[DeTomaso] Rear Window
jgkrenton at comcast.net
jgkrenton at comcast.net
Sat Mar 19 10:58:10 EDT 2016
Stephen:
It will be interesting to see what Tom Tjaarda has to say.
It's my recollection that the deck lid opening is in fact a big air exhaust duct, sort of based on the NACA duct principle. It was designed to help move high pressure air from under the car, up around the engine and out the back. This was intended to help with the cars stability and engine cooling at high speeds while reducing the overall aerodynamic drag of the car. Blocking the opening would impact these functions (as does removing the little side windows).
Somewhere around the industry is some information on wind tunnel tests that were done on the Pantera body back in the 70's. I just don't remember where now.
I did have the opportunity to be following a friends Pantera who was at 70 or 80 MPH when an air conditioning hose let go and filled the engine compartment with Freon mist (looked like smoke!). I can tell you from that experience that the deck lid opening is very efficient at moving large quantities of air from the engine compartment rapidly and smoothly.
Here is some information on NACA ducts which you might find interesting and useful in your explorations.
http://ntrs.nasa.gov/archive/nasa/ca...1993093623.pdf
http://ntrs.nasa.gov/archive/nasa/ca...1993091791.pdf
http://ntrs.nasa.gov/archive/nasa/ca...1993093645.pdf
I also seem to recall that the low pressure area behind the car, and the low pressure in the engine compartment are about equal in that 70 to 80 MPH zone. This means the stock Air Conditioning condenser has no airflow (the pressures are low enough the fan is ineffective) and the condenser can "freeze up" in certain conditions which causes over pressure and is why the hose broke on my friends Pantera.
FYI/Your experience may vary....
Jeff/2467
----- Original Message -----
From: "Stephen" <steve at snclocks.com>
To: detomaso at poca.com
Sent: Friday, March 18, 2016 8:35:52 AM
Subject: [DeTomaso] Rear Window
I've been pondering for quite a while now the idea of putting a piece of
glass in the "window" in the deck lid behind the engine in 5332. This would
keep the engine from getting wet every time it rains and would also keep
from sucking grit and water off the road when the car is running down the
road.
Both seem like decent goals. Oh, and it would also make it more difficult
for someone to just reach in and remove 5332's dog-bone air cleaner. OK -
no one has taken it so far. But, hey, it would be all too easy to remove.
Looking at the opening - it sure looks like the original intent was to put
in a window.
I have sent an e-mail to Tom Tjaarda (attached) asking his thoughts.
Hopefully he will respond.
In the interim, I put together a set of sensors so I could see what I could
learn about conditions in the engine compartment just inches in front of the
opening, to the right side of the dog-bone. The attached photo shows the
instrument cluster shot from inside the car.
I'm measuring air velocity through the rear window (vane anemometer)
Vacuum just in front of the window
Rotation of the a/c condenser fan (volt meter connected across the motor,
which acts as a generator when the fan turns)
Two of the above are fairly straight forward. The vacuum - not so much.
When one talks about a vacuum, one is usually talking about a vacuum,
relative to ambient atmospheric pressure. So, if a vacuum gauge is hooked
up to your intake manifold, it reads the difference in pressure between the
atmosphere where the gauge is sitting, and the inside of your manifold,
where the hose from the gauge is connected.
So, what does one compare to in a moving car? The interior of the car?
That is what one would get if one puts a vacuum gauge in the cab and routes
the hose to the area in question. But, what does the pressure (or vacuum)
inside the car do with the car moving down the road. Open the windows, your
ears pop, and the pressure changes. Most of us don't have an external air
inlet (at least an intentional one) to the cab except for the windows.
