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      08-14-2018, 03:55 PM   #1
Merlin_83
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Angry She no likey the heat - hot, hot weather and A/C

Greetings,

It's been 96-100 degrees Fahrenheit and very dry the last 2 weeks. I try to never run the A/C, but had to with the high temps.

In stop and go traffic the car feels so sluggish has F#$& ! On long runs not as much. What's the horse power penalty running either the air conditioning on MAX or AUTO (set to 60 degrees) ?

The car just feels like it doesn't like the high temp in general under 40 mph. At higher speeds it's better with ducted air flowing to the radiator, A/C condenser coil, oil cooler, transmission cooler, err...power steering cooler

Thank you,

Merlin
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      08-14-2018, 04:00 PM   #2
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Whats the under tray on the front of your car like? IIRC if it's beat to shit, it can cause your car to run hot.
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      08-14-2018, 04:25 PM   #3
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The AC compressor is quite efficient, I've run it when I lived in FL, when I picked the car up in Dallas and drove back, it causes very little parasitic loss. It was not uncommon for me to see 105+ temps on the roads around Tampa/Orlando and the AC not skip a beat and stay cold, however those higher air temps just heat soak the entire car and I felt that total heat soak is what just makes any car feel sluggish, not necessarily the drag of running a compressor.
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      08-14-2018, 05:11 PM   #4
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^ Agreed, I think heat soak just makes a car sluggish. E.g., AC off at 100F still feels slower than AC off and ripping it at 50F. I don't notice a difference with AC on, and I've also read that the AC and alternator are disconnected at WOT anyway.
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      08-14-2018, 05:18 PM   #5
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NEVER running the AC will cause the seals to fail!
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      08-14-2018, 05:41 PM   #6
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Quote:
Originally Posted by Merlin_83 View Post
I try to never run the A/C, but had to with the high temps.

In stop and go traffic the car feels so sluggish has F#$& ! On long runs not as much. What's the horse power penalty running either the air conditioning on MAX or AUTO (set to 60 degrees) ?
so you go from one extreme (never running the A/C) to the other (MAX A/C) ...
you should set it to a moderate 70-74 degrees and use the A/C more regularly
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      08-14-2018, 05:44 PM   #7
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Warm air is less dense. The car will always be more sluggish in hot weather.

Not running the a/c might get you 8-12 hp more. Hardly worth suffering for that.
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      08-14-2018, 06:55 PM   #8
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Regular use (once per month to a few times per year?) immediately followed by non-AC vent air flow before powering down is the best way to extend the life of the HVAC system. But that's not really the question here.
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      08-14-2018, 07:52 PM   #9
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Under WOT I believe ac disengages...

You are just probably experiencing hot IAT.

You should turn the heater on max. Race cars do that to cool the engine. Lol
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      08-14-2018, 09:02 PM   #10
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It’s pulling timing
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      08-15-2018, 06:58 AM   #11
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With my e46 M3 I could notice a real difference in acceleration with the A/C off vs. on. With my e92 M3 the difference is negligible. I leave it on (in South Florida it does get used year-round).
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      08-15-2018, 09:16 AM   #12
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Agree with DSilk, a total crap car can be sold in FL for sooo much more because in the description they have the two words everyone is looking for, "Working AC".
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      08-15-2018, 10:35 AM   #13
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Thanks guys !!

There's been some good advice, perspective and recommendations so far. And I think the best advice moving forward is putting the A/C setting to the AUTO mode and set to more moderate 68° or 70° F.

The heat soak thing also makes sense as I can tell even not running the A/C on hot, hot days the car just 'feels' slow. Not to digress the original topic but in reference to heat and air densities I know aircraft jet engines have to be 'de-rated' for certain conditions i.e. high and hot.

An example is a jet aircraft heavily loaded with fuel and payload taking off at Denver or other high elevation cities on a dry day. Air density is much less and the pilot has to de-rate take off thrust less than 100%. This done to minimize exhaust gas temperature [EGT] from being to high. Cars with turbocharges and inter-coolers allow the charge air to be cooled by the air-to air or air to water inter-cooler this has the affect of dropping temperatures to have denser [colder] and higher mass air. Basically more readily available oxygen. BMW takes it a step further buy dumping water into the fuel/air mixture, which is crazy when I think there adding water - But then I guess water injection aside from inter-cooling has been around for years in the hot rod community.

I may have stated some of that wrong so if there are any aerospace/aircraft engineers, please update or correct what I stated.


Thanks,
Merlin
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      08-15-2018, 12:29 PM   #14
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Well, derating is more of specifying a maximum power setting at less than max rated power to prolong its life, e.g., piston engines in a helicopter. This has the added benefit of being able to sustain max power higher up. Like if BMW derated the S65 to 300 horses. Obvi, you would be able to maintain that HP over a lot of altitudes and temps.

Your examples are more along the lines of not being able to use rated power because of another factor, such as temp limits. That certainly has the effect of derating it for that moment in time but it's not the normal usage of the term, derating applying beyond specific situations.

The bottom line is, all other things being equal, hot air is less dense (much like having a car at altitude). Less dense air makes less power.
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      08-15-2018, 12:34 PM   #15
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Quote:
Originally Posted by pasghetti View Post
Whats the under tray on the front of your car like? IIRC if it's beat to shit, it can cause your car to run hot.
+1

I had a cracked undertray and for a little while I ran without it while I awaited a replacement. The car ran much hotter without the undertray - you can actually see higher temps on the gauge cluster temp reading. After I put on the new undertray, it was back to normal.
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      08-15-2018, 04:30 PM   #16
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high IAT's = timing pull. it sucks but that's the hot summer.
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