JoeyVee

I live in NYC. I've noticed my car acting extremely different from one day to the next. Obviously when its 65 degrees and humid she runs liked a raped ape. Some days when its gets hotter 75-80 she runs REALLY slow. Like I don't have a turbo. But then on some warm days shes still got it. Also when it was VERY cold and crisp out she didn't really like that weather too much either. So my questions is this. Lets rate the performance robbers. At a perfect ten we have 65 degrees and humid. Running perfect Like this morning
Then we have these four to categorize. I'm new to forced induction and I am no physicist so I want to see what people think would hurt the twins more.
99 Degrees with 100% humidity
99 degrees with 40% humidity (yeah right. Not in NY)
25 Degrees and 90% humid (doesn't happen here either)
25 degrees and 10% humidity. (happens ALL too often this year) brrrr

I also start to see power dropping when driving through city streets after a long drive. I'm guessing I'm heat-soaked under the hood and she just needs to cool off a little..
So lets see if we can have a nice Thursday discussion. Thanks for letting me post ladies.

quickaudi:STFA

Oh, and stock intercoolers are overmatched for anything other than stock boost.

JoeyVee

Stock. RS6 exhaust but thats it. But in general. We know best conditions. Where do the others rate?

quickaudi:STFA

heat matters, humidity doesn't matter as much, altitude matters a little

remember, since you're not in the same place for these runs, you have to factor in gearing, throttle position, heat soak, etc. into the equation.

Now, if we were comparing runs at the same location with the above the same, then it'd be a different story

JoeyVee

Gotcha. say all things being equal. Ill give examples.
Sea level (thats me)
I wake up in the morning. cars cold. I wait for oil temp to creep up a little. I can tell by *** dyno that sometimes she feels sluggy. sometimes like she wants to run all god damned day. My main question was if I wake up and its 99 degrees and humid is it going to run better than it would on a day when its 75 and NO humidity. I know they love cold dense air. But thats rare in the summer here. So.. if you could pick your conditions to race. would you rather have 99 degrees and 99% humidity
or
72 degrees with 10% humidity?
At sea level. on a track say. Drag. not autox. ad optimal conditions to cool the car off after each run.

S4_Cinci (I S'edTFA)

specific heat capacity of the ambient air. This engine produces a lot of heat and higher intake air temps (IAT) tend to retard timing, thus robbing you of power. Also remember that being stock, once you hit max boost, that's all there is - thus you will hit max boost more readily on a cold day vs a hot one. But on to heat capacity - your body doesn't like humidity because you keep cool by sweat evaporation, which needs a big differential between dry bulb and wet bulb temperatures. On a hot and humid summer day, when the air is saturated with water vapor, cooling via the evaporation of water is quite inefficient. But humid air has a higher heat capacity than dry air, at the same temperatures.

Thus humid air is better at heat transfer than dry air for a given volume of air. But the 2.7t engine uses a mass air flow meter, so it compensates for the extra density in humid air vs dry air. Thus regarding intake air charge for a given IAT, the engine management system equals this out and makes sure you don't get overboosted. On an old, NA engine higher humidity likely resulted in more power due to the effects of the humidity making for a denser charge as the only thing that mattered with a carb was velocity thru the venturis.

All that being said, IMO the best performance for this engine is lower temps. Pay close attention to what your outside air temp is reading, as this can be a big difference between what the weather man says due to the sun heating the asphalt road you're driving on vs the "in the shade" air temp. On the same day, if you're on a freshly paved blacktop road, the temp can be 10 degress hotter than on a older, grey sun-bleached stretch of road.

The humidity thing comes into play just due to the fact that when there is high humidity, the temp is also quite high and even though humidity is expressed in relative terms (50% humidity on a summer day means more water vapor in the air on an absolute basis vs 50% on a winter day). So your performance will be sapped on high humidity days due to high ambient air temps. The higher heat capacity in humid air can help the intercoolers operate at higher efficiencies, but since the air passes so quickly I think most of the time this effect is negligible, but it may come into play at lower road speeds.

Heatsoak is a real killer with this engine, but if you are having widely variable performance that is not due to an improperly calibrated butt dyno, perhaps a sensor is going bad and itermittently gives garbage data to the ECU.

Rebuttals are welcome and encouraged!

JoeyVee

Rebuttals my ***. 90% of that was over my head. So I agree . hahaha. Really though Makes sense. Thanks for the explanation. I was wondering. I dont have wildly different performance on those days. But some days it seems like full throttle is a ***** out run where as some other days it seems like eh im faster than traffic but not by much.
I just love getting in it on a nice cool clear morning. So I can get in line in traffic.. UFCK.
Sometimes I also think I am getting used to the power and it is not so awe inspiring anymore.. Chip time. I know.. Lol.

S4_Cinci (I S'edTFA)

to baffle with b.s. In cold weather, it definitely seems to me there isn't the rush of power building like there is in 50 - 60 deg weather, but this is likely due to the fact that the turbos are so efficient in the colder temps that they get to max boost at lower rpms (engine and turbine speeds both) so it is not as noticeable to the butt dyno as in warmer weather. You might also want to make sure you don't have a boost leak or other problem that is mild. There are many issues with this engine that can sapp performace w/o throwing a CEL/MIL light - contrary to the belief of the service techs at my dealership.

Hope I prvoded more help than not. City driving is not this engine's strong suit - it likes the open road. She'll heatsoak quite rapidly in traffic.

JoeyVee

No no.. Dont worry about it. thats exactly what I was looking for. A very definitive thorough answer! Thanks Cinci

quickaudi:STFA

ever scanned your car for stored codes, just for giggles?