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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > How much power do we lose when its hot outside?



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      07-07-2007, 02:51 AM   #1
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How much power do we lose when its hot outside?

Where I live, most of the times its like in the mid 60s or 70s, but I must say, when it does get up in the 80s the car feels a little slower. The engine is not as responsive and more melow. Doesn't have that aggressiveness to it.

I know turbo cars lose more power than NA engines at higher temps, but does anyone have any idea how much power we are losing? It's a little frustrating...

Would a bigger intercooler help?

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      07-07-2007, 02:57 AM   #2
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You are lucky, its 115+ all summer over here !
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      07-07-2007, 03:12 AM   #3
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hotter IAT's....slower your car is... so when its hotter yes your car is slower.
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      07-07-2007, 03:12 AM   #4
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Check this out:

http://www.csgnetwork.com/relhumhpcalc.html
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      07-07-2007, 03:14 AM   #5
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If you want to do it manually, here's your formula:
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      07-07-2007, 03:41 AM   #6
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Quote:
Originally Posted by E90ice View Post
Where I live, most of the times its like in the mid 60s or 70s, but I must say, when it does get up in the 80s the car feels a little slower. The engine is not as responsive and more melow. Doesn't have that aggressiveness to it.

I know turbo cars loose more power than NA engines at higher temps, but does anyone have any idea how much power we are losing? It's a little frustrating...

Would a bigger intercooler help?

You are right. The equations are wrong (NA). Depends heavily on IC efficiency. The problem is the scarce room for intercooler. But by upgrading IC, also the flow of it can be enhanced quite a lot.
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      07-07-2007, 05:40 AM   #7
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I think humidity makes a big decrease as well. It was 90+ and really humid yesterday and the turbos didin't add much.

Moist Air is Less Dense...

As you may have noticed, moist air is less dense than dry air. It may seem reasonable to try to argue against that simple fact based on the observation that water is denser than dry air... which is certainly true, but irrelevant.

Solids, liquids and gasses each have their own unique laws, so it is not possible to equate the behavior of liquid water with the behavior of water vapor.

The ideal gas law says that a certain volume of air at a certain pressure has a certain number of molecules. That's just the way this world works, and that simple fact is expressed as the ideal gas law, which was shown above in equation 1.

Note that this is the gas law... not a liquid law, nor a solid law, but a gas law. Hence comparisons to a liquid are of little help in understanding what is going on in the air, and may simply result in more confusion.

According to the ideal gas law, a cubic meter of air around you, wherever you are right now, has a certain number of molecules in it, and each of those molecules has a certain weight.

Most of the air is made up of nitrogen molecules N2 with a somewhat lesser amount of oxygen O2 molecules, and then other molecules such as water vapor.

Since density is weight divided by volume, we need to consider the weight of each of the molecules in the air. Nitrogen has an atomic weight of 14, so an N2 molecule has a weight of 28. For oxygen, the atomic weight is 16, so an O2 molecule has a weight of 32.

Now along comes a water molecule, H2O. Hydrogen has an atomic weight of 1. So the molecule H20 has a weight of 18. Notice that a water molecule is lighter weight than either a nitrogen molecule or an oxygen molecule.

Therefore, when a given volume of air, which contains only a certain number of molecules, has some water molecules in it (which are very light weight), it will weight less than the same volume of air without any water molecules.

From: http://wahiduddin.net/calc/density_altitude.htm

More air molecules in the cyliinder = good, less = bad. So many variables, visit that web page.


ANother good site: http://www.ulpower.com/resources-isa.htm
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      07-07-2007, 06:52 AM   #8
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^^^ All that may be true...if we were talking about N/A engines. Turbos effectively counter the effect of operating in less dense air.
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      07-07-2007, 07:13 AM   #9
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Quote:
Originally Posted by elucas730 View Post
^^^ All that may be true...if we were talking about N/A engines. Turbos effectively counter the effect of operating in less dense air.
Why is that?

the amount of air may differ between N/A and turbo vehicles but both will have the same air density, for example take 1 cm3 of air, it will always be the same 1 cm3 of air, but when temp varies the number of air molecules varies, the hotter the air the less air molecules, and vice versa, the cooler it is the more air molecules are available in that same area (1 cm3) resulting in more air molecules inside the cylinder for better combustion and more power.


EDIT: both N/A and turbo engines lose power in less dense air but N/A tends to lose more
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      07-07-2007, 07:17 AM   #10
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Quote:
Originally Posted by elucas730 View Post
^^^ All that may be true...if we were talking about N/A engines. Turbos effectively counter the effect of operating in less dense air.
They counter the effect because of their nature to flow more air into the cylinder, but as the air that goes in becomes less dense (either by heat and/or humidity/more water molecules), there is no way to make more oxygen molecules fit in the air. So yes it counteracts, but the amount of the counteraction is reduced, which reduces power over more optimal conditions (more dense air with more oxygen molecules that will allow more fuel and a bigger explosion).
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      07-07-2007, 07:20 AM   #11
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Quote:
Originally Posted by scottp999 View Post
They counter the effect because of their nature to flow more air into the cylinder, but as the air that goes in becomes less dense (either by heat and/or humidity/more water molecules), there is no way to make more oxygen molecules fit in the air. So yes it counteracts, but the amount of the counteraction is reduced, which reduces power over more optimal conditions (more dense air with more oxygen molecules that will allow more fuel and a bigger explosion).
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      07-07-2007, 09:54 AM   #12
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This discussion has been carried out ad nauseum in a thread posted over a year ago. In situations where you have higher air density, more oxygen is in the air (per given volume), thus more fuel is needed (richer tune). The converse is true. Saturation vapor pressure varies with different temperatures. Anecdotally, you probably notice that when you drive in higher altitudes, even on windy roads on mountain inclines, your gas mileage increases. I would notice this when driving through the Rocky Mountains in Colorado or in the higher altitudes at Sequoia National Park.

