PHIL928

The best driving for us out here in Dubai is at sunrise dead in the winter when it is about 6 degrees celcuis (33F) Given that we're also situated at sea level, I reckon it's pretty much optimum! Can't wait for December

DaveT

So glad to read this. Always thought my car was in need of some work. Certainly a difference when the temps drop.

checkmate1996

good post! I've always wondered this myself. The car always seemed much more fun to drive in Spring and Fall for some reason...now I know!

kmascotto

Come to Canada!! you will make all kinds of Power!! at -20C

Randy V

IC engines 101.

Cooler air is denser, thus contains more oxygen by volume.

Oxygen is the catalyst that burns the fuel.

More oxygen equals better combustion, equals improved engine performance.

SMTCapeCod

I'm still puzzling over the F/I cars. Since they use mechanical means to, in effect, make the air more dense and are limited by pressure of the charge.

If 12psi is the limit at a given rpm/throttle opening, how does the cooler air provide an advantage?

I'm not understanding the relationship of pressure and density here, as it relates to ambient v. forced air charge, I guess.

hwyengr

To simplify it a little bit, as mentioned earlier, the ideal gas law says that PV=nRT. P=pressure, V = volume, n=well, it gets a little complicated but it's roughly the density of the air, R=constant, and T=temperature. P and V are effectively constant for a given engine. The only difference between FI and NA is P.

So, if T goes down, n has to go up. That means there's "more" air at lower temperatures, even with forced induction.

Jherriott

All cars do like cold air I believe.

My old VW GTI loved cold days, My 928 feels awesome on cold mornings, and the 951 is downright terrifying at -10! (That's Canadian autumn, just before P-car hibernation...). And it's not just tires sliding either... I get through the gears in no time! So the power is up for sure!

Now my MOPAR Hemi... it likes Damp for some reason? *shrug*

Augustomf

Denser air. That is turbo cars need intercoolers. Hot air, less dense less explosion, cool air more dense more explosion.
Also check you intake air sensor.

Captain_Slow

If this is true (the part about lots of horsepower) it's not because of the water evaporating. Lots of fog means the air is at dew point (saturated, 100% humidity) and condensation is either equal to evaporation or the condensation exceeds evaporation. It is true that high RH makes the air LESS dense, which at first seems counter intuitive because a bucket of water is very heavy. But dry air at ST&P will be about 78% N2 and 20% O2 molecules, which are both much heavier than a similar parcel saturated with H2O at a high temperature dew point (water being two parts hydrogen...the lightest element by far). Cold air always has a low specific humidity (not much water capacity), and will be predominantly N2 and O2. I've read or heard sources saying that water vapor in the air can enhance horsepower. I've even heard of water injection...I have never understood how the water helps.

uraniummetallurgist

From Wiki....

https://en.wikipedia.org/wiki/Water_...n_%28engine%29

Bmw635

Force induction cars compress air so air gets hotter. Additionally, the turbos or compressors build heats when it spins up to 60k rpm. Expanded hot air has less volume so intercooler or mist cooling help cools the air and condense it prior to ignition.

The sc in my a BMW produce about 30-50hp differences with coldest air (50f) and cold compressor (50f)vs hot air (80f) and hot compressor (140f).

Captain_Slow

Thanks Uranium....

The Theory section of the wiki makes sense. It sounds like it's a wash between water droplets evaporating inside the hot intake to produce cooling, while simultaneously reducing the density of the charge. The real benefit appears to come from water reducing pre detonation, and allowing timing to remain advanced where power is greatest.

Interesting how water injection was used during WW2 (presumably).

928wolf

The Engineering term is Volumetric Efficiency. The amount of oxygen taken into the cylinders versus the theoretical amount that could fit in the cylinder's volume (I am going from memory so that last part may not be precise). Increased density means more air per cubic inch times the cubic inches of the cylinder equals the amount of oxygen available to burn. Air density goes up as temperature goes down. Volumetric efficiency goes up as air gets denser.

As mentioned, the timing retards at 120F intake temperature. I'm sure some curious scientist has measured how low the ambient air has to be to stay below the 120F number, but my guess is it's in the 70's.

Switching off the air conditioner should help a little too. 10 hp? I don't know.