EB vs GT Summertime Powerloss

[boB]

Nope.

At any given temperature and pressure pair, dry air contains a higher partial pressure of oxygen than humid air, as with humid air the water vapor in the air exerts its own partial pressure, decreasing the absolute percentage and partial pressures of all other gases.

With respect to aircraft taking off, humidity increases the density altitude notably, lowering effective lift on humid days. Therefore, it is easier for a plane to take off from any particular location on a hot / dry day than it is on a hot / humid day at the same location.

There's a calculator for showing the effect of humidity on air density and density altitude here: https://wahiduddin.net/calc/calc_da.htm

Yes, but...
" Both an increase in the temperature and a decrease in the atmospheric pressure, and, *to a much lesser degree*, an increase in the humidity, will cause an increase in the density altitude. "
And then there is the effect of humidity on IC engines, almost like free water injection it lowers the temperature and allows more spark advance and more power.

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[vdin]

I live in the desert so DA goes up to 7500ft in the summer and the power loss is crazy, almost feels like I’m driving an Accord.

 

[ice445]

I live in the desert so DA goes up to 7500ft in the summer and the power loss is crazy, almost feels like I’m driving an Accord.

It's probably an octane/knock issue and not necessarily the DA. My car on the dyno put down 400 to the wheels at 6000ft DA, but it wasn't that hot out so it was able to run max timing. With low air density plus a lot of timing retard, it's probably like driving a dog with zero throttle response. What are your CHT's like in the summer?

 

[vdin]

It’s roughly around 215F when I’m cruising. It definitely could be knocking, I only have access to 91 octane here on the west coast.

 

[ice445]

It’s roughly around 215F when I’m cruising. It definitely could be knocking, I only have access to 91 octane here on the west coast.

Do you have the performance pack? I only ask because the radiator is about 30% larger on that and gives you some extra margin in hot temps.

 

[Zooks527]

Yes, but...
" Both an increase in the temperature and a decrease in the atmospheric pressure, and, *to a much lesser degree*, an increase in the humidity, will cause an increase in the density altitude. "

Sure, but the comment I was responding to stated that "hot and dry" was worse than "hot and humid", so for that comparison one would have assume that both were at the same temperature and pressure, where humidity would be the only variable acting on DA. In that case, increasing humidity would increase the DA.

And then there is the effect of humidity on IC engines, almost like free water injection it lowers the temperature and allows more spark advance and more power.

I'm not sure it does in this case.

You won't get the cooling effect, as that's a function of the energy required to evaporate the water after it's added to the intake charge. That doesn't happen in the case of changing humidity, as the water has already evaporated and is already in vapor form before the intake. You therefore don't get the concurrent increase in charge density that results in more oxygen being available for combustion.

WRT more spark advance, increasing humidity at any temperature results in a lower percentage of oxygen in the air (richening the mix for any fixed fuel flow) as well as reducing the density of the charge (also richening the mix at any given fuel flow). As the ECU's response in both cases will be to lower fuel flow to maintain its desired mixture regardless of timing, I would think a lower power output would be the result.

 

[boB]

Sure, but the comment I was responding to stated that "hot and dry" was worse than "hot and humid", so for that comparison one would have assume that both were at the same temperature and pressure, where humidity would be the only variable acting on DA. In that case, increasing humidity would increase the DA.



I'm not sure it does in this case.

You won't get the cooling effect, as that's a function of the energy required to evaporate the water after it's added to the intake charge. That doesn't happen in the case of changing humidity, as the water has already evaporated and is already in vapor form before the intake. You therefore don't get the concurrent increase in charge density that results in more oxygen being available for combustion.

WRT more spark advance, increasing humidity at any temperature results in a lower percentage of oxygen in the air (richening the mix for any fixed fuel flow) as well as reducing the density of the charge (also richening the mix at any given fuel flow). As the ECU's response in both cases will be to lower fuel flow to maintain its desired mixture regardless of timing, I would think a lower power output would be the result.

Yeah, I don't know enough about the process to know if the relation between temp and humidity skews one way or the other but as you note, more humidity does increase DA. I recall living in Phoenix and at 118 degrees many flights were delayed until the temp dropped "but it's a dry heat...". ;)
As for the cooling effect of humidity I am not certain about that either, I recall reading about it in a book by Julius Mackerle, he was a specialist on air-cooled engines and that may change how it works. Anyway, where I live it is usually hot and humid so I am probably down on power most of the time. :(

 

[Zooks527]

I recall living in Phoenix and at 118 degrees many flights were delayed until the temp dropped "but it's a dry heat...". ;)

It's 85 degrees in my back yard as I type this. I can't imagine adding another 33 degrees. dry or not !

