[DeTomaso] Rear Window

[Stephen]

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

 

-------------- next part --------------
   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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