Hey Tom
If the scoop opening is so large that flow is larger than the needs of the
engine, then you could be getting circulation in part of the opening and out
part of it, thus creating a negative pressure from the velocity. The rule of
thumb for carburetor hood scoop is 10% more area than the venturi area.
Don't ask me how I know, but if the opening is too big you can actually
siphon fuel out of the carburetor from the circulation air movement and
create a fire hazard and lean out the engine. A 5X5 inch opening sounds
large. Take a look at Al Teague's scoop opening. It might be half that
area.
Just a thought
Dave the Hayseed
----- Original Message -----
From: "Thomas E. Bryant" <saltracer@awwwsome.com>
To: "Land Speed" <land-speed@autox.team.net>
Sent: Monday, December 31, 2001 4:01 PM
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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