along with knowing the 'true' facts about Ram Air
it is also valuable
to be able to calculate how much
hp is gained with colder air
or lost with restriction from a filter.

old post:
---------------
Page 415 of the Bosch Automotive Handbook, 5th edition

http://tinyurl.com/yplr3d

has the two equations you need
to see how horsepower and torque change
when either temperature or pressure change.

For temperature the official equation
used by Americans (SAE) Europeans (ISO) and Japanese (JIN)
is:

{ Temperature Before + 460/Temperature After +460} raised to exponent 0.6

you need a scientific calculator to do this
but there is one built into every Windows computer
under Start...Accessories

As an example,
say that your re-locate your air inlet to a spot where the air temperature
drops to 80 degrees
when before the air coming in had been 100 degrees.

{100 +460 divided by 80 + 460} raised to exponent 0.6

={560/540} raised to exponent 0.6

= {1.037} raised to exponent 0.6

= 1.022

You multiply this number times your horsepower or torque
so if your engine.
If your engine makes 230 hp at peak
1.022 times 230 = 235 horsepower with the 20 degree lower air temperature.

You may have heard the 'Rule of Thumb' that each 10 degree F reduction in air
temperature improves horsepower by 'about' 1% and the equation above is where
that comes from, but is more accurate

For corrections when the pressure changes
the following equation is used:

{absolute pressure after/absolute pressure before} raised to exponent 1.2

As an example
say that the weather is changing where you live
and one day the weatherman says the pressure is
29.5 inches of Mercury as a stormy "Low" passes over
then the next day a clear sky "High Pressure Area" passes over
and the pressure rises to 30 inches of Mercury

{30/29.5} raised to exponent 1.2
{1.0169} raised to exponent 1.2
= 1.0204

It is important to realize
that built right into your engine
are IAT (intake air temperature)
and MAP (manifold absolute pressure)
sensors.

Your IAT and MAP sensor outputs can tell you whether an aftermarket air intake
has helped or hurt your power output.

You can read these sensors
yourself by using an OBD-II scanner, or a cheap electrical multimeter.

You are correct to suspect that most CAI's
only make more noise and have flashy colored parts.

Want confirmation about this from a CAI manufacturer?

Well right now KN Filters is running an advertisement in the various hot rod
magazine saying that a typical paper air filter from the factory creates a
restriction of about 2.8 inches of water....and when this filter gets really
dirty the restriction rises to about 12 inches of water restriction. The KN
advertisement has a dyno graph showing that this can cause about 11 hp power
loss on a high horsepower engine.

The pressure of the air around us is about 404 inches of water
so KN Filter is saying that the pressure is changing from
404 - 2.8 = 401.2
to a new pressure of
404 - 12 = 392 inches of water

Play around with the equations above
and you can find out what the actual horsepower of the engine KN was using
was....and then confirm it by looking at the dyno graph in the advertisement.

The old saying is:

Figures Never Lie
But Liars always are Figuring

very interesting! didn´t knew that.

but does someone knows a formula how to calculate
the needed diameter of an intkaehose?
no matter if cold air is really needed or not.
I have 2x600cfm edelbrock. that means, the hose
should be able to let 1200cfm air through, right?
but how to calculate the diameter? I mean if the pressure
is right, any hose can suck 1200cfm.

there are tables in HVAC handbooks that you can use to predict in advance
how much restriction will result
from so much volume
down a hose of a certain diameter

Marks Standard Handbook of Engineering
IIRC also has tables like that

the basic question:
if I hook a 600 cfm air filter to a 600 cfm carburetor,
then I have 600 cfm of capacity, right?

Unfortunately, wrong

you have to look up the restriction (pressure drop) across the two components at 600 cfm,
add them,
then re-calculate the lower cfm that results from that pressure drop 'pushing' the airflow

a similar situation occurs when you double the pressure feeding fuel injectors in efi systems

the flow of fuel through the injector does not double
but instead goes up by about the
square root of 2 (1.414)