|
|
Reposted to add to the confusion.
Brent
-------- Original Message --------
Art asks:
<<<<
Question for Brent Regan
Brent
Can you give me the formula's to figure CFM required
for a TSIO 550 and SCFM rating for a K & N filter?
>>>>
Calculating the Standard Cubic Foot per Minute (SCFM) is, at best, an
approximation. The basic strategy is to take the displacement of the
engine (550 cubic inches in the case of the Continental) and multiply
it how many "displacements" the engine uses in a minute to get the
volumetric needs of the engine. Remember that in a 4 stroke engine it
takes two revolutions to get one displacement so divide the RPM by two
and multiply by RPM to get the (theoretical) cubic inch per minute flow
through the engine. Divide this by 1,728 (cubic inches per cubic foot)
to get Cubic Feet per Minute (CFM). This assumes the engine has no
net restriction on airflow. There are a lot of things that effect the
air flow throughout the engine. Well designed "tuned" intake and
exhaust runners improve flow. So do superchargers (this includes
turbochargers) and a well designed cam. Filters, bends, friction,
valves and throttle plates all reduce flow.
The net effect of all these factors is defined at the Volumetric
Efficiency, which is the ratio of the actual flow versus the
theoretical flow. Altitude and temperature
have an effect as related to standard atmospheric conditions.
The wide open throttle (WOT) volumetric efficiency varies with the
engine and installation but a conservative (and easy) figure to use for
a naturally aspirated engine is 1.0 and a turbocharged engine is 1.1.
The volumetric efficiency of my TIO-540 at 350 Hp is 1.043. I know this
because I measured it on a calibrated dynamometer.
The airflow requirement of the TSIO-550 would therefore be 275
cuin/revolution x 2700 RPM /1728 cuin/cuft x 1.10 = 472 CFM. The
density of air at standard conditions is 0.07647 Lb/cuft. The mass flow
requirements of the engine at WOT and standard conditions is therefore
472 CFM x 0.07647 Lb/CFM = 36 pounds per minute. For a given power
setting the mass flow through the engine must remain the same. For a
turbocharged engine at a fixed power output the CFM flow through the
filter increases with altitude because the density of the air is
decreasing.
For the same TSIO-550 at 75% power you need about 27 pounds of air per
minute to feed the fire. At FL250 every cubic foot contains 0.0343 lbs
of air so you need to flow about 780 CFM through the filter. The air
pressure at FL250 is about 150 inches of water. A dirty 400 SCFM
filter may cost you 8 inches of water or better than 5% power.
Filter flow ratings are available from the manufacturer. Be sure you
get the pressure drop across the filter at rated flow so you can do an
accurate comparison between filters.
Regards
Brent Regan
|
|