It's now officially cold here in MN. My butt-dyno told me that something new and exciting was happening. My cold-blooded croc seems to be more aggressive in the cold.

So I dust off my high school chemistry and invoke the universal gas law. PV=nRT. Assuming atmospheric pressure is around 101.3 kPa - the only difference between summer and winter is temperature.

We're about -10 degrees C today. Summer is plus 20C. <<flurry of calculator work>> It seems like the number of O2 molecules flowing into my engine is around 11% more. Does that translate into 11% more HP?

My butt-dyno says I'm getting more. Does this math work? Does the physics work?

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-10C? Where are you, in Europe? I thought you said MN.

Of course the colder it is, the denser the air, meaning more O2 for combustion, but I doubt you really feel it on a NA engine. If it were supercharged or turbo then maybe.

Yes, you really would have 11% more HP if your math is correct. There is nothing quite like warming up a sports car properly, finding some bare pavement on a -40 day and just stomping on it... your car feels like it's found a secret nitrous button...

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Actually it's the opposite... a turbo or supercharged engine creates it's own atmosphere, so temperature and elevation changes don't have as large of an effect as they do on an NA motor. (they still play a roll, but not to the same degree).

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I haven't felt that. With NA cars, felt nothing. With Turbo or SC I have. That SC or Turbo is squeezing more of that cold air in causing higher boost.

I have lived in MN all my life and I have noticed the effect of cold temps on performance. The effect is not dramatic, but it is noticable.

Something else must be at play then. Turbo/supercharged vehicles are more popular in high elevation areas (Denver, Calgary) because they don't suffer from the thin air as much as an NA engine is.

When your engine is sucking in air without compressing it (NA), you're at the mercy of temperature, humidity and elevation. Where-as with compressed air (turbo,s/c), your car is going to create the atmosphere based on how it's programmed, so if your turbo is at 1.2 bar, it's going to get the atmosphere to 1.2 bar, regardless of the outside temp, humidity, elevation. The only difference is that at higher elevations, the turbo would have to work slightly harder to compensate for the thinner air.

An NA engine doesn't have this same ability to compensate.

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Degrees C work in MN too!

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I noticed this too, but attributed the extra wheelspin to my summer tires! Thanks for the post.

P.S. I have ordered snows and they should be on in the next week. That will be fun!

What's O2?

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Molecular oxygen.

Yeh, just looked it up - a single O represents an Oxygen molecule, ie, O on the periodic table but it cannot exist as a single molecule so it will always join with something else. O2 is a vapor


So now I know

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The Universal Gas Law is also called the Ideal Gas Law

ideal gas law
PV=nRT
where

P is the pressure (SI unit: pascal)
V is the volume (SI unit: cubic metre)
n is the amount of substance (loosely number of moles of gas)
R is the ideal (or universal) gas constant (SI: 8.3145 J/(mol K))
T is the thermodynamic temperature (SI unit: kelvin).
(The law works with any consistent set of units, provided that the temperature scale starts at absolute zero, and the appropriate gas constant is used.)

The ideal gas law mathematically follows from a statistical mechanical treatment of primitive identical particles (point particles without internal structure) which do not interact, but exchange momentum (and hence kinetic energy) in elastic collisions.

Since it neglects both molecular size and intermolecular attractions, the ideal gas law is most accurate for monoatomic gases at high temperatures and low pressures. Obviously the neglect of molecular size becomes less important for larger volumes, i.e., for lower pressures. The relative importance of intermolecular attractions diminishes with increasing thermal kinetic energy i.e., with increasing temperatures. More sophisticated equations of state, such as the van der Waals equation, allow deviations from ideality caused by molecular size and intermolecular forces to be taken into account.

The bottom line is that it's more complex to calculate any increase or decrease in HP.