The bottom line is that air density and oxygen saturation affect performance due to the combustion of air/fuel in an engine. Density altitude is calculated using temperature, percentage relative humidity, and barometric pressure.

When it comes to temperature, cool air is more dense than hot air as it holds less water vapor.

An intercooler functions to remove heat from intake air, increasing the air density, and thus power.

I have always wondered why, aside from the explosive dangers, why no one has devised an O2 concentrator or even O2 tank, that increases the O2 content of the air fuel mixture for race cars. The answer may lie in the type of fuels, or N2O injection, that is more combustible?
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      07-07-2007, 10:19 AM   #13
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Quote:
Originally Posted by ksfrogman View Post
This discussion has been carried out ad nauseum in a thread posted over a year ago. In situations where you have higher air density, more oxygen is in the air (per given volume), thus more fuel is needed (richer tune). The converse is true. Saturation vapor pressure varies with different temperatures. Anecdotally, you probably notice that when you drive in higher altitudes, even on windy roads on mountain inclines, your gas mileage increases. I would notice this when driving through the Rocky Mountains in Colorado or in the higher altitudes at Sequoia National Park.

The bottom line is that air density and oxygen saturation affect performance due to the combustion of air/fuel in an engine. Density altitude is calculated using temperature, percentage relative humidity, and barometric pressure.

When it comes to temperature, cool air is more dense than hot air as it holds less water vapor.

An intercooler functions to remove heat from intake air, increasing the air density, and thus power.

I have always wondered why, aside from the explosive dangers, why no one has devised an O2 concentrator or even O2 tank, that increases the O2 content of the air fuel mixture for race cars. The answer may lie in the type of fuels, or N2O injection, that is more combustible?

FWIW, http://auto.howstuffworks.com/question259.htm

mentions benefit of cooling. You mentioned "aside from explosive dangers", but I think that may very well be the primary reason nitrous is preferred to an oxygen tank.
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      07-07-2007, 10:23 AM   #14
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So NA engines lose less power when its hot outside, why?
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      07-07-2007, 10:31 AM   #15
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who said moist air is less dense? its much more dense, just not with oxygen
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      07-07-2007, 10:38 AM   #16
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Quote:
Originally Posted by E90ice View Post
but does anyone have any idea how much power we are losing?
I don't think you are going to get any exact numbers because no one wants to dyno a hot car.

As ambient temps rise, the I/C starts loosing its ability to dissipate heat. Acceleration on a heat soaked I/C is noticeabley less. Feels like about 5-7% for every 30 degree difference in ambient temp. In other words 60F to 90F would be 15-25HP difference, 30F to 90F may be 30-45HP.

But the ECU's safety measures could be also kicking in, lowering the boost, if charge becomes too hot.
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      07-07-2007, 10:46 AM   #17
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Quote:
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So NA engines loose less power when its hot outside, why?
Yes, and NA engines make less power (than FI) when it is cold outside. Air density is proportional to boost pressure. More dense charge (cooler air) = even more dense air under pressure = more power.
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      07-07-2007, 10:47 AM   #18
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The DME corrects for temperature with more boost, like altitude. Around 1 psi per 30 degrees, give or take.
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      07-07-2007, 10:51 AM   #19
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Covered at length many times. As has been the fact that the word is "lose" and not "loose"...
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      07-07-2007, 11:19 AM   #20
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Quote:
Originally Posted by kptaylor View Post
Covered at length many times.
What has been covered was how and why turbo engine loses power, but one of the questions was "I know turbo cars loose more power than NA engines at higher temps, but does anyone have any idea how much power we are losing". This has yet to be answered.
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      07-07-2007, 11:30 AM   #21
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Quote:
Originally Posted by zenmaster View Post
What has been covered was how and why turbo engine loses power, but one of the questions was "I know turbo cars loose more power than NA engines at higher temps, but does anyone have any idea how much power we are losing". This has yet to be answered.
As i recall in one of the japanese Best Motoring International episodes they mentioned that for every +1 degree Celsius above 23 or 25 your engine will lose -7hp, but im not sure of the info it doesnt seem right sorry

EDIT: Add +10 degrees and you lose 70 hp LOL , Forget about it i dont think i can remmber what they really mentioned, but it was something like this.
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      07-07-2007, 11:48 AM   #22
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As i recall in one of the japanese Best Motoring International episodes they mentioned that for every +1 degree Celsius above 23 or 25 your engine will lose -7hp, but im not sure of the info it doesnt seem right sorry
The 335i has its own threshhold of heat soak based on underhood temps and its I/C efficiency. The ability to provide cold air and dissipate heat is going to be particular to the car. Also, the actual percentage of power made from the turbo boost is also a huge factor. In the case of the 335i, that 8.5psi is about 100HP - or 30% of the total power. Some cars make more or less power from FI than others. So I doubt there is a practical rule of thumb that works for all FI cars.
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