911 A/C idling test

[Jim Sims]

Today at noon hour, I ran a simple test to check my 911 A/C system while idling with the car stationary.

Vehicle: 1976 coupe, 911S 2.7 l with SSI's and upgraded engine fan; factory external oil cooler lines and trombone oil cooler (20W50 dino oil). Minerva blue exterior with black interior (leather, vinyl and carpeting) with black Porsche full front bra. Single occupant (in driver's seat) in car. Windows (non-tinted) closed and ventilation system off.

A/C system "Koool" dealer add-on A/C system (1976 vintage) evaporator and blower (with added muffin fan booster behind passenger side floor board pushing return air into evaporator); underdash air distribution system (replaces knee pads), decklid condenser (single layer of tubing), Sanden compressor running ester oil (Griffith's compressor upgrade kit), new barrier hose with lines run up driver's side of car, Y/P products HTX-100-BK suction line heat exchanger (subcooler) just before receiver/drier (aftermarket r/d unit). Red Dot trinary pressure switch at output of compressor; intermediate pressure contacts not yet connected. Condenser and evaporator cleaned and flushed, system leaked checked (ultrasonic leak detector) and evacuated for 3 hours with 2-stage 25 micron level A/C servicing vacuum pump (new oil in pump) then charged with about 2 cans of R-134a.

External conditions: Full sun-clear sky, car facing west on asphalt
pavement (light colored aggregate), external air temp 91F, RH 30%, 6950 ft altitude (note that first Apple Mac computers operated at Los Alamos burned out due to the thin air prompting Apple to improve the natural convection cooling system), no wind. No other vehicles within 50 feet.

Initial car interior temperature upon entering was 105F; removed windshield sun shield and drove car at 35 to 60 mph on a 5-mile round trip to warmup engine and cool down evaporator. A/C vent air temperature dropped to 50F with evaporator fan on highest speed (there are 3-speeds plus off). 30 mph during last 1/2 mile to parking lot for test.

Start of test: engine oil at 197F, air into evaporator (passenger floor board area) @ 80F, A/C air vent outlet temperature 62F; A/C system delta T: 18F


Idled engine for 5 minutes at 500 rpm:
engine 205F, evap inlet 91F, A/C vent outlet 76F; A/C system delta T: 15F

Idled engine for 5 minutes at 1000 rpm:
engine 217F, evap inlet 90F, A/C vent outlet 78F; A/C system delta T: 12 F

Idled engine for 5 minutes at 1500 rpm:
engine 223F, evap inlet 89F, A/C vent outlet 70F; A/C system delta T: 19 F

Idled engine for 5 minutes at 2000 rpm:
engine 223F, evap inlet 86F, A/C vent outlet 64F; A/C system delta T: 22 F

Idled engine for 5 minutes at 2500 rpm:
engine 223F, evap inlet 86F, A/C vent outlet 60F; A/C system delta T: 26 F

I was intending to mount a medium sized axial fan under the condenser to pull air through it when the car was idling (as in traffic); the fan would be switched by the intermediate pressure contacts in the Red Dot trinary pressure switch. Given the available room and the proximity of the CIS system air cleaner to the condenser I was wondering how to do this effectively. Reading Charlie Griffith's comments that their tests indicated that this approach was only marginally effective and not at all much above idle caused me to consider using the Red Dot switch to kick the throttle up to 2000 rpm whenever the A/C condensing pressure rises. I may obtain one of the thottle body microswitch mounting brackets and mount a 12V "fast idle solenoid" on it.

Cheers, Jim

[bigchillcar]

jim,
i hope to study your data a little later this evening when i get more time..thanks for taking the time to post it..we need someone like you working the 'cutting edge' of improving 911 a/c. just as a quick thought, have you seen the aftermarket tops available for the cis airbox? they look like they would permit the necessary clearance for the fan you suggest at the rear deck..weltmeister has a simple kit and then another company has made (albeit expensive) something else in a kind of a silver plastic space-age looking thing..
ryan

[kuehl]

What I find interesting is the delta (air inlet vs. outlet) you had 1k rpms. I'm wondering what would throw off the curve here helter skelter? Maybe you caught the temp measurement while the expansion valve was hunting for a second or two?

