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    Results 21 to 28 of 28
    1. #21
      Join Date
      Sep 2007
      Posts
      424
      Regarding designing a mix plenum as suggested in posts 9-11- I think most would be surprised how much effort is put into this at an OEM level. Getting Temperature Control Curve slope positive so they don't invert (temp lever goes higher, actual outlet temperature drops), outlet airflow and temperature stratification (near equal, or specified, airflow from each outlet, and equal or specified temperature difference from outlet to outlet) is where months of development work take place. Start moving components around after the heater core, and these kind of go away. It was a problem that gave me grey hair in my 20's.
      Yeah, I had thought about inconsistent temp distribution issues that could crop up. As in, once the air is past the heat/evap cores & blend door and into the manifolding for the vents.

      But I hadn't heard of inverted temp control effects. Care to elaborate on that? I had thought it was a pretty straightforward matter of using a blend door.

      In the big picture, I was thinking about the airflow sort of like airflow in an engine. Try to keep the port size consistent throughout the trip. Tight-radius bends & sharp edges are bad. Avoid tall/narrow airflow areas (at least after the cores & blend door) to help keep the temp consistent throughout the air mass.

      I'm aware that there would be other issues. Colder air is denser, and denser air carries more inertia when it moves. But I'm less familiar with tackling that.

    2. #22
      Join Date
      Dec 2006
      Location
      Out of the Burbs of Detroit to SoCal, then onto my ancestral homeland, the woods of Cascadia
      Posts
      1,753
      Country Flag: United States
      Quote Originally Posted by mikedc View Post

      ......

      But I hadn't heard of inverted temp control effects. Care to elaborate on that? I had thought it was a pretty straightforward matter of using a blend door.

      ...
      You'll never see it in a production car because it's tuned out. It's a mistake. The temperature curve should look like a lazy S, the slope of the curve varying, but always upward to the right on an outlet temp vs control position graph.

      Air mixing in the plenum does funny things. Being an HVAC engineer became more about aerodynamics in the last years I was working, trying to get air around corners we were playing with Coanda (Romanian aerodynamacist) effect, and as cabins became progressively quieter, moving air silently became a moving target.

      Most people aren't as critical as the development specification for a vehicle. On test drives I'd frequently as the platform A/C guy about features I detected, and we'd discuss if I'd ask for warranty service if it was my car. We were tuned to be very critical of performance

      Quote Originally Posted by mikedc View Post

      ......

      In the big picture, I was thinking about the airflow sort of like airflow in an engine. Try to keep the port size consistent throughout the trip. Tight-radius bends & sharp edges are bad. Avoid tall/narrow airflow areas (at least after the cores & blend door) to help keep the temp consistent throughout the air mass.

      ...
      HVAC always sucked hind tit in packaging space. Ducts should be round, until you had a radio in your space, you'd put in a kerdunk (colloquial term for an irregularity), then the instruments woulod squeeze you, and the stylists had control over the outlets, a MOST IMPORTNT FEATURE. And they usually f****d them up. (Note here- mid 80's GM outlets ike the ones on my G body el Camino are the one's I like best- low restriction and good directional control)

      I was constantly amazed how little priority was put on HVAC, last time I looked (a good number of years back) about 30% of warranty dollars were spent on HVAC. And so important to customer satisfaction

      Quote Originally Posted by mikedc View Post

      ......

      I'm aware that there would be other issues. Colder air is denser, and denser air carries more inertia when it moves. But I'm less familiar with tackling that.
      Density difference between cold air and hot air are no so great, so not so relevent to planning airflow
      Greg Fast
      (yes, the last name is spelled correctly)

      1970 Camaro RS Clone
      1984 el Camino
      1973 MGB vintage E/Prod race car
      (Soon to be an SCCA H/Prod limited prep)

    3. #23
      Join Date
      Sep 2007
      Posts
      424
      You'll never see it in a production car because it's tuned out. It's a mistake.
      Yeah, I get that. I'm just struggling to figure out how it could happen at all. A blend door is a simple device.


