New idea to help lower A/C vent temps.
 

I have an idea for lowering vent temps that I have not seen here before so I though I would throw it out for discussion.

I have an 87 with the large vents so I get very good air flow and have had the bowtie vent blocked off for some time to force all of the flow through the dash vents. Furthermore, I just completed the Kuehl blower motor upgrade so now I have massive air flow.

On to the idea. What would happen if a hose was connected to the bowtie and run to the evap inlet in the passenger floor board. Cooler air on the input side should lead to cooler temps coming out, right?

I know that the purpose of the evaporator is heat exchange by circulating cabin air through the coil and removing heat. So long as the evaporator not sourcing all of its airflow from the bowtie hose (note that the evap sources cabin air from the triangular passenger footwell inlet and the round tunnel inlet) I would think that it would coninue to remove heat while also blowing incrementally colder air from the vents.

Thoughts?

Did someone say that Porsche a/c upgrades are a slippery slope? :D

It seems to me that it would be easy to temporarily test. Take some PVC pipe with sufficient inside diameter to fit over the bowtie "stub" and route the pipe over to the evaporator inlet. Do before and after vent temp measurements. If it works out, then you might consider using some flexible ducting like SCAT.

My thought, though was to wonder how much airflow you would lose by not having the bowtie blocked off. I suppose with the new blower it might not make a difference...

Sounds good, but I believe there is a flaw in your logic. The air entering the evaporator box is already coming from the inside of the car, after it's been through the evaporator. You can add a hose to route the air, but the air will still be coming from the same place.

....but haveing just come through the evaporator it will re-enter the evap inlet before mixing with the cabin air thereby lowering the inlet temps...

I believe I read a post a while back from Jim Sims regarding vent temps and airflow. IIRC the jist of the post was that vent temps are only part of the equation. Low vent temps with puny flow will never keep up with cabin heating and higher vent temps with more airflow will feel cooler recirculate cabin air at a higher rate and be more comfortable. I think I now buy into this argument as the impact of changing out the blower motor and increasing air flow has had a MASSIVE improvement in cooling performance. By comparison, while I have never measured vent tems in the wife's Mountaineer, it does not feel anywhere near the sub 40's I can get in the 911, but the volume of air coming through the vents recirculates the cabin quicker and will freeze you out in a heartbeat.

I think the concept is that cold air right off the evaporator would then be put back into the evaporator, thereby requiring less of a differential between the return air and the desired output temp. The cabin air that would otherwise go back into the evaporator return has already heated up, and would be much warmer than the air coming out of the bowtie port.

Mark, bright idea, but let's stop and give it a little thought before you spend you time to do the mod. I did measure the temperatures at the vent, comparing between my 911 and my lexus, more than once. Usually, it gets kreaking cold on my lexus only at second click; while it's still not that cold on my 911 even at max click. At those 2 positions, on these 2 cars, temp measured to be colder on my 911. Why? The only different I can see/feel is that the air volumn/flow are different. At max speed, I could feel the air flow on my 911 is max about 1.5 feet away from the vent; while I can feel it at the back seat on my lexus. So, it might not be the temperature that we have to deal with here. It should be the air flow and air volumn that we should increase, if posible.

I think the concept is that cold air right off the evaporator would then be put back into the evaporator, thereby requiring less of a differential between the return air and the desired output temp. The cabin air that would otherwise go back into the evaporator return has already heated up, and would be much warmer than the air coming out of the bowtie port.

I think that the longer you ran the smaller the difference would be. In our older cars we are basically allways recerculating the air no fresh air is coming in unless you open the vent.

I've tried a number of things to get colder AC temps. I removed the air pump, cut the nozzle off of the air box and relocated the coil so I could install an 880 CFM fan on the left/drivers side of the rear condenser.
I added an 80 CFM muffin fan behind the passenger floorboard to help return air to the evaporator.
I made duct work out of some 1.5" rubber hose that T's into the bow tie inlet than comes out about 12" on either side and makes an elbow to deliver air to both the driver and passenger "crotch area" from under the dash. If you bend down an look you can see it.
I replaced the motor on the front condenser and my system is fully charged with R12.
I'm sure if I lived anywhere but Arizona this would work well but here in 105 to 110 degree heat all I can get out of a car from the garage is 55 degree temps and if it was in the sun, yikes its 130 to 140 degrees in the cabin and 60 degrees is all its going to blow for about 45 minutes until the cabin cools off.
These temps are at highway speed, cruising at 60 MPH at 3000 RPM's.
After the R12 leaks out I am going to charge it with Benzomatic/Propane - Crazy? Another user Ed Bighi has me convinced, he wrote this to me.