They say in the world of ac the average person feels a comfort perse when there is a delta of at least 25 F degrees.
So, if you have an air inlet, I assume measured at the floor of the car of 90, you would need an outlet of 65 F to get to the hypothetical 25 delta. Then again, who ever dreamt that one up did not mention how many cfm's you need blowing at you !

I guess, in a perfect world, if you had the engine compartment "tight", meaning the rubber weather strip between the engine compartment and motor was good and no other openings (hose paths by the shock towers) were major, you could probably take an air measuring device and find the mean or average across the deck lid inlet. Figure out he square inches and do the math to guesstimate the cfm being drawn in by the motor fan. Plot it our for your 500 to say 2k rpm range and you might come up with the nominal amount of additional cfm you will need across the deck lid condenser at idle (this being path to choosing some type of fan), then you get to play with the cfm rating from the mfg'r' vs. what you really get when you take a round fan of xx diameter and mount it against a rectangle.

Have you ever considered "masking" the deck lid air opening to insure all the air has to move through the deck lid condenser coil (the stock arrangement is a bugger to have fun with).

A note on testing; when we have performed some tests while driving on a sunny day in the late afternoon, we have observed the vent temps rise when driving toward the sun vs. driving away from the sun. Unfortunately if I do that for too long of a period of time, here on the East coast you get get pretty wet.



Keep us posted,

Griff

[kuehl]

Forgot about the rpm solenoid thing:
What will happen if you have a device that increase the rpm's to 2500 when you are sitting at traffic light? Maybe a tap on peddle can release something. Kinda reminds me of the mechanical automatic choke on cars of yesterday; the spring sitting on the engine was contracted when cold, so that when you "set" the peddle the throttle linkage moved to a fix cam position and would high idle until the engine got warm. A tap on the gas peddle release the cam position. Something to ponder.

[cmonref]

"Idle up" could be achieved with a hand throttle. That's what I used for my old '68 912 years ago.

The factory Tech Spec manual for '78-81 SC shows 1500 liters/sec airflow thru the engine fan at 6000 rpm. Thats 3177 cfm. If it is linear, maybe 500 CFM per 1000 rpm. Then, as Keuhl says, what "losses" around the sheet metal and shock towers?

NAPA sells a solenoid that might do well for you. Echlin 2-2307, about $28. Ports for 4-5 mm hose.

[bigchillcar]

i'm not crazy about the idea of bumping up the idle to 2k at idle either..and the idea of masking off the area to make sure all air passes through the condenser makes me think we're restricting airflow to the engine..not good in my car without an aux oil cooler. i could picture a thin plastic shroud on the back side of the rear deck condenser with a puller fan centered in it to ensure that all air through the condenser passes through the fan, but this would thwart engine cooling. seems what we need is twice the area we have on the rear deck - half for the engine and half dedicated to the condenser..also not practical..both engine and compressor are fighting for this airflow.

perhaps for those of us who have at least one aux oil cooler that a warm engine isn't such a problem, but for most it definitely is. wonder..how would it do to move a rear deck condenser to the trunk area? any advantage/disadvantage to having it near the evaporator, but far from the compressor? i know it would require modding the trunk and taking away space, but could one be mounted with an electric fan attached somehow..i know the air would have to be vented..just thinking out loud (read out my ass).
ryan

[kuehl]

Masking off "around" the condenser will not inhibit air flow signficantly. I don't have the figures on engine's cooling fan but is pulls quite a lot of air. On a side note consider the 930's deck lid, half is intercooler and half is condenser and engine. A simple test would be to check your ac vent temps or hook up a set of gauges and take some quick temp and pressure measurements. Then take some duct tape and make a quick seal on the underside (engine side) and on the outside as you will have a major gap when you carefully lower the deck lid on your gauge hoses. Note the before and after.

Eliminating the deck lid condenser and moving it to the front trunk would be like the old 914 set up (which did not do much) however you have gas tank issues regarding placement. With respect to ac lines it would be alike a 964, you would only eliminate the capacity value deck lid condenser to compressor hose , however that is not the issue. You could consider a lower rider nose job as you would have with an aux. oil cooler, but you would still need major fans at idle or slow speeds to get the needed btu exchange. Your alternative in the front is the 964 approach if you have the time to do a nice hack job.

