperature that is typically applied in dyno testing. This should give you an idea of the temperature-horsepower relationship. If you want the details, go here.
In case you don't like to do the math, I created a chart from the formula in the article. I assumed 1 atmosphere of ambient atmospheric pressure.
The correction is calculated as a percentage deviation from the value at 25 deg C. For example the SAE correction subtracts 5% from the horsepower value at 0 deg C to get the value that would be obtained at 25 deg C. Or to state it differently, the engine makes 5% more horsepower at 0 deg than at 25 deg. I suppose this is based on some idealized assumptions, not all of which I pretend to understand in detail.
In case you don't like to do the math, I created a chart from the formula in the article. I assumed 1 atmosphere of ambient atmospheric pressure.
The correction is calculated as a percentage deviation from the value at 25 deg C. For example the SAE correction subtracts 5% from the horsepower value at 0 deg C to get the value that would be obtained at 25 deg C. Or to state it differently, the engine makes 5% more horsepower at 0 deg than at 25 deg. I suppose this is based on some idealized assumptions, not all of which I pretend to understand in detail.