My solution is to use a fairly sensitive pressure gauge (a sphygmomanometer
- gauge used to read blood pressure. It reads in inches of mercury. FYI,
there are roughly 50 inches of mercury per psi). I have connected the gauge
with small-diameter plastic tubing to a 200 ml stainless sample bomb. I
wrapped the bomb in towels and placed in a cooler to minimize temperature
changes. See, the pressure in the bomb will change by roughly .03 psi (1.5
mm Hg) for each 1 degree F change in temperature.
I then pressured up the bomb and let it sit overnight to stabilize. It
ended up at 280 mm. This is my "zero" at the then current barometric
pressure.
One has to view the pressure gauge as a delta-pressure device. It shows the
difference in pressure between the bomb (high pressure) and the atmosphere
around the gauge. So, if the reading on the gauge goes up, the differential
is increasing, which means the atmospheric pressure is going down. As in a
wee bit of a vacuum around the gauge. Flip side, if the reading on the
gauge goes down, well, there is less differential between the 280 mm Hg in
the bomb and the atmosphere around the gauge, so the pressure is going up.
OK - that is complex.
While a gauge that can be read down to around 1 mm hg (0.02 psi) is pretty
sensitive, I suspect we are talking pretty minimal vacuum levels.
Preliminary testing suggested a reading of 272 mm Hg at 80 mph. The
pressure around the gauge went up!
Huh. A lot to learn here.
I also noted that the volt meter did not show any rotation of the fan on the
a/c condenser. Need to make sure the wiring is ok. But, note, the meter
reads in mV, and, in my earlier testing, it showed voltage if the fan turned
at all.
The rubber for the new window shows up today, the window next Wednesday.
My plan is to get a decent video and stills of the test instruments at
several speeds. Then, when the new window is installed, rerun the test. I
suspect the anemometer will still be valuable, since I suspect there will be
a lot of turbulence in the air in the engine compartment even with the
window in place.
Needless to say, having the window in place will complicate putting the
deck-lid back in place.
Stephen Nelson
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-------------- next part --------------
Stephen:
It will be interesting to see what Tom Tjaarda has to say.
It's my recollection that the deck lid opening is in fact a big air
exhaust duct, sort of based on the NACA duct principle. It was
designed to help move high pressure air from under the car, up around
the engine and out the back. This was intended to help with the cars
stability and engine cooling at high speeds while reducing the overall
aerodynamic drag of the car. Blocking the opening would impact these
functions (as does removing the little side windows).
Somewhere around the industry is some information on wind tunnel tests
that were done on the Pantera body back in the 70's. I just don't
remember where now.
I did have the opportunity to be following a friends Pantera who was at
70 or 80 MPH when an air conditioning hose let go and filled the engine
compartment with Freon mist (looked like smoke!). I can tell you from
that experience that the deck lid opening is very efficient at moving
large quantities of air from the engine compartment rapidly and
smoothly.
Here is some information on NACA ducts which you might find
interesting and useful in your explorations.
[1]http://ntrs.nasa.gov/archive/nasa/ca...1993093623.pdf
[2]http://ntrs.nasa.gov/archive/nasa/ca...1993091791.pdf
[3]http://ntrs.nasa.gov/archive/nasa/ca...1993093645.pdf
I also seem to recall that the low pressure area behind the car, and
the low pressure in the engine compartment are about equal in that 70
to 80 MPH zone. This means the stock Air Conditioning condenser has no
airflow (the pressures are low enough the fan is ineffective) and the
condenser can "freeze up" in certain conditions which causes over
pressure and is why the hose broke on my friends Pantera.
FYI/Your experience may vary....
Jeff/2467
__________________________________________________________________
From: "Stephen" <steve at snclocks.com>
To: detomaso at poca.com
Sent: Friday, March 18, 2016 8:35:52 AM
Subject: [DeTomaso] Rear Window
I've been pondering for quite a while now the idea of putting a piece
of
glass in the "window" in the deck lid behind the engine in 5332. This
would
keep the engine from getting wet every time it rains and would also
keep
from sucking grit and water off the road when the car is running down
the
road.