Consider that after the first design with the single deck lid condenser got into the field, the consumer probably said "he, thanks for the ac, but it does not blow as cold as my Buick". So Sales finally twisted Engineering's arm and got them to make a small improvement with the front condenser. I suppose the intake through the fender with the blower pushing down on the front condenser was a bandaid for (those who knew they needed) atypical large surface as you would find in most front engine cars with grills.

Moving the condenser closer to the evaporator, "could" an effect with respect to the temperature of the refrigerant.

So surface area with good air movement is the key. I would imagine we could dig around and find the formula but we know that with this vehicle and body design you are limited. What's been proven to work without major hacking is the is the rear fender condenser system or the under the body armadillo killer; you choice a is matter of philosphy.

Another note of moving things around, would be a Sanden SHS electric Scroll compressor; eliminating the belt driven compressor in the rear and placing the dc electric unit in the trunk. Then again you would need a nice alternator for the amp draw, cables front to rear, etc. but I'm getting off the track here, time to find some coffee.

[bigchillcar]

kuehl,
all good points..i agree that the basic rear-engined, air-cooled design lends little to the possibilities for a/c..many have come and 'died' trying to work a better solution so it's true that it's unlikely to come easily..but we keep trying don't we?!

your mention of a dc trunk unit i think was also mentioned by jim sims in another thread. what about mounting a small, dedicated generator/alternator where the current compressor is mounted as a stand alone source of power for this trunk-mounted motor? i don't know if one exists, but just thinking out loud that it could be isolated from the rest of the electrical system to power this unit (unless there's an advantage to having it available to the rest of the car). you're right - sounds like major cutting/mods would be required to put the dc motor PLUS an airflow-ducted condenser/fan arrangement..might as well say you don't have a trunk anymore. i need to go outside and stare into my trunk for a little while and try to find some inspiration..
ryan

[Nitrometano]

Jim, your car have tail?

[kuehl]

bigchilli,

We have investigated various means and ways of condenser, evaporator, compressor and vent arrangements. And naturally much of our same efforts are duplicated, however we always want to be sure. What you see in our line up of ac improvements for the 911/930 came about when we went "through it all" so to speak and finalized what is "reasonable" given all the demands from the consumer. There is no utopia.


We considered the electric compressor as well electric hydraulic motors for power steering a few times. It all boils down to: cost, marketability and "energy" (you can't be a direct source as close to the engine). Naturally there is no cost benefit in moving a working compressor design up front, as you know you need a larger elec. cable to carry the juice and a larger power generator. You could remove the alt/fan assy., drop an alt. in the place of the compressor, but what do you gain? Maybe if you want to use the fan as a supercharger it would make sense. The design behind the electric compressor I would imagine was pushed by the move to alternative fuels or means of powering the car: just imagine how much complaining the market is going to generate when the first frontier OEM builds an all electric car and Mrs. Jones gets stuck in the middle of LA traffic cause the auxiliary applicance options are sucking all the amps.

On the far side is the battle brewing over CO2 refrigerant systems being dreamt up across the East pond vs. the new replacement for R134a here on the West side.

Ahhhh, let the games begin!

[R. Aster]

Originally posted by kuehl
What I find interesting is the delta (air inlet vs. outlet) you had 1k rpms. I'm wondering what would throw off the curve here helter skelter? Maybe you caught the temp measurement while the expansion valve was hunting for a second or two?
...
Griff

On my 89, vent temp drops 2 or 3 degrees when I go to idle after typical road speeds, at least for a minute or so. I had assumed that it was because the evaporator fan slows a few RPM due to voltage sag from the alternator at idle.

[As yet another data point for guys with cars from my era, I am getting vent temps as low as 39F (fan position III) on mid-eighties days with a stock system, R12 topped off until to top of sight glass and then maybe 2 oz more. I just cleaned both condensors (Simple Green) and got another 3 degrees (from 42 to 39).]