Hey Rob. I hope it's rob I'm no expert, but I certainly tried a few things in this 88 carrera that I had. On my current car, an 80 turbo, I took out all the ac stuff and interior. But anyway, in my carrera, I always felt that fanwise, there was plenty. Maybe could use a little more, but it was suffecient IF i could get the outlet temps cold enough. At the time, it ran R12 but the system was empty. I took it to a buddy to pull vacuum from the system to evacuate, then put in some Envirosafe, which you can get in Phoenix from I guy I know. It's an R12 replacement which is also compatible with R134. Now, there are a number of opinions on this, all mostly based on bull****, stupidity, and outright moronness, but, after much research on my part, I said screw it and put in a couple of cans. It worked. Barely decent, but worked. Now, keep in mind, Envirosafe is a mixture of propane for cooling, isobutane to bring compressor head pressures back to normal R12 levels (R134 head pressures are way higher, so if your car is converted, you are good to go), and a pine odor for instant leak detection by nose. All good, but, here is what I found out. If the cooling is only done by the propane alone, why not go full propane? Well, I converted a Benzomatic nozzle into a R12 hookup and dumped an entire bottle of benzomatic in my car. Boom, under 50 degrees at the vents on the freeway. So long as the car was moving, I was good. No tints even. Now, the molecules for this gas are tiny, so, it will leak out faster. But, if you are converted to R134......no prob. So, if it leaks out slowly, you gotta make the sacrifice to spend another 6 bucks in a fresh bottle of benzomatic from Home Depot. What the propane did was mask in the innefficiencies of the system by being a superior refrigerant. Now, fire risk issue, if you are wondering. A couple of universities in Australia tested this extensively and the conclusion was that there was not enough in the system and even in a bad crash, those tiny ammounts would exit so fast that even IF there was a flame, it would last a fraction of a second. I used it all the time, my sister for 8 years in her carrera with no problems. Now, this isn't a miracle cure and you definitily have to learn a few things as far as charging. When I pop back in Phoenix, I'll help a friend charge his, so, maybe I'll give you a ring if you are interested. Also, another thing to keep in mind. The issue with these SCs and Carreras is not the fan speeds. It's the lack of condensor size. Put a steel underbelly condenser in there and your problems are over. Some say, oh, it's right next to the pavement. Nah. Been there done that. I had a friend in Miami with a 3.3 turbo engined 911SC that, even on a blistering hot day, we would sit inside the car while idling and the thing was ice cold inside. I mean, a 2 meter wide condenser is a 2 meter wide condenser. End of story. Have a look at this website. These guys sell underbelly condensers www.scottsind.com

The use of flamable alternative refrigerants is generally not recommended for good reason. I personally would never risk it.


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Mark - how much of a difference did you notice by plugging up the bow-tie hole? I added another condenser in the left rear wheel well and am pleased with the temps, but not the airflow. I removed the bow-tie and saw a huge difference in the amount of air coming from underneath the dash. But, if plugging the bowtie hole to force the air out of the dash vents really works, I would much prefer that solution than to have the majority of the air coming from underneath the dash.

Thanks, Farrell

I do not recall that there was that much of a difference. It's easy to plug - just stuff some paper towel in there from both sides and give it a try.

i understand your idea to be that you want to divert air before leaving the vent back into the evap, correct? you would be better off turning the fan down. if you divert air, that is less air coming out of the vent, it will be colder most likely but again, the same as turning the fan speed down. if you want colder air, increase your condensor or the air flow acorss it.

Done and done. Condenser capacity has been increased (dual rear fender condensers) and high capacity evap motor has been installed to increase air flow.

This is just an idea to to "tweak" vent temps/system performance. I hope to do some testing on Sunday.

Very interesting concept.

What I do is turn on the fresh air vent to the "FLOOR" setting and it seems to help circulate more air to the intake inside the cab.

Please keep us posted.

the high capacity fan will lower your vent temps. the evap is designed to remove heat from a certain amount of air, if you increase the amount of air going across it, the evap can not keep up. that is why when you go to a lower fan speed, the vent air is colder.

The bowtie stub is 1-3/8" OD which is the approx ID of bathroom sink PVC drain pipe. To plumb the bowtie opening to the floorboard I picked up two flanged 90 degree drain elbows and a 2" length of pool hose from Home Depot.

http://forums.pelicanparts.com/uploa...1248649383.jpg

I made four relief cuts in the PVC so that it would fit over the bowtie stub. In my application, my center channel speaker rested on the bowtie. Now it rests against the PVC elbow keeping it from slipping off. I plan on going back an modding a bracket to hold it in place using the bowtie cover mounting screws.

http://forums.pelicanparts.com/uploa...1248649565.jpg

Not pretty but this does not show when seated. Cannot even see the speaker
http://forums.pelicanparts.com/uploa...1248649727.jpg

I got lucky here. The vent holes previously drilled in the floorboard were the correct size for the PVC elbow. :)

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http://forums.pelicanparts.com/uploa...1248649918.jpg

92 degrees ambient today. Just went for a 15 minute test drive. Vent temp when I pulled into the garage - 25.7 degrees. My glasses fogged up completely when I got out of the car :eek: I'll play with this over the next couple of weeks to measure the effect connected vs disconnected. Initial results look pretty good! Total cost $16.98

http://forums.pelicanparts.com/uploa...1248650031.jpg

Whoa WTF?

That is crazy cool :eek:

Is there a way you can check to see if the condensor is freezing up on you?

Wow congrats! I freaked when I got 18 degrees once.

You are the Man!

So the previous best was 29 deg?

What was the fan speed?