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Based
upon these discussions I had with Brent 6 yrs ago, I have been flying behind a
K&E 1560 reusable filter for the last 600hrs in my TSIO-550B2B. It is
rated at 530cfpm, and seems to work well in the space I had already committed to
for the original WIX filter.
Doug
Johnson
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
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