A/C on vs. A/C off

[avecta]

Originally Posted by Tom K.

While the A/C compressor increases engine drag, there is no speed difference as long as the mechanical (notice the spelling) connection between drives wheels and engine is unchanged - which is the case in a MT unless a gearshift has been made or the clutch is slipping.

Tom

I'm sorry mr english teacher. Typing on the phone whilst driving*

You know there's a clutch on the a/c compressor right?

There's a rotor friction plate and a a/c compressor friction plate, and when the a/c is off only the rotor plate is spinning freely to the compressor plate (a/c compressor)..
When it's turned on, there is electro magnetic field through the rotor which pulls together the rotor and compression plate thus MORE drag on the engine than just a rotor pulley it is now spinning the compressor.....

[Exite]

One thing to note is the OP probably drives a manual car. A manual car is a lot more sluggish with the ac on. You have to rev the gears higher before shifting to avoid jerking the car, rev match when shifting to avoid jerking the car, etc... You have to drive the car way different when the ac is on. With an auto it doesn't matter hit the gas and it goes the same ac on or off.

My car is manual and every manual car I've owned with the a/c on you have to drive the car a little different to not make jerky shifts. Just the way it is.

[2012_135i]

Originally Posted by Exite

One thing to note is the OP probably drives a manual car. A manual car is a lot more sluggish with the ac on. You have to rev the gears higher before shifting to avoid jerking the car, rev match when shifting to avoid jerking the car, etc... You have to drive the car way different when the ac is on. With an auto it doesn't matter hit the gas and it goes the same ac on or off.

My car is manual and every manual car I've owned with the a/c on you have to drive the car a little different to not make jerky shifts. Just the way it is.

That is correct. I have a 6MT. I was wondering if that was a factor because "sluggish" definitely describes the way the car feels with the A/C on. I don't have any problem with jerkiness ("I'm an excellent driver" - Rain Man ) but the sluggish performance compared to A/C off is a real drag. Are you saying if I had a DCT I wouldn't notice any degradation of performance? No sluggishness? The car would go go go just the same as A/C off? Or are you specifically addressing the jerkiness issue that others have which would not be there with an auto?

[2012_135i]

By the way, Exite. Love your avatar. One of the greatest games ever made.

[ShocknAwe]

Ah, so this 1-2 and 2-3 clutch disengagement jerk on the 6MT is normal with the A/C on? I couldn't even figure out how to turn the A/C off this afternoon when I was driving around with the windows open.

[RPM90]

Originally Posted by 2012_135i

I'm surprised at the difference in performance when running the A/C as opposed to when it's off. Naturally there's going to be a difference, but the degree of difference is what I'm surprised at. All my previous vehicles were 4 cylinders that suffered a notable power drag with the A/C on. I figured the BMW's much more powerful turbo inline 6 would mitigate this, not completely, but at least to where it wouldn't be all that noticeable. Well, it's really noticeable. With the A/C off the car is alive, eager, and ready to rumble. With the A/C on it feels like driving through molasses, at least when starting off and getting up to speed. Anybody else notice this?

There is a slight difference when you turn on AC, but it's only noticeable at low rpm before boost kicks in. At idle when I turn the AC on, my idle rpm rises by about 100 or so rpm. Does yours? It should.
Over all, no I don't notice the type of power loss that you describe.

Remember, a turbo engine puts out low/modes torque when there is no boost. It's the nature of a turbo engine. If you use very light throttle where you're not building much boost, then power is modest, so if you're using very light throttle and AC is on, then you'll notice the drag more so.
If you drive it normally, or at least normal for enthusiast driving , then you're using more throttle and thus more boost. That boost creates more power and it easily has plenty of power to overcome the AC drag.
Even with AC on I can still smoke the rear tires off the line, if I want to.
I've done it with AC on an off. I don't do that often though as it's wasteful.

Maybe you've got a boost leak somewhere, or something else is wrong?

[Lucky1]

Originally Posted by Exite

One thing to note is the OP probably drives a manual car. A manual car is a lot more sluggish with the ac on. You have to rev the gears higher before shifting to avoid jerking the car, rev match when shifting to avoid jerking the car, etc... You have to drive the car way different when the ac is on. With an auto it doesn't matter hit the gas and it goes the same ac on or off.

