powdrhound

Very good info. Thx Macster! It would make sense that the car would be designed to operate safely regardless whether the AC was on or off..

993GT

No issue running with A/C on or off, on street or track, it is SAFE...and Macster is right about the compressor shutting off under heavy load/full throttle, I should have talked about that-no power loss under heavy acceleration due direct to compressor draw...but still interested in seeing any data about effects of the fuel chilling and any possible negatives effects due to loss of radiator efficiency

mdd

Macster, if your AC compressor is running without the snowflake indicator in the dash being lit up, its broke. You should take it to the shop and have that fixed right away before your car blows up. Please report back.

I agree with jpflip, Macster's assumptions and conclusions about the compressor running all the time to support the fuel cooler function are false.

Macster

Sure feels like the line is cool at the cooler. I mean there were plenty of warmer lines, and other hardware to compare to.

The A/C system (autoclimate style) otherwise works just fine and has since I've owned the car. Hard to image a failure that would cause the compressor to be on or something on to account for the cool line.

Anyhow, I've sent an email to a tech I know asking him about this. If I hear from him I'll report back.

Kevinmacd

If the compressor was running and the fan is off the evaporator would be freezing up and you would definitely have telltale sign of water drainage under the car when parked.
Auto compressors are either on or off. The thermostat engages and disengages the compressor clutch.
I would think Porsches take on cooling the fuel is based on ambient temps vs racing at the track. On those hot summer days when you are in need of AC the thought to cool the fuel is based on the fact it is hot enough to use the AC.
BTW just as a side note there are races that require some of the classes to be air conditioned during the race, as in the ALMS.

mdd

The relatively cool fuel from the tank being continuously pumped through the housing would make the housing and lines slightly cooler than the other components in the engine compartment.
That doesn't mean the AC compressor is running or that the fuel is being cooled as you stated as a fact.
In reality, the fuel is being slightly heated by having to pass through the extra lines and housing that are exposed to the hot engine compartment. Basic thermodynamics.

powdrhound

So, back to square one. Lol ! Should I run the AC at the track to cool the fuel? The answer seems to be leaning towards the YES camp..

993GT

we need to datalog(or run a proper water temp. gauge) to see if water temperatures are rising/above 'optimal' when using the A/C, if the cooling system is not able to keep water temps correct at any point then should try running without A/C on and re-test, a cooler running engine=higher detonation point/avoidance=safer/more power
My bet is A/C OFF is better for track-use, A/C ON in hot stop-n-go traffic

Carlo_Carrera

Read your manual. If you leave the HVAC control in "auto" mode the AC is always on all the time unless it is very cold outside. The HVAC system will use both heat and AC at the same time to keep the temperature at the selected degree.

Macster

Yes. After I sent the email off to the tech I got to thinking about this some more. (Too bad I didn't think more about this before I posted!)

Anyhow, the other day the weather was cool. I noted on the way home the outside temp just a few blocks from home was only 64F. It has been cool a while now, for several days. The car sits outside and well, the fuel in the nearly full tank was cool, has been cool and has not had a chance to heat up.

The line I felt was cool to the touch because it was just cool from the cool air blowing past it from the engine compartment fan. The fuel lines and in fact the fuel cooler felt cool because the fuel (return fuel) was cool, was keeping the hardware cool.

I will get a chance this weekend to run the A/C -- forecast has Saturday highs in the high 80's (with rain(!) forecast Sunday) -- and after having the A/C on a while then I'll feel the lines. I suspect they'll be more than cool to the touch, but downright cold.

It makes sense now -- in fact rather obviously -- that if the snowflake was off the compressor would not be on. My mistake was not taking the time to go outside and fire up the car and turn on the A/C and then check the lines which would have made it clear my info was wrong.

Every time I fail to confirm something I embarrass myself. When will I learn?

Anyhow, thanks for the post and to all the others that took the time to respond to my erroneous info and offer the correct info.

Macster

If I have an credibility left I don't think it matters that much.

First my preference would be to have the A/C off. Even though hard acceleration disengages the compressor the times I have had the A/C on and engaged in hard acceleration the change in the air temperature and humidity when this happens is a bit distracting.

Also, having the A/C off so it isn't constantly disengaging then engaging again reduces whatever wear and tear there is to this mechanism. This reasoning would also have me manually deploying the spoiler too and leave it manually deployed the whole track session.

As for the fuel cooling I wonder how quickly this cooler would cool the fuel in the tank since as best I can tell it only works on fuel that is returned back to the tank?

If one starts out with hot fuel I would hazard a guess given the relatively short sessions on the track the amount of cooling would be low.

Or maybe not. Hard to know without some instrumentation.

Still, my thinking would be ideally to start out with cool fuel. If it is cool where you are there is probably no problem.

If it is hot where you are try to arrange the fuel tank to be low (say around 1/4 tank) then fill up before heading to the track. The fuel in the storage tank is probably as cool as any fuel can be in your area and as long as the drive to the track isn't that long the fuel will not heat up much. In fact if it is that warm you'd run the A/C and this during the drive would work to cool the fuel in the tank.

Be sure to leave some room for adding enough 100 octane fuel at the track to bring up the average octane of the fuel in the tank to 93 or 94 octane or more if you want it.

You can keep the A/C on say while the engine is running before you head out on the track, during your familiarization/warm up lap. Towards the end of this then switch off the A/C.

Then after the session turn the A/C on during the cool down lap to among other things start cooling the fuel again.

Back in the pits as you let the engine idle at least the 2 minutes recommended by the owners manual the A/C is left on.

But of course you are free to leave the A/C off all the time, leave it on all the time, even during your session, or any combination you feel is right for you.

My feeling is whatever gives you peace of mind so your focus in on your driving, not on the fuel and its temperature, or on the compressor and what it is or is not doing, etc.