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.


http://forums.pelicanparts.com/uploa...1247082141.jpg

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. :)

http://forums.pelicanparts.com/uploa...1248649673.jpg

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?

At that vent temp reading and very high humidity the evap was surely beginning to freeze up. Temp setting was max cold and the compressor was just starting to cut out.

Fan speed was max for most of the test drive. Cut it to about 50% for the temp readings. In terms of previous bests, I have gotten temps down to the high teens at much cooler ambients though. But yes, I did get a reading of 29 on Friday after you told me you got yours down to 31 because I could not let you win - I would not have been able to sleep! That's one degree for each addl fender condenser vs your setup!:D :D

huh... umm... manufacture was missing something?
Mark, can you please test measure the temp. differences with and without this mod, at the same time?
Thank you very much.

Just completed two 5 mile test runs testing bowtie diversion to evap intake.

Run #1 (connected) starting vent temp 82 degrees, bowtie connected to floorboard. Temp setting at max cold. Fan speed on high for first 2 1/2 miles, 50% thereafter. Vent temp on arrival 28 degrees.

Run #2 (disconnected) 30 minutes later. Starting vent temps 76 degrees with bowtie line disconnected. Vent temps on arrival 31 degrees - 3 degrees warmer even with a 6 degree lower ambient cabin temp head start!

Wife is out of town so I can do crazy stuff like this in the wee hours!:D The dogs, however, think I'm perfectly normal.

congratulation, look like you are getting around 3 to 5 degrees (maybe 3 to 10?) for this mod.
Thanks Mark.

first, air temps are always checked with the fan on hi, not 50%. that is also how the pressures are checked. ducting air back into the evap is doing nothing more than what turning the fan speed to 50% or so does, which is reduce the heat load on the evap and reduce the air out of the vents and lower the temp out of the vents. running an AC system at <32 deg is going to freeze it up, then you have no AC. you should also check your low side pressure at these temps, if it goes into a vacuum, it will kill the compressor.


if you really want to see if you have acomplished anything, you need to measure the volume of air out of the vents with and without your mod. then reduce the fan speed without the mod to equal the air volume with the mod, then measure the air temp.
you dont gain anything unless you drop the air temp for the same amount of air flow out the vents with and without the mod.

EDITED - T77911S edited his post so no need for my comment. We'll still have to disagree on the effectiveness though.

Mark added his elbows and hose to test his idea. He hasn't cut up his car at all; he just removed the bowtie vent cover and added his pieces. I'd also say this is in the prototype stage - I expect the bowtie vent could be ducted behind the floorboard for a more tidy installation.

He also lowered his vent temps, at least 3 degrees and probably more, so he's certainly gained something. He's a long way from freezing the evaporator.

Thanks for the thread Mark - keep it up.

yes, i realize it now, i am sorry annd have edited my post.

he has not gained anything until he compares air volumes.

Isn't this what HVAC guys refer to as 'short cycling'?? What we really want to do is pull the warm air from behind the front seats into the system. This would bring the cabin temps down and eventually the vent temps, don't you think?

Have you tried to do a test run like:

Start off in your driveway and have the bowtie hose routed into the return, drive a specific route say for 15 minutes, record the vent temperature.

Stop the car, undo the butterfly hose and let the bowtie blow out to the cabin, drive home the 15 minute route, record the temperature.

I'm curious to know if the temp would be as low as it was on the 1st leg of the trip.

Lord knows if you follow some of my posts on A/C I've tried many different things including a rear condenser fan, a return air fan behind the passenger floorboard, plugged up the bowtie to send more air through the dashboard vents,3 additional small 34 CFM fans sitting on top of the front condenser in addition to the factory fan mounted on top of them and a full R12 charge. Next I'm going to an alternative refrigerant ES12a as another user did and got lower temps than R12.
I'm in Phoenix and my SC's A/C stinks. It is my only car and when its 110+ degrees I sweat like a pig in it, maybe it'll blow 60 degree's. At night when the temps are in the 90's I can get 50-55 degrees pot of the vents.

azrob, I've been following your posts too. I was also thinking of adding a booster fan under the floorboard, into the evaporator pickup, as an easier/cheaper alternative to replacing the evaporator fan control. I don't ever want to deal with pulling the evaporator box again if I can avoid it. How did that single mod work for you?

My system (OEM compressor and rear condenser, new barrier hoses, Procooler, Rennaire serpentine evaporator, Rennaire desert duty front condenser, and R12) is adequate, but it could be better. I currently get 40 degrees at the center vent.

It seems to me that Mark's mod could help reduce the cabin temps a little faster, but ultimately not lower than without.