As for the cooling effect of humidity I am not certain about that either, I recall reading about it in a book by Julius Mackerle, he was a specialist on air-cooled engines and that may change how it works.

There is absolutely a difference as increasing humidity increases the heat capacity of air at any specific temperature, meaning that a set volume of air is able to absorb more energy per degree of temperature rise.

That being said, the effect is pretty small (perhaps a couple of percent increase in heat capacity going from dry to saturated at normal outside temperatures) and it would be a trick to actually notice the difference.

 

[Jaymar]

Driving a turbo car now I feel like it takes more of a hot day penalty than any NA motor I had before. Yes the turbo can make up for the lack of air density but the performance of that is limited by the intercooler which is also limited by the hot air.

Altitude, much like in aviation motors as mentioned, is far less detrimental. Taking my old 302 up into the mountains made it feel incredibly sluggish and this car barely noticed.

 

[Garfy]

Ding, ding, ding…. We have a winnnnnnaaaaah!

Open element CAIs are not beneficial on vehicles where they don’t see constant maintained speeds (Highway use), sit in traffic or are only driven short distances.

There’s been many facts posted about this across many auto forums, it’s nothing new. Even on this site alone it’s been proven that a closed airbox on a N/A S550 being used for daily driving in dense traffic has more benefits.

You might as well just call an open element CAI a HAI, because that’s exactly what it does… it sucks hot air building up under the hood right into the intake due to overwhelming heat soak. Heat soak is worse in spring/summer months due to higher exterior temps and far worse if the vehicle sits in stop/go traffic.

A vehicle with an open element CAI only benefits from it when and if the vehicle is used in a track environment, on highways or open rural roads where the vehicle is constantly moving and the air is cooled due to maintained speeds.

Yeah, most of the so-called CAI systems I've seen for newer cars are all drawing air from underhood. I recall some years ago (don't recall what kind of cars, maybe some 4-banger imports?) that had a system where they used a low restriction filter element but it was in a housing that included an air duct that ran out to the front of the vehicle, either through the grill or just under the bumper area. That was truly drawing ambient temp air into the engine, not hot underhood air. I wondered if it might have helped when you were at highway speeds in that the air was being pushed into the intake as well.

 

[gadgtfreek]

Thats why I bought a CAI with a lid. Those "seal against the hood" things or open-air cones are dumb. They all get some heat, but why seal against the hot ass hood, or have an open cone for engine heat. Did anyone pop the hood on a 5.0 after driving for 15 minutes? You can fry and egg on it.

 

[ice445]

Thats why I bought a CAI with a lid. Those "seal against the hood" things or open-air cones are dumb. They all get some heat, but why seal against the hot ass hood, or have an open cone for engine heat. Did anyone pop the hood on a 5.0 after driving for 15 minutes? You can fry and egg on it.

I'm sure many learned the hard way by grabbing the prop rod with a bare hand, lmao.

 

[gadgtfreek]

I'm sure many learned the hard way by grabbing the prop rod with a bare hand, lmao.

It's a hot sum-bit LMAO. You will only do it once...

 

[NoVaGT]

Yes, but...
" Both an increase in the temperature and a decrease in the atmospheric pressure, and, *to a much lesser degree*, an increase in the humidity, will cause an increase in the density altitude. "
And then there is the effect of humidity on IC engines, almost like free water injection it lowers the temperature and allows more spark advance and more power.

There is absolutely a difference as increasing humidity increases the heat capacity of air at any specific temperature, meaning that a set volume of air is able to absorb more energy per degree of temperature rise.

That being said, the effect is pretty small (perhaps a couple of percent increase in heat capacity going from dry to saturated at normal outside temperatures) and it would be a trick to actually notice the difference.

None of this means anything to the amount of power an ICE can create, in relation to humid atmosphere versus dry atmosphere. Humidity means the atmosphere contains that much less oxygen.

Have you ever noticed you can't fucking breath on a hot and humid day? That's because there's no oxygen in the atmosphere, your lungs are sucking in atmosphere that's packed with humidity instead of the oxygen you need.

And the same thing applies to your engine.

 

[Qcman17]

I'm sure many learned the hard way by grabbing the prop rod with a bare hand, lmao.

Actually first they were blinded by the tail light flash when locking the car and as they staggered around disoriented they grabbed the hot prop rod in the hopes of stabilizing themselves.

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