This morning the car's thermometer said it was 49. That is the coldest it has been since I got the car. I think I read (back when it was real hot) that cold air makes it run better. I had forgotten about that until this morning. Once I got out of the neighborhood, and the engine temperature was up to normal, I gave the car some gas and ZOOM! It feels like 20 more hp.

Can someone tell me this is not my imagination and why it does this?

 

It was 37 in my car today, I guess winter is here and is going to stay. I like it though, I am looking forward to driving in the snow.

Medic

 

OK, it's not your imagination

Seriously, I have found that around 50 degrees seems to be the optimal temp performance-wise. This is purely subjective, but I have read the explanation on other threads (it involves math and physics, topics which make my brain hurt).

 

It has to due with the density of air and oxygen for cumbustion and the loss of energy through heat. Or, i could be completely off.

I'm having problems trying to articulate the correct answer. I hope someone can help by chiming in for you.

 

Colder air is more dense and makes more power. You'll notice it to a lesser extent on a cool summer evening.

 

Ok kids, take notes this time.

Cold air has two advantages for it. It is denser, and also has more moisture.

Being denser, you will get a better air-fuel mix with less effort. Mainly it's the moisture. It's akin to nature's own water injection. Horsepower is directly linked to compression pressure in the cylinder. The more pressure in the cylinder on the compression stroke, the greater the reaction during the ignition (expansion) stroke. During the compression stroke, the air-fuel mixture is compressed into the combustion chamber in the cylinder head. Water (moisture) will not compress. So by not compressing like the air-fuel mix, it helps to create a higher pressure in the cylinder. Higher pressure, faster downstroke on the ignition. It also acts as a natural anti-knock by slightly cooling things down, preventing pre-ignition. A lot of competition cars have used water-injection, as did WWII aircraft for an emergency boost.

 

If it has to do with air density, that means less power at high altitude, more power at sea level ? Good for me in south Louisiana , bad for someone in the mountains - justice, finally

 

There you go...and that is why colder is better

 

Partly. Engines always run better at sea level. But in this case, it's moisture. So your humidity is doing something good for a change, besides rusting out your car!

 

It is not the humidity that causes the rust it is the salts on the iced roads that do the damage. Rust is of no concern in South Louisiana..

Forest

 

Gromit, just out of curiosity, how on EARTH did you know that?

That "cold air has more moisture" seems wrong to me at first, because I know that air can be dried by cooling (that's why ac coils get wet), so I looked at the thermodynamics section of Wikipedia to try to support my point.
http://en.wikipedia.org/wiki/Saturation_vapor_pressure
Now my head hurts and I still don't get it.

 

Your supercharger makes more horsepower by compressing air, forcing it into the cumbsustion chamber. The additional oxygen in the copressed air allows the engine to burn more fuel in given cycle which in turn makes more horsepower.

But to answer you question (i hope) when air is copmressed it creates heat which is a loss (or use) of energy.

Hot air (molecules at an active state) take more energy to compress, and when compressed it makes more heat. Meaning supercharger has to work that much harder to compress the air into a given volume and a loss in energy.

When you have cooler air (molecules at rest) it takes less enregy to compress the the air meaning you can compress more oxygen into a given volume than possible if you had hot air.

Ok, Have you ever bought a balloon outside in the warm air then brought it into an air conditioned house or somewhere there is a large drop in temp? Well, if not, the balloon seems a though it has deflated and gotten smaller. But what has happened is that the molecules of air within with in the balloon have cooled down (in rest) but you still have the same amount of air molecules you started with. So now you have the same amount of air molecules in a smaller volume.

So, back to the supercharger. Now you have colder air which which is being compressed by a supercharger (think about the ballon). The cold air is taking up less volume now you are using less energy to compress the cold air and feed it through for cumbustion and you are also able to compress more air within a given volume. Which means a lower loss in energy through heat by compression and more oxygen in a given volume for cumbustion.

Does that that make sense? or am I just talking out my @ss?

 

Colder air also makes the IC much more efficient. I'm betting that you would REALLY notice it if you had an aftermarket setup such as the DFIC.

 

I vote that cold air causes the intercooler to be more efficient, cooling the air and allowing more air volume to get into the cylinders. YOur engine controls know the mass of the air, so if there is more mass of air, it can inject more fuel...therefore better performance.

On the subject of cold air holding more moisture, sorry, that is exactly backwards. The warmer the air, the more moisture it can hold.

YD

 

So i think that is why when the temperature is cooler i hear the supercharger whine being louder than when its hot

 

"On the subject of cold air holding more moisture, sorry, that is exactly backwards. The warmer the air, the more moisture it can hold."

Thank you Yo'sDad, my head feels a little better now.

And thank you, goldenchild. "more oxygen in less space" is simple enough to understand.

 

Brain hurts. Must go drive MINI through twisties for relief.

 

So when it is cold and rainy the cold air does not have more moisture than when it is warm?

I no air conditioning removes moisture to create the cold air but does naturally cold air necessarally have to be dry?

Also do cars perform better in the humid east coast vs. the dryer west coast? Or did we decide that moisture has nothing to do with this and it is that more cold air can fit in to a given area than hot air and that was all that was to it?

 

I think this is it:
1. Rain is condensed moisture. We are speaking here of the water vapor in the air (uncondensed).
2. No, but its capacity to hold water vapor is less than warm air.
3. This water injection stuff is news to me, so I can't speak to this last, except to say that my understanding is that they perform better at sea level than at altitude. I think this might be why some planes have a maximum altitude.

 

The air we breathe is not artificially dried! Natural air always has moisture, even in the desert (just dang little of it). But colder air closer to the earth, will condense the water vapor into a smaller volume. In other words, say you have a two story home. The bottom floor will always be cooler than the top floor, yes? That cooler air is denser, and as the air cools and settles lower, it still contains the same amount of H2O it had when it was warmer, but now that has become denser as well.

But it isn't a matter of thermodynamics (completely). Water does not compress. Ever. So water (moisture) being present in the cylinder takes up volume as surely as having a smaller combustion chamber in the head. Smaller combustion area - a more compressed air/fuel mix - more explosive force directed to the crank - more power to the wheels.

One of my degrees is in Automotive Technology with engineering electives.

 

The joys of a supercharger and denser air :D
Look at it this way...Superchargers pressurises air intake, colder air is more dense...denser the air=more boost. More boost=more fuel More fuel= More BOOM! (charge)

Grrr I want an MCS so bad :D Although I will miss my twin turbo S80 :(

 

OK, so how does cold air effect the MINI COOPER with no super charger? I assume the same with a little less pop then the MCS.

 

is this going to be on the exam?...I got in late....can I copy someone's notes?

 

You hit in on the head. Colder air is more dense, thus there is more oxygen in a given volume of colder air. Hence why we all try to install cold air intakes and bigger intercoolers to lower the charge temp. 60 degree air has a lot more o2 in it than 120 degree air. Not really double, but there's a big difference. A 40 degree temperature delta can be 15-20 HP. That's like installing a pulley!

 

Turbocharged and Supercharged engines are more sensitive to air temperatures, but there's definately a difference still. Maybe 7-10 HP instead of 15-20, but you can feel it. It's almost as big of a change as shutting your AC off.