      Being an HVAC engineer became more about aerodynamics in the last years I was working, trying to get air around corners we were playing with Coanda (Romanian aerodynamacist) effect, and as cabins became progressively quieter, moving air silently became a moving target.

      Most people aren't as critical as the development specification for a vehicle. On test drives I'd frequently as the platform A/C guy about features I detected, and we'd discuss if I'd ask for warranty service if it was my car. We were tuned to be very critical of performance
      I've noticed modern HVACs running quieter even when the fan is blowing hard.

      Behind a modern dashboard you find big molded-plastic ductwork. I assume the quietness is mainly accomplished with larger duct sizes + smoother internal flow.

      I suppose if the passages are too large then the airflow won't exit the vents fast enough to reach back into the cabin. But with modern dashboard packaging demands, that issue may not come up very much.

      HVAC always sucked hind tit in packaging space. Ducts should be round, until you had a radio in your space, you'd put in a kerdunk (colloquial term for an irregularity), then the instruments woulod squeeze you, and the stylists had control over the outlets, a MOST IMPORTNT FEATURE. And they usually f****d them up. (Note here- mid 80's GM outlets ike the ones on my G body el Camino are the one's I like best- low restriction and good directional control)

      I was constantly amazed how little priority was put on HVAC, last time I looked (a good number of years back) about 30% of warranty dollars were spent on HVAC. And so important to customer satisfaction
      What amazes me, is that there's no decent dashboard access to service the HVAC systems. Imagine building a car without an opening hood on the front end.


      Density difference between cold air and hot air are no so great, so not so relevent to planning airflow
      That makes sense.

    4. #24
      Join Date
      Dec 2006
      Location
      Out of the Burbs of Detroit to SoCal, then onto my ancestral homeland, the woods of Cascadia
      Posts
      1,753
      Country Flag: United States
      You'll never see it in a production car because it's tuned out. It's a mistake.

      Quote Originally Posted by mikedc View Post
      Yeah, I get that. I'm just struggling to figure out how it could happen at all. A blend door is a simple device.
      Until you consider that you are sending a fraction of the air thru the heater core and a fraction bypasses the core, then they re-blend in the distribution plenum. What are the chances that there will be some eddy currents, or regions of high velocity where the entry air 'blows over' the directed air.

      The condition is heavily pronounced on side window demisters. You have a channel directing air near the front base of the side window to clear glass for rear view mirror visibility, and defrost air from the windshield will bend that air current mitigating it's effectiveness replacing it with air that has cooled defrosting front glass. You could close off the defrost duct- nah, frosty windshield is a bad tradeoff for clear sideglass.
      Greg Fast
      (yes, the last name is spelled correctly)

      1970 Camaro RS Clone
      1984 el Camino
      1973 MGB vintage E/Prod race car
      (Soon to be an SCCA H/Prod limited prep)

    5. #25
      Join Date
      Nov 2010
      Location
      Ventura County CA
      Posts
      556
      Country Flag: United States
      Hi Greg, just want to say thank you for all the information and sharing your experience/background engineering AC. I became fascinated with the original AC system on my 70 Nova from an engineering perspective and got it up and running to my satisfaction, but imagine there is still a lot I could do to improve performance on the air handling side of things. You've given me a few things to think about, so thank you for that!

      Are you still working in this space or have you moved on/retired?
      Clint - '70 Nova "restomod" cruiser & autocross family car

    6. #26
      Join Date
      Dec 2006
      Location
      Out of the Burbs of Detroit to SoCal, then onto my ancestral homeland, the woods of Cascadia
      Posts
      1,753
      Country Flag: United States
      AIRFLOW AIRFLOW AIRFLOW. CONTROLABILITY CONTROLABILITY CONTROLABILITY

      Don't know what the current airflow of your Chevy Twice is, but if it was my car I'd look at a blower motor and wheel from a late 70's early 80's A, B, or G body w/ HVAC and try to adapt it to your scroll housing. Add a relay so you feed the blower full voltage on high. Cars start getting comfortable about 225 cfm total airflow at zero body pressure. I like a car with about 250 cfm @ ZBP.