My car is manual and every manual car I've owned with the a/c on you have to drive the car a little different to not make jerky shifts. Just the way it is.

This describes my experience with MT and the A/C on exactly. IMO the 128i turns into a bit of a dog with the A/C on and carrying extra people in the car makes it even worse. Most noticeably there is a lot less power down low and the throttle response is crap. I definitely feel I have to downshift a gear lower to get similar performance with A/C off especially when the speedo hits 100km/hr or higher. When my CPO warranty is over I'm installing the BMS powerbox and hopefully it will improve the throttle response at least.

Driving my Civic with the A/C on in the summer really annoyed me because of the performance hit, its a bit disappointing to feel the same even with an inline six under the hood. I thought the 135i would fare better in terms of how noticeable the performance drop is but it doesn't sound like that is the case from this thread.

Its too bad because summer is the best time to drive the car (drier roads, no snow, summer tires are on) but if it gets too hot and the A/C goes on it kind of robs the experience.

[RPM90]

Originally Posted by avecta

I'm sorry mr english teacher. Typing on the phone whilst driving*

You know there's a clutch on the a/c compressor right?

There's a rotor friction plate and a a/c compressor friction plate, and when the a/c is off only the rotor plate is spinning freely to the compressor plate (a/c compressor)..
When it's turned on, there is electro magnetic field through the rotor which pulls together the rotor and compression plate thus MORE drag on the engine than just a rotor pulley it is now spinning the compressor.....

Jim was responding to your overly simplified comment.
You said to drive at a "constant" speed, then turn the AC on and see what happens to RPM.
Well, vehicle speed is a function of engine rpm and gear ratio.
Thus, if you're driving at a 'constant', say, 55mph, then even if AC kicks on it has no bearing on gear ratio and engine rpm.

I think what you wanted to say, is that when the AC kicks in the compressor drag on the engine will bring engine rpm down, and it will.
But then, you're not going to moving at that "constant" speed, your speed will decrease if you don't compensate by giving more throttle in bring engine rpm and speed back to where it was.

Your comment should have included the whole point you were trying to make. You are correct that AC compressor drag will result in putting a greater load on the engine, and if the driver doesn't compensate, then engine rpm will come down, and speed will decrease.

But, the question at hand is how great is that drag?
Most don't notice a big drag, especially with the 135i and it's more powerful engine.

You can see and feel this drag when at idle. When idling note the rpm, now turn the AC on. You will feel the compressor kick in and the engine slow down, but quickly the idle speed control kicks in and raises idle rpm to compensate. In some cars, the throttle increases but engine rpm is brought to the same rpm as before the AC kicked in. In other cars, like our 135i, the engine rpm actually goes higher by about 100-150rpm, at least in an MT 1.


I understand the point you were trying to make. Jim was clarifying your comments overly simple wording.
My intent is not a dig on you, I'm just saying that the way you worded your comment left out much that would have helped clarify your message.

I'm a verbose person in conversation and in writing. Often I write too much.
The converse of that is people who write too little and their message doesn't get across as it should.

[Exite]

Originally Posted by 2012_135i

That is correct. I have a 6MT. I was wondering if that was a factor because "sluggish" definitely describes the way the car feels with the A/C on. I don't have any problem with jerkiness ("I'm an excellent driver" - Rain Man ) but the sluggish performance compared to A/C off is a real drag. Are you saying if I had a DCT I wouldn't notice any degradation of performance? No sluggishness? The car would go go go just the same as A/C off? Or are you specifically addressing the jerkiness issue that others have which would not be there with an auto?

In an auto/DCT the loss of performance/sluggish I'm sure is still there just not as noticeable since the car is shifting for you. Mostly why it makes me drive the car jerky with the ac on is because the rpms drop so fast. So I either have to shift really fast (an auto tranny will also do this), shift at 4k rpm instead of 3k rpm or keep on the gas a little bit during the shift.

