Tom;
The placement and shape of the "tube going to the scoop" is critical. You
may be getting a reading based on velocity through the scoop rather than on
its static pressure-- remember Bernoulli. Perhaps your MAP (Manifold
Absolute Pressure) would be a better measure of how effective your scoop is
but the pressure measurement needs to have pretty high resolution.
I recommend a shareware program called "Master Converter" by Rodney Savard
for conversions such as you asked about. This program can convert almost
ANYTHING and you can find it by doing a search on Altavista, etc.
1" H2O @ 60F = 0.03609119066 psi
1" H2O @ 60F = 0.07348242091" Hg @ 32F
Regards, Neil Tucson, AZ
-----Original Message-----
From: Thomas E. Bryant [mailto:saltracer@awwwsome.com]
Sent: Monday, December 31, 2001 3:01 PM
To: Land Speed
Subject: Air Scoops
List:
I would like to start a new thread discussing air scoops. Many of you
know that I have been experimenting with a variety of scoops for the
last few years with questionable success.
The last meet at Bonneville (the World Finals) I had installed a new
RacePak data collection system. When ordering the system, the sales
person attempted to discourage me from using a scoop pressure measuring
sensor. He gave several good reasons why it was difficult to attain
reliable information. I still was not convinced to leave this
information to chance, so it was included in the package.
The scoop pressure system consists of a tube out the front of the nose
of the car (a reference pressure) and a tube going to the scoop. The
reading should be zero or a bit positive if the scoop is working as
designed. The results were as follows: Once the car was put in gear and
started to pull, the scoop pressure went negative. The higher the RPM
the more negative until it reached 9.6-9.7 in. water, then it leveled
out for the remainder of the run. (It reached 9.6 @ 6400 RPM in low gear
and @ 6200 RPM in high gear.)
This didn't seem right to me, but I didn't fully understand what might
be going on. Before going to the Lakes in November, I did two
things...First, with the car running and the computer recording, I blew
into the scoop tube by mouth and registered a positive reading, so now I
knew what pressure should look like on the readout. I also move the
scoop pressure pickup from the side of the scoop to the center rear just
below the stacks so that movement of air into the stacks would have less
effect on the reading.
At the Lakes, on the first two runs, I repeated the same readings that I
was getting at the World Finals. The 3rd and 4th runs at the lakes were
made after cutting the nose off the scoop, giving about a 5"X5" opening.
With that modification the pressures went a maximum of 0.1 inches
negative in low, 2nd and the bottom of high gear. Above 5000 RPM in high
gear the readings were from 0 to 0.5 inches positive.
I don't know how much this was hurting our top speeds (according to my
calculations, we were pulling about 3 inches of vacuum in the scoop),
nor do I know why the scoop wasn't delivering the air needed. The
opening was actually larger than the calculations called for, almost
five square inches. Possibly, for some reason, the air was being
deflected away from the scoop opening. What do you think?
I also have a couple of question for the engineers on the list. (1) What
is the conversion of inches of water to psi?
(2) What is the conversion of inches of water to inches of mercury?
Tom, Redding CA - #216 D/GCC
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