Wavy, I'll be honest with you, collectively everything I did helped a degree or two for each at best, don't get your hopes up to high on that return air fan mod. I used an 88 CFM fan. My blower is loud because I have the bow tie plugged up and the return air fan. A '79 has a small center vent and its like trying to shove 2 pounds of something into a 1 pound container.

Phoenix is hot if you never have been here walking outside feels like opening the oven door on Thanksgiving to check on the Turkey, a flash of hot air.
I've woke up at 4AM on a few occasions when its just 88-90 degrees out and went for a ride and shoved my Taylor digital thermometer in the vent and hit 50 degrees. Before I did all of the mods my car would never go below 60 so yes each thing helps but as I mentioned if the cabin heats up in the Sun its a lost cause, sweat pours out of me and it never cools off. Driving in town, stop and go is the same story, never gets cool.

My kids and my girlfriend never want to go out in my car, "its to hot in the Porsche", "lets take Elaine's car" - - ->2003 BMW 325i.

The A/C thing has me frustrated. Its 110-115 degrees every day and the coolest cabin temp I get is 90 degrees, that's hot and no fun.

I'm hoping that this mod does work!

i dont know if this is the right formula, but it may help.

Btu/hr=CFM x .075 x (H1-H2) x 60.

H1-H2 is the total heat removed per pound of air, Btu.lb (?)

what is says is, if you reduce the air flow, the BTU's or cooling capacity of the AC system goes down. so to determine if any improvements are made, the CFM out of the vents has to be constant.
if you want to make improvements, find a way to use the cold water condensation out of the evap air box to cool the lines going into the evap. the more you sub cool the hi side (basically what the condensor does), the better it should work.

I think you have other issues. I have a 930, which has 1/2 the rear A/C cond that you have, and at 90 degrees, with 90% humidity, i blow 45 with the fan on one speed below high. If I get in the clouds, out of direct sunlight, I'll get upper 30's. My system is stock, except I also have the rennaire evap, and I use freeze12. I cannot believe at night, in the upper 80's, you only blow 50. That just does not sound right.

As far as the above "mod" helping you out, I would not count on it. As until he performs a test, as stated above, where he measures the temps with the same amount of airflow across the evap, with mod and without mod, then his numbers are inconclusive. Hell, I can get 10 degree temp reduction just by lowering my fan speed, which in effect lowers the airflow across the evap, which is really all he is doing. He may cool faster as stated above, but provide a significant decrease in vent temps to justify the work, I doubt it.

Now, having said the above, I'd love to eat everything I said, and be proven wrong, and have him see 10-15 degree drop in vent temps due to this, so keep going at it!!!!!

First of all thanks for the PM and the edit on the "hack".

Regarding the comment that I have not gained anything until I compare air volumes, if you look back at the beginning of this thread the "concept" was a recirc of airflow from the bowtie to the evap intake for the sole purpose of seeing if it would lower vent temps. I mentioned that I already have massive air flow now due to the Kuehl evap motor upgrade so losing some volume in the diversion was planned.

Regarding testing vent temps at full vs 50% fan speed, for the purpose of my test it is irrevalent so long as I am consistent in my test which I was. On both 5 mile test runs I was at full fan speed for the first 2-1/2 miles and 50% for the last 2-1/2 miles after which I took the ending vent temp reading.

Here is an old thread of mine with a floorboard fan project. I ended up removing it to make room for some electronic equip. While I never did any testing I'm not sure it made all that much of a difference. I did get a "nice work" comment from Jim Sims which made the project worth it in my book!:D

http://forums.pelicanparts.com/porsche-911-technical-forum/336402-evap-assist-fan-install-floorboard.html?highlight=assist+fan

My tests described in post #24 were conducted with the same fan speeds......see my clarification in post #35.

That's a great thread Mark - thanks for posting! I'm wondering if a fan of the correct size could be installed directly onto the evaporator intake port.

Just reporting back. This mod will stay. I haven't done any more testing but I believe it is contributing to lower vent temps and over all improved A/C performance. Its been in the mid 90's the past couple of weeks and I'm getting consistent vent temps below 40 degrees. Did a little clean up today by trimming the PVC elbow for a better fit and painting the PVC black for a stealthier look. I may go back and shoot the blue hose with some paint.

http://forums.pelicanparts.com/uploa...1250362816.jpg


http://forums.pelicanparts.com/uploa...1250362877.jpg


http://forums.pelicanparts.com/uploa...1250363152.jpg

Thanks for this great thread Mark, and thanks for reporting back. Sounds like it's worth doing, and as we all know, every little bit helps with these systems. Also nice to see the installation can be tidied up and nearly hidden - thanks for the new pics.