Ram air box truths (real PSI gain)
Gentlemen,
This issue has been explored and many of us have spent hundreds of

dollars

or many hours to install a RAM air box in our aircraft to gain an

expected

increase in performance. Truth be told, there isn't much gain under

200

mph. At least not enough to even register on the airspeed

indicator.

I'll

explain why and you can decide for yourself.  This is probably why

no

one

will quote any increase in speed when you purchase a RAM airbox

from

them,

because there won't be any.
Here are the true hard facts using the following criteria:

(Publication:

Aerodynamics and Flight)
At   MSL Pressure 14.7 psi  (densest air possible on the planet)
At   200 MPH (if your plane will go this fast) direct air flow (non
obstructed, perfect flow)
At   59.02 degrees F  (15 degrees C)  Standard aeronautical

calculation

temp at MSL

TOTAL INCREASE IN PRESSURE =  0.710 psi

By the way, any performance increase is assuming that there are

zero

bends,

curves, voids, filters elements or any other

disturbance/blockage of

the

flow into the cylinder head ports. And we all know that isn't the

case

in

our aircraft or engine induction design.  Now, go ahead and factor

in

actual conditions and temp at 2000 to 8000 ft MSL along with your

actual

airspeed (indicated pressure) and the fact that you are not getting
true/undisturbed RAM air into your PERFORMANCE DESIGNED Lycoming

induction system.

You might see 1.5 HP at 2900 RPM's on an 0-320 engine running Wide
open Throttle.  Just don't pack a lunch before flight, you'll get

the

same

results.
Just passing along mathematical facts.

This guy betrays his own lack of 'mathmatical facts'.  Since engine HP is
almost directly related to available manifold pressure, the HP increase
using HIS numbers would be 7.727 HP.  (.710 / 14.7 =  4.829%   * 160 HP =
7.727)

Ever seen a formula 1 racer NOT use a ram air inlet?

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

Every time I take off I see a half inch gain in manifold pressure from the
time I start rolling at full throttle until I start climbing and thus see
the pressure start back down.  At about 120 on the deck I show about 3/4"
more MP than a full throttle run-up.  (No points for precision here it's a
cheap MP gauge).   This is with a bone stock somewhat rough RV-4 induction
system with a carb and Van's FAB air box.  It is sealed well with only the
tiny rain drain hole in the bottom.

The pressure increase should be the same through the entire induction pipe
from snorkel to port.  I get 28 inches at static full throttle run-up when
the barometer says 30.  Stuff a half inch more in the nose and the port
will see 28.5 and still have the 2 inch loss from filter, carb, and that
atrocious manifold.

At 120 mph dynamic pressure is (6.9 IN-H2O * .073423) = about 0.5 IN
Hg.  If I had a CS prop this would be a more accurate example allowing
static rpm to match that at 120 mph.  I get 2050 static and about 2300 at
120.  At 175 you should see about an inch NET improvement with a good
snorkel and no air leaks.

Mike McGee, RV-4 N996RV, O320-E2G, Aurora, OR
13B in gestation mode

 
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