cold air

And that's fine, but to do as you did, to answer a question wholly different to the question that was asked...why? What's the point?

DS

So did most others who replied to my thread. Some spawned other discussions, which I didn't really care about so long as the main question was answered by guys like you, Geoff (interesting point about engine block temp), fbloogyudsr (fuel doesn't atomize in cold), Brandon, etc.

I just ignored the other responses about road conditions, mileage, and someone calling me a troll. I guess it takes more self-control for some people to harness the power of usenet effectively :-)

So if this is a linear relationship, very close to zero kelvin the power could easily double.... (obviously assuming the 'air' is still a gas)

I suppose a similar relationship applies to altitude........along with a plethora of other factors like drag, etc ;-)

Speaking of humidity, I never knew that water displaced air. Aren't there the same number of O2 molecules in 1cc of dry air as there are in 1cc of saturated air?

The Society of Automotive Engineers has a test standard (J1349) that applies to net power testing of automotive engines. The standard includes a formula for an atmospheric correction factor for WOT power in spark-ignition engines.

Roughly speaking(*), the calculated net power ratio (between power outputs at two temperatures) is inversely proportional to the square root of the absolute temperature ratio, i.e., colder air produces more power.

To use one of your examples for numbers: power at -15F vs

(*) The correction formula assumes that *indicated power* (i.e., total engine power, including internal friction power) varies with the reciprocal of the square root of the temperature ratio. It also assumes that friction power does not change with temperature, so the change in *net power* is actually greater than is suggested by the temperature ratio. The effect is fairly small but in your examples, the net power ratio would increase by 2%-3% using the SAE formula. IOW, the SAE formula shows a 13% increase in power just by changing air temperature from 90F to -15F, assuming a mechanical efficiency of 85% (i.e., a friction loss of 15%).

-- Chuck Tomlinson