intake air temp VS power??

[Jeff Alton]

Just curious if those in the know can state what the average drop in HP is with temp change, by degree??

For instance, how much decrease for 20 degrees F increase in intake temp? Or any other example....

Not looking for guesses..... Just curious.

Cheers

[JohnJL]

Hi Jeff,
I dont know how accurate this is but it shows ~5% change for 20 degrees. That's at 100 to 120 degrees but I havent played with anywhere else on the curve.

http://wahiduddin.net/calc/calc_hp_dp.htm

[hcoles]

you might look at drag racing pages... they do a lot of calculation re. corrected density alititude which takes into account temp./humidity/pressure.

[rnln]

I read somewhere stated about 4% to 5% too, other cars not 911. Once, I was looking into this. I couldn't find and specific data but someone did some extend tests. I forgot all the detail but here is the link if you haven't read and interested.
If you find anything, please post.
Experimental data for Cold air intake

[RWebb]

Jeff, I would assume it is tightly correlated with air density. You can get air density vs. air temp. from psychrometric curves. I may have something laying around - if so, will post. Or you can search the net or visit a good engineering library.

[RWebb]

try this

http://www.engineeringtoolbox.com/air-temperature-pressure-density-d_771.html

[Jeff Alton]

Thanks guys, making for good reading!

Cheers

[DG624]

How does the under hood air reduce density and HP?

[rnln]

hot temp from the engine does.

[dentist90]

Not just heat from warmed manifolds... ambient air temp and altitude affect density and O2 content in the charge air (assuming non-turbocharged).

[burgermeister]

This has a nice equation:

http://wahiduddin.net/calc/cf.htm

[burgermeister]

This has a nice equation:

http://wahiduddin.net/calc/cf.htm

[NathanR]

I researched this question a few years ago. I don't remember the source, but according to the spreadsheet I built at the time, but my calculations used a .1852 percent drop in potential HP for every degree increase in intake air temps. I got that number from a book somewhere, don't ask where.

The fun part of that project was I actually instrumented the intake and drove around and took temp measurements in various driving conditions. I was trying to decide if it was worth it to create a cold air intake for the car. I determined, for that car, the maximum potential for power improvement was about 8%, in stop and go driving conditions. I never got around to building the cold air intake.

Cheers,
Nathan

[Jack Olsen]

Exactly. When in stop-and-go traffic are you ever going to need your engine's maximum horsepower? I built a cold air intake and did temperature measurements. In extremely hot and slow traffic, there was a pretty significant gap in temps. As soon as you step on the loud pedal, though, that gap starts to disappear as the volume of air being moved by the engine blower renders radiant engine heating of the air insignificant. On the track, the difference dropped to essentially nothing.

It's the nice thing about the air-cooled 911 engine design -- when you need power, it'll be there.

[burgermeister]

I knew I ran across this recently ... this is from an article by Alan Caldwell, Panorama, Feb 98.


And, for what it's not worth, I agree that a cold air intake is pointless on a 911. With 1500 liters (around 54 cu. ft) of air being moved by the cooling fan each second (I think that's the spec for the later fans), that probably replaces all the air above the engine every 0.15 seconds ... not likely to heat up much in that time.

[Jeff Alton]

I was not thinking about intake air temp vs ambient. My ECU displays intake temp and it is always with in 2C of ambient when the car is moving.

I was more curious about say 40F vs 85F of ambient temp and what its effects would be.

Cheers

[dentist90]

It looks to me like going up into the mountains affects the correction factor more than the ambient temp. 30*C at sea level is roughly equiv. to freezing temp at 1500'. I live at 3000'... no wonder I can't get 150mph from my wagon!

I know our Sea Doo will rev at 7000rpm at full throttle at lower altitudes, but only 6800rpm where we live. It is also faster on colder days than hot, but never as fast as at lower altitude.

[burgermeister]

I think on modern EFI cars (especially the 3.2's w/out knock sensors) the difference with temperature is more pronounced than the graph would lead you to believe, as the ECU dials back timing with rising intake temp to avoid detonation.

At least my 3.2 butt-dynos significantly better on a 40 degree day than on a 90 degree day.

[Wavey]

Additional reading here, Pelican thread from 6 years ago. Doesn't exactly address Jeff's question but does address underhood temps VS intake temps.

Experimental data for Cold air intake