andymlow

In the market for a later 928. Reading this forum has taught me a lot. Thanks you to all contributors. Wondering if the rear or second A/C unit makes that much difference as I live in FL and the A/C is really critical to enjoying the car much of the year. Comments appreciated.

Also is it inappropriate to place a post asking about cars for sale such as "does anybody know this car or seller?"

Thx

Speedtoys

Makes a large difference for me. I'll use it to keep the back of my head cool/dry when really hot.

MainePorsche

I use mine on really extreme days up here. I would think it would be crucial down by you.

pumpedTSI

The single AC on my 85 Euro is sufficient for me......Miami...

Speedtoys

So you have never used your rear AC?

pumpedTSI

Dont have it......making the point that single AC setup should be fine IF its working properly...

Speedtoys

Yes, I know you've never used it.

The question was for people with rear AC and what THEY think.


And a properly working rear AC, adds a lot to a properly working front AC. Howbouthtat.

pumpedTSI

Yeah.....cash me outside...

MainePorsche

When on a long highway trip I'll just have the rear AC on to keep cool without having the front and side air streams flowing at me. Is very convenient.

FredR

The rear ac is a useful addition to the package and I have had such on both my 928's but it should be understood that when the system is on the limit of its performance the front ac suffers a loss of performance because the system is limited by condenser duty and that is the same for both systems.

Thus the real question becomes what are the limits and that is a bit more difficult to answer as it depends on humidity and heat accumulation. Given you are in Florida and as I understand it is pretty humid there most of the time, the system struggles a bit once the temperature hits 35C and that number drops in traffic.

If you intend to cart little ones around in the back I would suggest it is essential to have the rear ac and a good heat rejection film system as without such I fear it would be a torture chamber for them.

As and when the system struggles one can turn off the rear unit if there are no passengers.

Wisconsin Joe

Can't comment on rear A/C, my front doesn't even work.

But to answer your other question:

Well within 'forum decorum' to inquire about cars and/or sellers.

There are a few cars out there that should really be avoided. They pop up for sale every so often. The white GT with the cracked & welded trans case comes to mind.

Other times someone will ask about a car from a well known member and get solid assurances that it's as represented.

docmirror

Here's the engineering principal of the deal.

The limit of the AC is a function of the limit of the condenser to exchange hot for cold air. With only the front AC engaged, the air from the front AC will reach the cold setpoint of the expansion valve and the air from the front AC won't get any colder, or it would freeze the evaporator(triggering the freeze switch). Thus - the condenser can fully subscribe the front evap unit, and have reserve cooling capacity left over.

When the AC in the rear is also enable, neither of the evaporators will now reach their low temp setpoint, and the restrictor valves will remain in full exchange mode. This means the exchange temp at the condenser has reached its maximum thermal exchange for the size and fluid flow.

One of our erstwhile mechanic/engineers has developed a larger, or more efficient condensing unit which will increase the exchange capacity of the condenser so that both front and rear evap units will be able to become fully subscribed(the expansion valve will regulate the temp setpoint just above freezing) for maximum cooling effect.

Given enough time, the rear AC will provide added cooling to the entire car, but due to the blower location, the gains may not be felt right away from the rear AC, and the front AC will be noticeably warmer air coming out, giving the impression that the AC unit is not working well. This is simply a function of the condensing side becoming oversubscribed by both evap units, and not regulating the evaporation of the fluid very well.

YMMV, don't try this at home, contents have settled, and may cause **** leakage.

Speedtoys

No leakage here Doc...

100% agreed, but when its truly hot in the car, I dont run both at full speed, _I_ seem to feel better physical results with the increased coo air MOVEMENT behind me, not just in my face while I bake behind me.

Im sure there is SOME more total cooling, but it's mostly a rear pax comfort unit, or..if you're like me, you enjoy the additional air movement from behind you

Although..with Han's new fans installed, and a 12" pusher fan for city driving, I feel I may experience better total cooling with both units that I was before. We'll some come June/July.

FredR

There is a caveat to all this- if you were to acquire a manual S4 or GT those cars have a full size condenser whereas the automatics have a smaller condenser because of the real estate occupied by the ATF cooler- this would better support the rear ac option so that it can perform better at higher ambient temps.

The manual GTS has a transmission cooler so loses that benefit.

dr bob

While living in the Los Angeles basin, I'd use the rear AC to help counter some heat-soak issues if the car had been sitting out for a bit. Humidity there is significantly lower than what you enjoy in Florida though. Plus mine is a black car with no window tinting.

As Doc Mirror shares, running the front system alone has the compressor cycling, bouncing off the anti-freeze switch setting in the front evaporator. Spreading the capacity between the front and rear systems means the front evaporator temp doesn't get to freezing temps, so the compressor can run continuously. The rear evaporator suffers a lot more from recirculation, as the inlet fro the rear evaporator is right behind the outlets. As Jeff shares, that's great for cooling towards the driver's head.

Regardless, at some point after the car has cooled some with both units in service, you'll reach a point where getting the front of the cabin completely cooled off will require shutting off the rear system. In my car, dry climate with the anti-freeze switch jumpered, the front vent temp rises almost 20ºF (to about 40ºF) when the rear is active. Front system alone will easily make ice cubes.

At this point I wouldn't make rear air a deal breaker as you shop. It might sway a choice that's otherwise pretty close, but it's not on my critical-features list so long as I have an otherwise fully functional system.