[RPM90]

Originally Posted by 2012_135i

That is correct. I have a 6MT. I was wondering if that was a factor because "sluggish" definitely describes the way the car feels with the A/C on. I don't have any problem with jerkiness ("I'm an excellent driver" - Rain Man ) but the sluggish performance compared to A/C off is a real drag. Are you saying if I had a DCT I wouldn't notice any degradation of performance? No sluggishness? The car would go go go just the same as A/C off? Or are you specifically addressing the jerkiness issue that others have which would not be there with an auto?

The drag on the engine will be the same whether it's MT or AT.
In either car you have to adjust your throttle input.
In an MT you simply use more throttle to compensate for the added drag.
In an AT, as well, you have to use more throttle to compensate or the added drag.

As for the "jerkiness", that's up to the MT driver. If the driver has a lazy shift, then he may get a more jerky feeling as the rpms will drop a bit quicker compared to no AC drag.

In an AT there is of course no tendency for a "jerky" shift as the shift is done differently.

More throttle. That's what is needed. OH, and no lazy shifting, which one shouldn't be practicing in the first place.

[Exite]

Originally Posted by 2012_135i

By the way, Exite. Love your avatar. One of the greatest games ever made.

Fo sho half-life one of the best fps games.

Now we just need a movie. It should be directed by James Cameron and Hugh Laurie should play Gordon Freeman.

[jkoral]

6MT here and was also surprised by the impact of A/C. Don't notice it on my family cars, but they have all been automatic (and not driven as spiritedly).

I have an old habit (started a while back and continued with my past few cars) of turning off A/C when merging onto one particular highway (almost always go WOT). Turn it back on once I've merged and at cruising speed. Even in the 135i, I still do this.

[avecta]

Originally Posted by RPM90

Jim was responding to your overly simplified comment.
You said to drive at a "constant" speed, then turn the AC on and see what happens to RPM.
Well, vehicle speed is a function of engine rpm and gear ratio.
Thus, if you're driving at a 'constant', say, 55mph, then even if AC kicks on it has no bearing on gear ratio and engine rpm.

I think what you wanted to say, is that when the AC kicks in the compressor drag on the engine will bring engine rpm down, and it will.
But then, you're not going to moving at that "constant" speed, your speed will decrease if you don't compensate by giving more throttle in bring engine rpm and speed back to where it was.

Your comment should have included the whole point you were trying to make. You are correct that AC compressor drag will result in putting a greater load on the engine, and if the driver doesn't compensate, then engine rpm will come down, and speed will decrease.

But, the question at hand is how great is that drag?
Most don't notice a big drag, especially with the 135i and it's more powerful engine.

You can see and feel this drag when at idle. When idling note the rpm, now turn the AC on. You will feel the compressor kick in and the engine slow down, but quickly the idle speed control kicks in and raises idle rpm to compensate. In some cars, the throttle increases but engine rpm is brought to the same rpm as before the AC kicked in. In other cars, like our 135i, the engine rpm actually goes higher by about 100-150rpm, at least in an MT 1.


I understand the point you were trying to make. Jim was clarifying your comments overly simple wording.
My intent is not a dig on you, I'm just saying that the way you worded your comment left out much that would have helped clarify your message.

I'm a verbose person in conversation and in writing. Often I write too much.
The converse of that is people who write too little and their message doesn't get across as it should.

True, true. Thanks for clarifying

[RPM90]

Originally Posted by ShocknAwe

Ah, so this 1-2 and 2-3 clutch disengagement jerk on the 6MT is normal with the A/C on? I couldn't even figure out how to turn the A/C off this afternoon when I was driving around with the windows open.

Normal? NO.
Common? Yes, if your MT skills need work.

You need to practice your MT shifts.
No lazy shifting. Learn to shift quickly all the time.
That way, when the AC brings the engine rpm down quicker, you can shift quickly and smoothly.
Since you know the rpm will drop quicker, then pick a higher rpm to shift.
If you use 2000rpm for a smooth shift, then use 2300 or 2500 when the AC is on, or shift quicker.

