Vintage Air experts -- question about the Gen IV

[parsonsj]

After months of on-again/off-again I have finally all the fabrication, wiring, and install of my V/A setup completed

This past week I got all the wiring sorted (the documentation was wrong on the FAST XFI, and the FAST techs finally figured it all out). So now I've got the A/C fully integrated with the XFI, trinary switch, and fan controller. All the electrical stuff is perfect.

I vacuumed and charged the system, and have been driving around with the windows rolled up and the A/C on. It works awesome... but it is doing something I didn't expect. I wonder if it is working as intended, or if I have a charge problem.

At cruising speeds (2500 rpm) it seems to have a higher vent temperature -- say 44*. At idle (1100 rpm) it seems to have a lower vent temp, like 38*. Does the evaporator (a Gen IV) take cabin temp into consideration, and perhaps put out lower vent temps when the heat load in the cabin is higher (like sitting at a stop light at idle) vs a lower heat load when the car is going 75 mph? Perhaps to prevent ice-up of the evaporator? The cabin is perfectly comfortable, it's just that the vent temps seem to vary quite a bit.

Does that make any sense?

[ProdigyCustoms]

Try running both fans when the A/C is on instead of one. You may be building more high pressure cruising but not pushing enough air through the grill. What are your pressures?

[parsonsj]

Yeah, the fans work great: they come on at 220 and go off at 170. Both of them at full speed. It takes about 10s for the pressure to drop back.

This is with the DC Controls fan controller and the V/A trinary switch. I also got the a/c integrated with the FAST XFI, once we jointly discovered that the input signal to indicate a/c on is 12V and not ground, lol.

So the idle kicks up when I turn the a/c, the fans regulate the high side pressure, and I'm just chasing this odd higher temp at higher speed.

As for pressures: at idle around 85 ambient, I've got 175 high side, 28 low side. I've no idea at cruising speed. :)

jp

[KUL FIR CHICK N]

The vent temps should go down @ higher engine speed if you have the cool knob to max cold. If you have a little heat blended, you're probably seeing higher flow through the heater. Does it do this when parked (running the engine up to 2200-2500), or just when you're driving?

[parsonsj]

I haven't done a test at 2200 rpm at idle, so I don't know. My report at 2500 rpm was while cruising the I-4 at 70 - 75 mph.

I was on max cold all the time.

jp

[Twentyover]

On full cold?

I'm wonering if the higher pressure in the heater line is bypassing/forcing open the heater valve?

As long as the unit is getting a solid column of liquid to the TXV (no gas), it should cool approximately the same, regardless of the temp of the liquid (emthalpy difference between saturated and subcooled liquids are pretty small, relatively speaking). This would leave the reheat function as the suspected cause of the higher temp (at least to me)

[ProdigyCustoms]

We talked and he is a tennie low on freon also. So maybe is is cycling a bit at speed?

[Twentyover]

Dunno- On mine (in a 70 Camaro) I can feel the temp swings during cycling- but as previously stated, I'm more sensitive to this feature because I was in the auto HVAC biz for a while.

As stated I read the problem as a consistent higher temp. Also, from a discussion I had w/ VA, they shut the hot water valve off on full cold. I guess I'd make sure it was closing and not bleeding coolant into the heater

jp- where's your binary switch? do you have any low pressure cut function on the suction side of the system?

[KUL FIR CHICK N]

I mean the temp knob cranked to full cold position. The system is a re-heat blend system where the full air flow bypasses the heater when the temp knob is all the way to cold. At this point, hot water flow through the heater, even if coolant is leaking past the valve seat will have a very minimal effect on the air temp.

What I was referring to is that if he is running with the knob backed off of full cold in order to regulate the duct temp, varying flow through the heater can have an effect on the vent temp stability. We see this most on vehicles with high flow radiators, and/or removed thermostats where the flow across the heater is reduced to a trickle at idle, then comes on hard at higher engine speeds.

Since John states that the only test was on the highway, the first thing I'd be looking for is outside air being forced into the cabin through leaks in the firewall, kick panel vents, etc.

I know you have done huge fab on this car John, but is the original cowl vent completely blocked off and sealed. I don't remember the exact configuration of the fresh air vents on the Novas, but I'd check it.

Basically, if you're idling in the driveway and have the interior cooled down, the inlet air to the evaporator may be 75 or so degrees, allowing you to pull the outlet air down to 38. If there is a hot air leak near the evaporator, raising the inlet air temp, the outlet air temp will rise proportionally.

One last thing. When the clutch cycles to prevent evaporator freeze up, the vent temps will rise for a few seconds, then once the compressor comes back on, the vent temp should fall again.

And of course like Frank said, if you are low on charge, you could experience some surging of temps.