      In the early 80's were were testing an economy Japanese car, and it's HVAC performance sucked. As a less to my young self, a mentor had us get a Cavalier blower mottor and scroll and cobble it in. Marked improvement. This car also had 'rubber vents'- didn't matter how you positioned them, within a couple minutes they were in THEIR happy place not yours. We taped some GM vents in place and the deal became very comfortable. We had some Japanese guys in the back- on the initial run without change they were sweating like pigs- but they were, of course, Very Comfortable in subjective ratings (Alot of ride testing uses subjective comfort ratings. Good engineers turn subjective ratings into objective performance specifications.) American rear seat passengers were screaming about comfort levels. After the change in blower and outlets, everybody was happy

      Another thing is- make sure your ducting is sealed. Sucks when you loose 20-30 cfm in duct leaks


      Quote Originally Posted by TheBandit View Post
      Hi Greg, just want to say thank you for all the information and sharing your experience/background engineering AC. I became fascinated with the original AC system on my 70 Nova from an engineering perspective and got it up and running to my satisfaction, but imagine there is still a lot I could do to improve performance on the air handling side of things. You've given me a few things to think about, so thank you for that!

      Are you still working in this space or have you moved on/retired?
      Greg Fast
      (yes, the last name is spelled correctly)

      1970 Camaro RS Clone
      1984 el Camino
      1973 MGB vintage E/Prod race car
      (Soon to be an SCCA H/Prod limited prep)

    7. #27
      Join Date
      Nov 2010
      Location
      Ventura County CA
      Posts
      556
      Country Flag: United States
      Thank you for the kind response Greg. I'm actually facing a failing blower motor right now. It is drawing too much current and blowing the fuse on high, and it seems the airflow is a little weak. The system has a relay to provide full voltage on high speed. I have a handheld anemometer but I'm not sure how to get to a total system airflow. Would I estimate that based on approximate vent area and air speed at each vent?

      I do need to go through the system and replace all the duct seals. The interior stuff needs some refurb.

      I'll see how much work it would be to adapt a later model blower. Just depends on the dimensions of course.
      Clint - '70 Nova "restomod" cruiser & autocross family car

    8. #28
      Join Date
      Dec 2006
      Location
      Out of the Burbs of Detroit to SoCal, then onto my ancestral homeland, the woods of Cascadia
      Posts
      1,753
      Country Flag: United States
      Quote Originally Posted by TheBandit View Post
      .... I have a handheld anemometer but I'm not sure how to get to a total system airflow. Would I estimate that based on approximate vent area and air speed at each vent?
      It's pretty much an OEM test. The GM variant seals the entire vehicle cabin (including exhausters). A large dual plenum with a large industrial blower on one plenum sits beside the car. The is a venturi plate between the plenums. The other plenum is ducted to the car interior. A crude method is open one window, seal with a plate (we frequently cut a cardboard plate) and a gauge to measure pressure between the cabin and the outside. This is going to sound hokey, but we would use a piece of saran wrap loosely taped to a hole in the plate, It's incredibly sensetive to small (really small) pressure changes.



      The car was powered to 13.8V, and the HVAC blower on outside air and full blower speed

      The pressure is adjusted on the blower plenum until the saran wrap was fluttering between positive and negative cabin pressure (I told you it would sound hokey.) There is a pressure differential between the car interior plenum and the blower plenum to keep the cabin pressure at zero with respect to the environment when the HVAC blower is running flat out. The venturi plate between the two plenums has a calibrated curve for that venturi, so you read the pressure differential vs airflow to get the airflow at zero body pressure.

      That's how we did it in the 80's- now days the machine directly reads the pressure differential across the venturi and goes to a look up table programmed in and sends a number to a digital readout. Alot simpler, but alot less fun
      Greg Fast
      (yes, the last name is spelled correctly)

      1970 Camaro RS Clone
      1984 el Camino
      1973 MGB vintage E/Prod race car
      (Soon to be an SCCA H/Prod limited prep)


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