[Lucky1]

Originally Posted by ShocknAwe

Ah, so this 1-2 and 2-3 clutch disengagement jerk on the 6MT is normal with the A/C on? I couldn't even figure out how to turn the A/C off this afternoon when I was driving around with the windows open.

It turns off the same way it turns on?
Do you drive around with A/C on at all times?

[2012_135i]

Originally Posted by RPM90

There is a slight difference when you turn on AC, but it's only noticeable at low rpm before boost kicks in. At idle when I turn the AC on, my idle rpm rises by about 100 or so rpm. Does yours? It should.
Over all, no I don't notice the type of power loss that you describe.

Remember, a turbo engine puts out low/modes torque when there is no boost. It's the nature of a turbo engine. If you use very light throttle where you're not building much boost, then power is modest, so if you're using very light throttle and AC is on, then you'll notice the drag more so.
If you drive it normally, or at least normal for enthusiast driving , then you're using more throttle and thus more boost. That boost creates more power and it easily has plenty of power to overcome the AC drag.
Even with AC on I can still smoke the rear tires off the line, if I want to.
I've done it with AC on an off. I don't do that often though as it's wasteful.

Maybe you've got a boost leak somewhere, or something else is wrong?

I understand what you're saying. I'm going to pay more attention to what the RPMs do when I turn the A/C on and off. Usually I don't change it. I either have A/C on or off when I start the car and drive off and keep it that way. On nice days just motoring around town I leave it off. On hot or sticky days I have it on at start up and don't change it.

Not sure about a boost leak but maybe that's something I'll ask the dealer about. Sometimes even with A/C off I don't feel I'm getting the response and power that I should. And like I said in one of my previous replies, I don't know if I have DME programming issues possibly reducing power and throttle response to some extent or not.

[2012_135i]

Originally Posted by jkoral

...I have an old habit (started a while back and continued with my past few cars) of turning off A/C when merging onto one particular highway (almost always go WOT). Turn it back on once I've merged and at cruising speed. Even in the 135i, I still do this.

I've always done this as well, especially in my '01 Prelude SH. I was hoping I wouldn't need to with the 135i. Don't actually need to, but it feels better if I do.

[alvitdk]

Originally Posted by Lucky1

It turns off the same way it turns on?
Do you drive around with A/C on at all times?

Actually it's good to drive with the AC on all the time, even in the winter the automatic mode turns the AC on. This way all the gaskets and o-rings in the AC lines are kept well lubricated. Never using the AC is a big part of failing AC systems.

[Lucky1]

Originally Posted by alvitdk

Actually it's good to drive with the AC on all the time, even in the winter the automatic mode turns the AC on. This way all the gaskets and o-rings in the AC lines are kept well lubricated. Never using the AC is a big part of failing AC systems.

Thanks, that is a good point. My Honda Civic's manual recommended turning on the A/C a few times during the winter to keep it in good working condition but leaving it on all the time? Sounds like it might be good for the A/C but wasteful of gasoline and just robs the driving experience (car drives better with it off). I'll turn it on occasionally but won't drive with it on all the time in the winter.

[alvitdk]

Originally Posted by Lucky1

Thanks, that is a good point. My Honda Civic's manual recommended turning on the A/C a few times during the winter to keep it in good working condition but leaving it on all the time? Sounds like it might be good for the A/C but wasteful of gasoline and just robs the driving experience (car drives better with it off). I'll turn it on occasionally but won't drive with it on all the time in the winter.

Understood, I always have it on, unless the top is down. I did make a test just recently and the difference over a whole fill of gas was less than 1mpg in average. So in that regard it is less fuel heavy than I thought. Top down on the other hand cuts about 2mpg of my fuel economy.

[Ammonia]

I drive with A/C on all the time. Unless its time to show someone what I've got.

Then I come home, sweaty, and the wife goes

"Who did you beat this time?"

"Well, there was this guy..."

[Lucky1]

Originally Posted by Ammonia

I drive with A/C on all the time. Unless its time to show someone what I've got.

Then I come home, sweaty, and the wife goes

"Who did you beat this time?"

"Well, there was this guy..."

LOL, thanks for that. Sweaty but worth it right?