[bigchillcar]

kuehl,
not sure if i was clear in my question. i was wondering if a basic alternator (or generator) could be mounted where the existing compressor is now as a dedicated powers source for the electric compressor in the front of the car..i'm guessing that it would take a lot of energy to run it, more than the car's 'regular alternator' could handle..or is this idea not really practical? you'd still have the compressor belt to drive this hypothetical second alternator..
ryan

[kuehl]

bigchill,
I guess you can do just about anything if you have the time, money, tools and sweat.

It takes xx amount of energy to turn a compressor, and it takes xx amount of energy to turn an alternator. Which is more energy efficient? Who knows, usually the closer you are to the power source the more efficient you are (not to say which is more efficient). From a practical standpoint I don't see a need, or better put, "a market" to engineer it, package it and sell it.

We have had a few requesting us to build an elec. power assist system using a dc /hydraulic source. Market? Maybe. Problems? I ran out of fingers thinking about the MTBF.


When I look at the electric compressor I say "gee, that's a great idea for when I get my electric car". To strip out what is in the car today, invest in an elec. compressor, 2nd alternator or a big dude to handle everything, simply is not practical..... at least for me. That is not to say it won't float someone elses boat.

[Jim Sims]

No, Nitrometano my car doesn't have a tail just a mid-year (74-77) basic engine decklid and grille.

I've been doing some research and consideration on this topic of improving A/C idling performance based on people's comments. "Masking" the decklid condenser to ensure that more of the air inducted in by the engine fan passes through the condenser fins is a good idea. Inspection of the perimeter of my decklid condenser leads me to the idea of stretching a wide rubber strip or band around the condenser edge. My condenser essentially covers the whole inside of the rear deck grill area. Therefore, if the band is the correct width, one edge will rest and seal against the decklid greatly reducing air leakage around the sides of the condenser. Such a sealing appliance, if made well and from black rubber would not look like a "Kludge" and could easily be removed.

On the topic of a solenoid to raise the idle engine speed to make more air flow when the intermediate pressure contacts close on my system's trinary switch: I noted that the engine speed when I reach the friction point when engaging my clutch is about 2500 rpm. My engine doesn't operate well below 2800 rpm when propelling the car so I will be above 2500 rpm when moving. In addition, experience driving my son's 73.5T during warmup with the hand throttle pulled up indicates this will not be much of a problem. The issue of the A/C suddenly raising the idle when not expected may be addressed through the use of an "anti-dieseling" idle stop soleniod. If I correctly understand the operation of these idle stop solenoids; they will not advance the throttle up on their own but will follow a driver throttle advance and then "latch" at the set, raised idle speed until their electric power is cut. Then they fall back until energized again and will again follow a driver produced throttle increase.

I'm trying to make incremental A/C improvements without doing a lot of additional sheet metal cutting on my car (the butcher job done by the "dealer A/C add-on" was bad enough). I may end up eventually adding a condenser in the rear driver's side wheel well.

The issue of electric car heating and A/C is an interesting problem. We had a small experimental fleet of electric Ford Rangers here at the Lab with a 1-ton battery pack under the bed! Running the defroster in the winter cut into range significantly and the full heater drained the battery in a hurry reducing range to around 20 miles. This issue alone may prompt the use of hybrid configurations over full electric.

Cheers, Jim

[bigchillcar]

hi jim,
my rear deck condenser is essentially the same..it cover the entire are of the external grill. you're thinking that still could be significant airflow losses around the edges? enough that you're hoping would be noticeable, measurable i assume. looking at my mom's toyota avalon, her darn condenser is 3x the size of mine and she has two what llok to be 12" inch, shrouded puller fans helping out..sure would be nice if the grill on the back didn;t have to service both airflow to the engine AND to the condenser, but i suppose i'm just preaching to the choir director, huh? as ugly as it would be and as wet as it could get inside, the whole darn rear lid needs to be a grill!
ryan

[cmonref]

Jim,

The "idle up solenoid" could be switched with the compressor clutch. With the solenoid cycling with the clutch, you should see minimal disturbances in idle speed. If you put a small manual metering device in the air line serving the solenoid, you could adjust the effects of the solenoid to suit.