cfm calc

[Greg Hermann]

>velocity pressure VP = (total pressure - static pressure).   if you assume a
>MAX VP possible (normally aspirated) of 1 Atmosphere moving to a perfect
>vacuum thru the opening  ..28"hg (gotta convert to "wc, I dont have the conv
>here) you can get the highest CFM possible thru the opening.
>
>feet per minute FPM = 4005 * SQRT(VP).. VP expressed in inches water column
>("Wc)
>CFM = FPM * port area in square feet.
>
>hope I didnt mess that up -

You did.

At an absolute pressure ratio across an orifice in the neighborhood of 2:1,
the velocity through the orifice will reach what is called a "choked"
condition--the flow velocity will be at Mach 1. NO MORE mass flow can pass
through the orifice once this condition is reached unless the DENSITY of
the upstream gas is increased. Doesn't matter how much more the pressure
ratio is increased above this "critical" ratio.

You are on the right track for figuring out the velocity through an
orifice, though. Figure out the kinetic energy of a given flow velocity,
(rho x V2 / 2 x g) and convert it to heat (778 ft lbs = 1 BTU), figure out
how much the gas cools off due to the gain in its velocity (the
acceleration process is almost perfectly adiabatic)., etc.,etc. (Make sure
to keep the units consistent!!!) The adiabatic cooling effect is ONE of the
reasons behind carburettor icing--it can and does freeze the humidity out
of the air. (the latent heat of evaporation of the fuel is the other major
cause of icing.)

BTW--Mach 1 for any gas is a function of the molecular weight and the
(square rott of the?) absolute temperature, nothing else.

You want to size a throttle body or a carb so that you are getting design
MAXIMUM air flow at WOT with a pressure ratio across it of somewhere
between 1.02 and 1.1 to 1. The former would apply to extreme high
performance engines with Individual runner manifolds and throttles, the
latter to "cooking" type engines--such as tractors and 2 barrels on V-8's.

Carbe function best (most accurately) with an air flow velocity at the
narrowest point of the venturi aomewhere in between Mach 0.3 and 0.6.

Greg
>
>----- Original Message -----
>From: Mike Swayze <mswayze at truswood.com>
>To: GMECM_DIY_EFI <gmecm at efi332.eng.ohio-state.edu>
>Sent: Friday, November 26, 1999 8:31 AM
>Subject: cfm calc
>
>
>> I vaguely remember bernoulli(sp.) and sink
>> equations-per chance is there an equation in the
>> archives for calculating cfm for a throttle body
>> based on opening size at WOT?
>> Mike Swayze
>>