List:
Some of you on the list have asked that I write about my experiences
with the air scoop. I don't claim to be an expert on this subject, just
a willing student. I certainly am not an expert on aerodynamics either.
So consider this while reading this.
First, I can tell you from my talking with the engineering community,
and I have spoken with a dozen or so, there isn't much data out there
that is conclusive about scoop design. Lots of "think so", but little
facts. You can come up with the pressures against a moving object at a
given speed, but that does not translate into absolutes at the scoop on
a race car. It appears that aircraft information cannot be translated to
the car since they are different aerodynamically than a land bound
object. I think we all agree that anytime you gather air from outside
the engine compartment there should be an improvement. I personally
believe that argument is enough to make a scoop desirable.
Now to what I have found in my experimentation, which at this point is
ongoing. I have played with the scoop idea for many years in a "guess
so" atmosphere. I have seen cars with scoop close to the salt for a
cooler and more moist air. I suppose there is an argument for that, but
I would be concerned about picking up the salt. My first scoop was on a
'29 roadster. It protruded out of the grille shell as most roadster
scoops do. I haven't a clue whether or not it worked, but I thought it
did. I did the same thing on the Pierson Coupe. These scoops were about
four inches forward of the grille shell which in all probability was in
dead air since I understand that the air parts to go around a blunt
object well ahead of the object its self. The Pierson Coupe had a
rounded nose, so it would have been more likely to gather some clean air.
On my present car, I first gathered the air through two openings in the
hood about 18 inches back of the front wheel humps, the ducting was
crude and certainly not air tight. At the World Finals in 1992, the
car's first season, we ran a 233 MPH 3rd mile with a top speed
(according to the tach) of 237 MPH in D/FCC. After that I tried putting
small scoops over the original openings with little or no change, I went
to a Harwood scoop that did gather pressure, I had three fires on shut
down because the fuel was being pushed through the nozzle air bleeds. I
talked with Mr. Hurt who was an engineer on the Herbert/Steen car and he
told me that they had experienced the same thing with their car. He
recommended plugging the air bleeds, which I did and ended that problem.
Another problem I had with that scoop was it tried to lift the hood off
the car, on one run I could actually see under the back of the hood from
the driver's seat. The parachute also flew very high on deployment,
also. These two things combined convinced me that the scoop was a
aerodynamic disaster. I went back to the openings in the hood that I had
originally, this time using Naca ducts. The car didn't run well at the
next meet so I naturally air intake was suspect (I later found that
there were actually other problems). After talking with Tom Burkland at
length, I decided to take his advice and build a scoop with an
calculated opening. With this setup, there wasn't much change in
performance, but I had numerous other engine problems to work out, so
who knows?
At the World Finals 2001, I made my first runs with data acquisition on
board. I can tell you that this was a great improvement from "seat of
the pants" info. This is when I realized that the scoop was not all it
could be. I have spoken of negative readings in the scoop (-9.7 inches
of water with this scoop above 6000 RPMs ) I need to explain what this
really means. The RacePak system measures scoop pressure by using a
reference tube out the nose of the car to measure pressure against the
vehicle. There is another pressure pickup inside the scoop (mine is
mounted in the rear of the scoop between and about an inch below the top
of the rear stacks) this reads the pressure and compares it to the
reference pressure. A negative pressure does not mean that there is not
pressure in the scoop, ( if there was 1 psi (27.68") on the front of the
car -9.7" in the scoop would be 0.64 psi (17.98)), it just means it is
not as much as the pressure on the front of the car.
The scoop we ran at Speedweek was first equipped with a K&N filter in to
very front of the scoop. With this setup we had the -9.7 readings again,
we took the filter out and the readings rose to -4+. The other
difference between the last two scoops was..last years scoop was reading
-9.7 negative, but rose to zero the instant the engine was shut down.
(RPM related) This years scoop , with and without the filter, did not go
back to zero for about two miles after shut down. (Speed related)
Looking at the graphs @ approx.120 MPH, (at wide open throttle), the
pressure going negative crossed the rising speed and @ approx.120 MPH on
shut down, (engine off) the graphs again crossed speed reducing and
pressure rising. Interesting to say the least.
Now my "think so"... last year's scoop was probably not in clean air
because of the flat surface below the opening and maybe the air box was
too small. At the Lakes in November, after cutting the nose off the
scoop which left roughly a five by five inch opening starting a hood
level, pressures were acceptable, zero to positive 0.6 inches of water
in high gear. This year's scoop was likely in turbulent air which caused
it not to receive an adequate supply.
We are in the process of raising the front of the scoop to get it into
cleaner air, if this works we probably will have to reduce the scoop
opening next. As you may have noted, I am enjoying trying to get a scoop
that really works. I believe that the monitoring system that I have will
allow me to do that with confidence.
Tom, Redding CA - #216 D/GCC
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