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.