BTW Greg, I still owe you that blower.

[MonzaRacer]

Ok first of all your probably low.
Hook up gauges and run system and let stabilize, now with air running on max ac fans running, bring your engine rpms up to around 2000-2500, if all is good it should NOT go so low as to cycle off. Most setups cycle off at anywhere from 20-25 on low side.
R134a may run as low as 28 to 32, BUT depending on system you may see higher than low speed pressures. Also ihave seen several aftermarket units requiring more than instructions say, I routinely see Ford cars saying needing anywhere from 1/4 to a 1lb or even more than published.
My moms 85 Ford LTD had AC and called for like 1.5 lbs of 212 but to0 function properly it would take around 2.25 to 2.5 lbs so it wouldnt cycle out at cruise speed.
Now if the high side runs high during testing sitting still, and climbs it needs more air flow.
Another issue you can have is underhood overpressure, to find this if your hood has safety catch,just pop hood on first stage to see if it stays cool or heat climbs.
Good luck

[ProdigyCustoms]

He has had gauges on it, he is under 28, I told him to ease it into the low 30s.

[KUL FIR CHICK N]

The Gen 4 is a TXV system with a thermister mounted in the evaporator coil for freeze protection. It should pull down to 12-15 psi before it cycles the clutch. CCOT systems typically cycle the clutch in the 25-30 range because they're cycling the clutch on low side pressure instead of actual evaporator surface temp.

[parsonsj]

Hey guys, thanks for all the help. Just knowing it is a TXV system will help me some.

I'm in Montreal this week, so I haven't done anything yet...

Yes, my cowl is completely sealed, and I have nothing that would allow warm air into the evaporator inlet. Yes, the knob is on full cold, and no, there is no hot water in the evaporator (I have hardlines for the heater hose so I can tell).

I'm thinking I'm low on charge. I've got about 20oz. in it, so another 8oz is my next step. I'll update with my results.

thanks!

jp

[MonzaRacer]

OK So first of all go in 2 oz increments, if it pulls lower than 25 reved up fans on its still lowwhile a TXV system is better you still need to watch the amounts installed as VA doesnt use large amounts if its 15 to 20 oz system 1 or 2 oz will be lots of freon.
DO NOT dump in another 8 oz. The trick is watching guages and vent temps versus ambient.

RULE ONE your only gonna get 30 degree drop, dont be obssesed with ice cubes out the vents.
If its 99degrees, look for a 55 to 60 degree on recirculate.
BIG ISSUE, AC will not to refrigerate your car it removes heat and moisture its not going to hang icicles its not designed to do this.
IT conditions the air.
BUT what we look for is outlet temp to know we have a proper operation, there is also a proper drop across the condenser required, if it doesnt perform you need a larger one or more air flow.

[GetMore]

O
RULE ONE your only gonna get 30 degree drop, dont be obssesed with ice cubes out the vents.
If its 99degrees, look for a 55 to 60 degree on recirculate.
BIG ISSUE, AC will not to refrigerate your car it removes heat and moisture its not going to hang icicles its not designed to do this.
IT conditions the air.

One question regarding those statements: Since Vintage Air is a recirculating system, where it does not allow outside air in, won't it continue to cool that air until it is near that "icicle" stage?
I understand that a unit that conditions outside air will only drop temps about 30°, and that's why "Max A/C" recirculates cabin air.

Of course, even a system that doesn't allow outside air in has to deal with heat gain through cabin panels. Undercar heat, engine compartment heat, solar heat gain, etc.

[MonzaRacer]

OK car emanates heat from engine, interior, windows, passengers, you may get it down to frosty cold but it will take time, it is removing heat and it will need to move heat overtime.
Also I dont care what system it is, your still only going to get a 30 degree drop.
ANYTIME your servicing an AC system you need windows or doors open you need to stress to system to make sure it functions under normal conditions.

[GetMore]

Is that 30° drop the difference between the air entering and then exiting the evaporator, or is it between the outside air and the air exiting the evaporator?
I'm thinking (and may be wrong) that it's the potential difference between the air entering and exiting the evaporator.
So, if you pull in 100° outside air you will have an output of 70°. Recirculate some of that air and you will get the temps lower.
On the other hand, if all you do is recirculate the air you should get the temps rather low. It might start off at 100°, but as the air keeps passing through the evaporator and drops 30° it will eventually get down near the freezing point, or whatever the critical temperature is for the evaporator.

Have I got this right, or am I missing something important?

[Twentyover]

Ability of the evap to remove heat from air is based on an equation giving a value called log mean temperature difference, and airflow. This (LMTD) is a function of the inlet and outlet temperatures of both the air and refrigerant, which is dependent of geometry of the system, airflow, and a pile of other stuff.