Both seem like decent goals. Oh, and it would also make it more
difficult
for someone to just reach in and remove 5332's dog-bone air cleaner.
OK -
no one has taken it so far. But, hey, it would be all too easy to
remove.
Looking at the opening - it sure looks like the original intent was to
put
in a window.
I have sent an e-mail to Tom Tjaarda (attached) asking his thoughts.
Hopefully he will respond.
In the interim, I put together a set of sensors so I could see what I
could
learn about conditions in the engine compartment just inches in front
of the
opening, to the right side of the dog-bone. The attached photo shows
the
instrument cluster shot from inside the car.
I'm measuring air velocity through the rear window (vane anemometer)
Vacuum just in front of the window
Rotation of the a/c condenser fan (volt meter connected across the
motor,
which acts as a generator when the fan turns)
Two of the above are fairly straight forward. The vacuum - not so
much.
When one talks about a vacuum, one is usually talking about a vacuum,
relative to ambient atmospheric pressure. So, if a vacuum gauge is
hooked
up to your intake manifold, it reads the difference in pressure between
the
atmosphere where the gauge is sitting, and the inside of your manifold,
where the hose from the gauge is connected.
So, what does one compare to in a moving car? The interior of the car?
That is what one would get if one puts a vacuum gauge in the cab and
routes
the hose to the area in question. But, what does the pressure (or
vacuum)
inside the car do with the car moving down the road. Open the windows,
your
ears pop, and the pressure changes. Most of us don't have an external
air
inlet (at least an intentional one) to the cab except for the windows.
My solution is to use a fairly sensitive pressure gauge (a
sphygmomanometer
- gauge used to read blood pressure. It reads in inches of mercury.
FYI,
there are roughly 50 inches of mercury per psi). I have connected the
gauge
with small-diameter plastic tubing to a 200 ml stainless sample bomb.
I
wrapped the bomb in towels and placed in a cooler to minimize
temperature
changes. See, the pressure in the bomb will change by roughly .03 psi
(1.5
mm Hg) for each 1 degree F change in temperature.
I then pressured up the bomb and let it sit overnight to stabilize. It
ended up at 280 mm. This is my "zero" at the then current barometric
pressure.
One has to view the pressure gauge as a delta-pressure device. It
shows the
difference in pressure between the bomb (high pressure) and the
atmosphere
around the gauge. So, if the reading on the gauge goes up, the
differential
is increasing, which means the atmospheric pressure is going down. As
in a
wee bit of a vacuum around the gauge. Flip side, if the reading on the
gauge goes down, well, there is less differential between the 280 mm Hg
in
the bomb and the atmosphere around the gauge, so the pressure is going
up.
OK - that is complex.
While a gauge that can be read down to around 1 mm hg (0.02 psi) is
pretty
sensitive, I suspect we are talking pretty minimal vacuum levels.
Preliminary testing suggested a reading of 272 mm Hg at 80 mph. The
pressure around the gauge went up!
Huh. A lot to learn here.
I also noted that the volt meter did not show any rotation of the fan
on the
a/c condenser. Need to make sure the wiring is ok. But, note, the
meter
reads in mV, and, in my earlier testing, it showed voltage if the fan
turned
at all.
The rubber for the new window shows up today, the window next
Wednesday.
My plan is to get a decent video and stills of the test instruments at
several speeds. Then, when the new window is installed, rerun the
test. I
suspect the anemometer will still be valuable, since I suspect there
will be
a lot of turbulence in the air in the engine compartment even with the
window in place.
Needless to say, having the window in place will complicate putting the
deck-lid back in place.
Stephen Nelson
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1. http://ntrs.nasa.gov/archive/nasa/ca...1993093623.pdf
2. http://ntrs.nasa.gov/archive/nasa/ca...1993091791.pdf
3. http://ntrs.nasa.gov/archive/nasa/ca...1993093645.pdf
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5. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091791_1993091791.pdf
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