[DeTomaso] Rear Window
Stephen
steve at snclocks.com
Thu Mar 24 23:52:11 EDT 2016
My wife helped me run tests on 5332’s a/c condenser fan today. We did a couple of things – Firstly we mounted a dust collector hood that I use in my wood shop. The attached picture shows it held in place by blue masking tape. It was a near perfect fit! Then we measured air velocity at the hood’s opening with the condenser fan pulling air into the car (normal direction) and pushing air out of the car (backwards).
The outlet was 3 5/8 inches (0.07 square feet) so a bit of a restriction for a 10 inch fan. The condenser outlet grill is 9 x 16 inches, so right at 1 square foot. Measured CFM were:
Pulling air into car: 86 cfm
Pushing air out of car: 112 cfm
Ratio out to in: 1.3
Ratio from previous work using computer fan to estimate air flows: 1.45
We then removed the dust collector hood and measured air flows for 30 seconds at each of 9 positions on the condenser discharge grill. The results of these measurements are in the Condenser Survey picture.
Pulling air into car: 330 cfm
Pushing air out of car: 480 cfm
Ratio out to in: 1.45
It looks like the restriction at the outlet of the dust collector caused a bit of a shift for the overall ratio, but the 9 individual measurements tie up very well with what the small computer fan showed.
It is also informative to see how the airflow is distributed across the condenser. When pulling air into the car the main flow is in the center. When pushing air out of the car the flow is more at the edges with the center being the least. I could argue that the distribution is better with the air being pushed out of the car. Or it is a wash.
I gave some thought to the concept that a pusher fan is less efficient than a puller - as in 80% of the efficiency of a puller. I suspect Joseph Byrd has it right – the assumption is that a pusher is mounted right up against the radiator while a puller has a shroud. At least this makes sense, given the ratio of a 12 inch circle (the flow area for a pusher) vs a 12 inch square (the area of a shroud for a puller) is 0.785. None the less, our work shows that, if both a pusher and a puller are shrouded, the puller is quite a bit less effective. To my mind, this relates to the fact that if you throttle a fan on the inlet you decrease the density of the air going to the fan, which reduces the flow considerably. Flip side, if you have the same resistance on the discharge of a fan you have full density going into the fan and greater output.
None the less, to my mind all of the data, that taken at speed, using a collector and measuring 9 points on the discharge screen, point to a much more effective system with the fan pushing air out of the car.
When Kelly looked at the grid of data for pushing air out of the car she asked if the center point might be wrong. Given the center hub of the fan is around 4 inches in diameter, and given that the fan cannot blow air at the center, it is not too surprising to find a relatively dead spot in the center. It is also worth noting that the periphery data points are quite a bit higher – which likely reflects the centrifugal action of the fan when it moves air, resulting in more air at the periphery of the condenser.
Flip side, when the fan is blowing toward the condenser (and out of the car), the pressure drop created by the condenser helps to distribute the air from the fan more evenly across the condenser.
The condenser in 5332 is brand new, bought from Pantera Performance.
For those who haven’t investigated the original plugs that connect the fan motor to the car’s wiring harness – it is possible, with a jewelers screwdriver or the like, to slip the spade connectors out of the plug body. Then one can simply reverse the positions of the two wires in one side of the plug and you have the motor spinning in the more effective direction.
Stephen Nelson
From: Stephen [mailto:steve at snclocks.com]
Sent: Tuesday, March 22, 2016 11:58 AM
To: 'detomaso at poca.com' <detomaso at poca.com>
Cc: 'tjaarda at astranet.it' <tjaarda at astranet.it>
Subject: RE: [DeTomaso] Rear Window
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.
See attachment for table of results.
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 --------------
My wife helped me run tests on 5332's a/c condenser fan today. We did
a couple of things - Firstly we mounted a dust collector hood that I
use in my wood shop. The attached picture shows it held in place by
blue masking tape. It was a near perfect fit! Then we measured air
velocity at the hood's opening with the condenser fan pulling air into
the car (normal direction) and pushing air out of the car (backwards).
The outlet was 3 5/8 inches (0.07 square feet) so a bit of a
restriction for a 10 inch fan. The condenser outlet grill is 9 x 16
inches, so right at 1 square foot. Measured CFM were:
Pulling air into car: 86 cfm
Pushing air out of car: 112 cfm
Ratio out to in: 1.3
Ratio from previous work using computer fan to estimate air flows:
1.45
We then removed the dust collector hood and measured air flows for 30
seconds at each of 9 positions on the condenser discharge grill. The
results of these measurements are in the Condenser Survey picture.
Pulling air into car: 330 cfm
Pushing air out of car: 480 cfm
Ratio out to in: 1.45
It looks like the restriction at the outlet of the dust collector
caused a bit of a shift for the overall ratio, but the 9 individual
measurements tie up very well with what the small computer fan showed.
It is also informative to see how the airflow is distributed across the
condenser. When pulling air into the car the main flow is in the
center. When pushing air out of the car the flow is more at the edges
with the center being the least. I could argue that the distribution
is better with the air being pushed out of the car. Or it is a wash.
I gave some thought to the concept that a pusher fan is less efficient
than a puller - as in 80% of the efficiency of a puller. I suspect
Joseph Byrd has it right - the assumption is that a pusher is mounted
right up against the radiator while a puller has a shroud. At least
this makes sense, given the ratio of a 12 inch circle (the flow area
for a pusher) vs a 12 inch square (the area of a shroud for a puller)
is 0.785. None the less, our work shows that, if both a pusher and a
puller are shrouded, the puller is quite a bit less effective. To my
mind, this relates to the fact that if you throttle a fan on the inlet
you decrease the density of the air going to the fan, which reduces the
flow considerably. Flip side, if you have the same resistance on the
discharge of a fan you have full density going into the fan and greater
output.
None the less, to my mind all of the data, that taken at speed, using a
collector and measuring 9 points on the discharge screen, point to a
much more effective system with the fan pushing air out of the car.
When Kelly looked at the grid of data for pushing air out of the car
she asked if the center point might be wrong. Given the center hub of
the fan is around 4 inches in diameter, and given that the fan cannot
blow air at the center, it is not too surprising to find a relatively
dead spot in the center. It is also worth noting that the periphery
data points are quite a bit higher - which likely reflects the
centrifugal action of the fan when it moves air, resulting in more air
at the periphery of the condenser.
Flip side, when the fan is blowing toward the condenser (and out of the
car), the pressure drop created by the condenser helps to distribute
the air from the fan more evenly across the condenser.
The condenser in 5332 is brand new, bought from Pantera Performance.
For those who haven't investigated the original plugs that connect the
fan motor to the car's wiring harness - it is possible, with a jewelers
screwdriver or the like, to slip the spade connectors out of the plug
body. Then one can simply reverse the positions of the two wires in
one side of the plug and you have the motor spinning in the more
effective direction.
Stephen Nelson
From: Stephen [mailto:steve at snclocks.com]
Sent: Tuesday, March 22, 2016 11:58 AM
To: 'detomaso at poca.com' <detomaso at poca.com>
Cc: 'tjaarda at astranet.it' <tjaarda at astranet.it>
Subject: RE: [DeTomaso] Rear Window
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.
See attachment for table of results.
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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