First pass you may get a 30 degree temp drop, the next pass maybe 20 degrees, and each successive pass will yield a lower value of incremental cooling. This statement is based on the incorrect assumption that there is no incremental heat added to the interior air from engine, solar, radiant from the road surface, etc.

As the interior temp drops, usually the driver will reduce airflow or add reheat, or a combination of both.

Eventually get near the temperature of the cycling temp of the evap? Sure, eventually, given a set of unusul conditions (cool or mid ambients, low or non existent solar load, few or no bodies in the shell radiating heat and aspirating moisture, no lights or electronics in the shell adding heat, and low conduction into the shell), but by that time the compressor is being cycled off a significant fraction of the time to prevent evaporator freeze (ice formation on the face of the evaporator, preventing airflow through the evap.)

[MonzaRacer]

Is that 30° drop the difference between the air entering and then exiting the evaporator, or is it between the outside air and the air exiting the evaporator?
I'm thinking (and may be wrong) that it's the potential difference between the air entering and exiting the evaporator.
So, if you pull in 100° outside air you will have an output of 70°. Recirculate some of that air and you will get the temps lower.
On the other hand, if all you do is recirculate the air you should get the temps rather low. It might start off at 100°, but as the air keeps passing through the evaporator and drops 30° it will eventually get down near the freezing point, or whatever the critical temperature is for the evaporator.

Have I got this right, or am I missing something important?



Pretty much dead on, but remember this is conditioning the air , not making cold air.
But for the most part from inlet to outlet will still make a 30degree drop, as the interior gets cooler the system becomes more efficient at keeping you comfy and dry. AND not let any outside heat radiating inside cause you discomfort.
Now as for freezing no the system isnt designed to go below freeze point, thats a freezer.
Most if not all are set up to keep the evaporator from freezing, hence the TXV.
With a closed system where no outside air can come in it becomes much more important to use TXV over CCOT.
BUT then I do prefer a system that can also pull in some outside air once in a while if I want, if my passenger gets too cold you take it off max AC/recirculate and let it simply cool outside air such as when cruising just before/after dark when its not too hot nor cool but a little cooling is desired.
And if I remember the proper split off the inlet/outlet of a properly sized condenser is 40 degrees if its less it may be stopped up if its molre it may be too small or not enough air flow.

Lee Abel
AFTERMARKET PERFORMANCE

[parsonsj]

I think the problem was just low charge. I added another 4 oz. or so, and just took it for an extended drive (heavy traffic, highway cruising) for about an hour. 85 degree day, high humidity. We've got about 200 of them left this year. :(

36* temps when the compressor cycled on, then it would go to about 42* when the compressor cycled off. Cabin temps were very nice, plenty of air movement from the vents, and I am pronouncing the AC as working. I'm very pleased.

Thanks for all the help.

jp

[compos mentis]

John,

Besides keeping you cool of course, have you noticed any other ways adding the a/c changed the feel of the car at all?

[parsonsj]

Hi Skip,

It's more civilized. I was riding around with the windows rolled up (it was 93 here the other day... but you'd never know it inside the car), and the stereo off. I was just enjoying the car, and its noises (muted of course with the windows up), without listening to the roar of the wind. I was practicing my 2-3 shift (push straight up... let the shifter do the work), and just having fun. I just moved my IAT to the intake manifold plenum from its original location in the air cleaner, so I was making sure the car still ran right. I did a 30-80 banzai on the freeway on-ramp, and the car still has that "keep it straight or we are gonna swap ends" feel along with a howl of rubber barely keeping traction.

Or I can turn on the iPhone stereo and rock out, or (my latest) listen to the Cincinnati Reds on the new iPhone MLB application.

It's fun. And when I get home, I'm not exhausted from dealing with all that heat. :)

Thanks for asking,

jp

[compos mentis]

It's fun. And when I get home, I'm not exhausted from dealing with all that heat. :)

Thanks for asking,

You're welcome JP.

That's a good recommendation for sure.

Reading between the lines, it seems the added weight/running it hasn't noticeably effected handling or power output...

[parsonsj]

it seems the added weight/running it hasn't noticeably affected handling or power output...Not that I can tell. The Vintage Air alternator puts out more amperage and so I'm running at 13V+ at idle (rather than 12.5V- and dropping). The EFI is much happier at the higher voltages. I swear it has more power than ever, but the butt-o-meter isn't known for accuracy. One of these days, Mike and I will put it back on the dyno to see for sure.

jp

[compos mentis]

I swear it has more power than ever, but the butt-o-meter isn't known for accuracy. One of these days, Mike and I will put it back on the dyno to see for sure. jp

Look forward to reading about your dyno findings.

Nice also that you've now got a much more family friendly car to cruise around in.