Why do cooler ambient air temps make my 928 run better?
#16
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
#20
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From: Insane Diego, California
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.
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.
#21
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.
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.
#22
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.
So, if T goes down, n has to go up. That means there's "more" air at lower temperatures, even with forced induction.
#23
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*
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*
#25
Once you get to the point where you have fog droplets, the cooling effect of the water evaporating brings the density back up some. So a cool evening in the fog, Lion's Dragstrip near the harbor, lots of horsepower.
#26
#27
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.
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.
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).
#28
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).
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).
#29
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.
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.