[DeTomaso] Rear Window

Stephen steve at snclocks.com
Tue Mar 22 14:25:46 EDT 2016


I am amazed at how much knowledge there is out there regarding air flow and the issues surrounding a/c performance.  Thank you to all who have sent in their part of this collective wisdom.  I will do my best to pull together all the comments in a summary before I get done with this issue.

 

On 3/19 my wife and I were able to do another series of runs up and down the highway.  The wind had dropped to around 9 mph.  We made runs both with the a/c running and shut off, as well as runs with the a/c condenser fan sucking in and blowing out.  My conclusions follow:

 

At speeds of 50 to 90 mph, the air velocity through the deck-lid opening is about 20% of road speed (assuming minimal wind and that my anemometer location is representative of the entire opening).

 

The pressure just inside of the deck-lid opening is slightly positive in the 50 to 90 mph range.  While my gauge location is flawed – right beside the dog-bone air cleaner inlet (my bad) – at speeds in the 50 to 70 mph range there is around 0.1 psi positive pressure.

 

The positive pressure under the rear of the car when it is moving at speed, coupled with the likely vacuum behind the car reduces the efficiency of the a/c condenser fan.  My tests suggest that around 60 mph the air-flow through the condenser is reduced by about 40%.  At around 80 mph it is reduced by over 50%.  While I don’t have a massive amount of data, what I have suggests that the decrease in flow through the condenser is not directly proportional to car speed, and that, in the 90 to 100 mph range the flow through the a/c condenser drops to around nothing.

 

Reversing the a/c condenser fan direction, so that it blows air through the condenser and out the back of the car, resulted in a 45% increase in voltage generated by the small computer fan I have mounted to the a/c condenser fan.  This increase is likely because there is no restriction to air-flow into the condenser fan when it is blowing toward the condenser.  When blowing in the other direction the condenser restricts air flow, resulting in less air being blown past the small computer fan.

 

	
Volts read on small computer fan monitoring airflow


Car Speed relative to air

0

50

60

70

80

90


Condenser Fan Direction

						

Suck air from the condenser

1.00

	0.60

0.55

0.45

	

Blow air through the condenser

1.45

1.58

1.59

1.60

1.60

1.60

 

The voltage generated by the small computer fan when the car was moving faster than 50 mph and the condenser fan was blowing air through the condenser reflected a:

 

- 10% increase over the stationary reading with the condenser fan blowing air through the condenser

- 60% increase over the stationary reading with the condenser fan sucking air through the condenser

- 3 to 4 times the airflow with the  when the car was moving around 80 mph and the condenser fan was blowing air through the condenser

 

For the engineers out there, I fully understand that the number of significant figures I am quoting are way beyond what my test rig justifies.  Please view this as raw data.

 

When it heats up around here I plan to stick a thermometer in the a/c vent outlets and see what impact reversing the condenser fan has on vent temperatures.  

 

Back to the rear window.  I should be getting a call in the next day or so telling me my new “window” is ready.  I will then be doing runs to learn more about its impact on air circulation and temperatures and pressure in the engine compartment.  

 

I did look at the original engine cover and noted that the open area in the screened section is roughly 50% of the open area of the window in the deck lid.

 

 

Stephen Nelson

 

-------------- next part --------------
   I am amazed at how much knowledge there is out there regarding air flow
   and the issues surrounding a/c performance.  Thank you to all who have
   sent in their part of this collective wisdom.  I will do my best to
   pull together all the comments in a summary before I get done with this
   issue.


   On 3/19 my wife and I were able to do another series of runs up and
   down the highway.  The wind had dropped to around 9 mph.  We made runs
   both with the a/c running and shut off, as well as runs with the a/c
   condenser fan sucking in and blowing out.  My conclusions follow:


   At speeds of 50 to 90 mph, the air velocity through the deck-lid
   opening is about 20% of road speed (assuming minimal wind and that my
   anemometer location is representative of the entire opening).


   The pressure just inside of the deck-lid opening is slightly positive
   in the 50 to 90 mph range.  While my gauge location is flawed - right
   beside the dog-bone air cleaner inlet (my bad) - at speeds in the 50 to
   70 mph range there is around 0.1 psi positive pressure.


   The positive pressure under the rear of the car when it is moving at
   speed, coupled with the likely vacuum behind the car reduces the
   efficiency of the a/c condenser fan.  My tests suggest that around 60
   mph the air-flow through the condenser is reduced by about 40%.  At
   around 80 mph it is reduced by over 50%.  While I don't have a massive
   amount of data, what I have suggests that the decrease in flow through
   the condenser is not directly proportional to car speed, and that, in
   the 90 to 100 mph range the flow through the a/c condenser drops to
   around nothing.


   Reversing the a/c condenser fan direction, so that it blows air through
   the condenser and out the back of the car, resulted in a 45% increase
   in voltage generated by the small computer fan I have mounted to the
   a/c condenser fan.  This increase is likely because there is no
   restriction to air-flow into the condenser fan when it is blowing
   toward the condenser.  When blowing in the other direction the
   condenser restricts air flow, resulting in less air being blown past
   the small computer fan.


   Volts read on small computer fan monitoring airflow

   Car Speed relative to air

                                      0

                                     50

                                     60

                                     70

                                     80

                                     90

   Condenser Fan Direction

   Suck air from the condenser

                                    1.00

                                    0.60

                                    0.55

                                    0.45

   Blow air through the condenser

                                    1.45

                                    1.58

                                    1.59

                                    1.60

                                    1.60

                                    1.60


   The voltage generated by the small computer fan when the car was moving
   faster than 50 mph and the condenser fan was blowing air through the
   condenser reflected a:


   - 10% increase over the stationary reading with the condenser fan
   blowing air through the condenser

   - 60% increase over the stationary reading with the condenser fan
   sucking air through the condenser

   - 3 to 4 times the airflow with the  when the car was moving around 80
   mph and the condenser fan was blowing air through the condenser


   For the engineers out there, I fully understand that the number of
   significant figures I am quoting are way beyond what my test rig
   justifies.  Please view this as raw data.


   When it heats up around here I plan to stick a thermometer in the a/c
   vent outlets and see what impact reversing the condenser fan has on
   vent temperatures.


   Back to the rear window.  I should be getting a call in the next day or
   so telling me my new "window" is ready.  I will then be doing runs to
   learn more about its impact on air circulation and temperatures and
   pressure in the engine compartment.


   I did look at the original engine cover and noted that the open area in
   the screened section is roughly 50% of the open area of the window in
   the deck lid.



   Stephen Nelson


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