Hi Guys,



are you guys noticing a lot of load on your engine when the A/C on?
'yesterday I got my car back from dealer and noticed this..

I was having a noise ( kind of chatter noise) when the car was cold in the mornings..

fellow member ChuckJ was very helpful, he did a special trip to looked at my car, what a great guy he is..

he thought maybe it was my gas, because my RPM needle was kind of bouncing I thought I was using 91oct, I wasn't checking the pump, today I did checked and gas is 93oct ..

yesterday I got the car back and still hear the noise, they couldn't duplicate the noise

but after they kept the car for a week I think the noise is just louder

the previews dealer told me they did heard the noise, thinking it was the DMF...so I'm going back and see if they can get that taken care, but I did noticed something that it wasn't there before...

when having the car on D and the engine cold right after RPM needle drops the noise will start, if I start the A/C the noise will get louder ( before it wasn't there) after turning the A/C off this noise will go away for maybe 2 seconds and the engine would just be quiet as it should be and then the noise will return ..

in case this has something to do with the DMF.. does the A/C gets more load on this? and strange thing is turning a/c off this will go away just for few seconds and then it would be like some engine load returns and this starts again

 

That's a very confusing post. Is this a PDK car? Are these things happening in park, or just in drive?

To answer the original question: no. I don't notice any impact on engine performance with the A/C compressor on. I'm sure there is an impact, but it's just not noticeable either in performance or sound.

 

Solrac6262:
There should not be appreciable performance difference since the compressor is disengaged upon rapid acceleration. If the compressor has not been lubricated properly or has a malfunctioning compressor there would be a drag, however.

ChuckJ

 

I did not know that. That is just cool!

 

Its PDK, happening with the car in Drive, as soon as I get the A/C on I can hear and feel more load and some grinding noise, right after turning the a/c off the car is quiet for maybe 2 seconds and then this grinding noise starts again, after the car is warmed up, its hard to listen this noise

 

You feel more "load" as in your performance suffers?

And it is hard to listen to meaning it is difficult to hear?

With this size engine, I wouldn't expect to be able to feel any loading, as the energy to turn the compressor is rather low as compared to the overall engine output. Not sure exactly what the 991 compressor consumes as it depends on it's BTU output .. But somewhere between 5 and 10 max probably much less than that typically when it isn't blasting.. My guess based on what I remember from older GM units years ago. It isn't trivial but with a 400HP engine it isn't as significant as it would be in a 100HP honda engine..

It might show up as measurable with fuel economy or overall top speed but I seriously doubt you could tell just from driving. I certainly can't...

As far as noise.. I don't really notice much.. but it wouldn't surprise me if you could hear the compressor clutch engaging and disengaging. And generally you see a slight RPM drop as well as hear the compressor when that happens. Honestly I'm so used to that kind of sound I'm not even sure I'd even notice that on a car.. But I wouldn't describe that as "grinding"...

 

I could feel it on my '93 Tercel. It only had 93hp at best. I had to turn the air off and catch the back end of overpasses to even approach 95mph!
The Carrera, I don't feel any difference. I've turned off the air when on the track, but I'm just torturing myself when I do that.

 

Solrac6262:
This AC works differently than cars back in the 70s. During that time you may remember, the AC brought down the idle when the AC compressor turned on. In the 991 the engine is programmed to run at a set speed on idle so you really shouldn't notice much RPM change (assuming the car is operating properly). If the compressor was damaged and providing much more drag than normal, it might exceed the automated compensating throttle input used to keep the speed constant.

ChuckJ

 

I have the Dual-path exhaust and do notice a slight difference in sound during idle when the AC is running. The sound gets throatier and the engine does perk-up a bit when I switch the AC off (at idle). There is no added noise when the compressor is running.

Personally, I wouldn't hesitate to have a trained professional look at my car if it was making any type of a grinding noise. If your dealer missed something the first time, perhaps you should try them again or take the car someplace else.

I hope your problem gets resolved soon so you can get back to enjoying your 911!

 

There was never any noticeable noise or engine load/bogging on any of my three (base, S and now TTS) with the AC on. I go directly from AC always on to heater always on (we don't get 4 seasons in Texas). I agree to being persistent with the dealer checking this for you, does not sound normal. In fact, my 991 S would just laugh at me, speeding up on a hill with AC on max.......