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BMW 3-Series and 4-Series Forum (F30 / F32) | F30POST > Technical Forums > B58 Turbo Engine / Drivetrain / Exhaust Modifications > Change boost when AC on
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      01-11-2019, 10:17 PM   #1
bimmer456
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Change boost when AC on

You would think BMW would program the DME to change boost level when AC is on to compensate for parasitic losses. Perhaps the aftermarket can make a flash tune that takes into account AC on and adjust boost.
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      01-11-2019, 10:29 PM   #2
RTPenland
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I was thinking the same but for when I roll my windows down and the car experiences more aerodynamic drag
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      01-12-2019, 06:06 PM   #3
Wires
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Why would you need to do this? It's not that drastic of a loss with the AC compressor running. If it was, you'd feel the car speed up and slow down as the compressor cycled, and I haven't felt that on any vehicle. (maybe on a NA 1.0L?)

When you are cruising on the highway, you're typically not in boost mode. When you are WOT, the AC is turned off so the point is moot.
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      01-12-2019, 10:42 PM   #4
bimmer456
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Quote:
Originally Posted by Wires View Post
Why would you need to do this? It's not that drastic of a loss with the AC compressor running. If it was, you'd feel the car speed up and slow down as the compressor cycled, and I haven't felt that on any vehicle. (maybe on a NA 1.0L?)

When you are cruising on the highway, you're typically not in boost mode. When you are WOT, the AC is turned off so the point is moot.
I feel like it drags the engine especially at launch. Not looking to go WOT all the time. I've also noticed it on Hondas but not as much on my Toyota.
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      01-13-2019, 03:40 PM   #5
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Do you notice any difference with AC off at launch? Anytime I notice a lag off the line it's typically due to higher intake temps from doing a lot of stop/start which gets you a boost retard. These turbo engines target torque, so even a quick 1/2 throttle blip will try to get you the 300+ ft/lbs of torque (hence why I'm saying I doubt its the AC that's dragging down the motor).
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      01-14-2019, 04:41 PM   #6
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Quote:
Originally Posted by Wires View Post
Do you notice any difference with AC off at launch? Anytime I notice a lag off the line it's typically due to higher intake temps from doing a lot of stop/start which gets you a boost retard. These turbo engines target torque, so even a quick 1/2 throttle blip will try to get you the 300+ ft/lbs of torque (hence why I'm saying I doubt its the AC that's dragging down the motor).
I notice lag when the ac is on at launch, like it's a b48. In sport plus there is no lag with ac off.
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      01-16-2019, 10:32 AM   #7
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Quote:
Originally Posted by bimmer456 View Post
You would think BMW would program the DME to change boost level when AC is on to compensate for parasitic losses. Perhaps the aftermarket can make a flash tune that takes into account AC on and adjust boost.
I believe on the //M cars the AC gets turned off when you nail the gas..

Mike
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      01-16-2019, 05:38 PM   #8
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As mentioned it's less so about the parasitic losses of the AC compressor but more so dependent on ambient temp and how hard the AC has to work (heat exchanger/radiator). This is all heat generation and removal based which drives the same cooling circuit as your Water to Air Intercooler thus affecting IAT's. Generally speaking when you have the AC off the weather is probably cooler and the air more dense making it seem more noticeable. Take a hot day with the AC on and then turn it off...I notice very little difference in my car. The ~5hp the AC Compressor takes to spin I doubt you can feel - especially as mentioned it's off during WOT.
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      01-18-2019, 05:16 AM   #9
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I have never, on any car, felt a difference in performance with AC on/off!!
Furthermore, modern cars (my 2011 Audi at least) disengages the AC compressor during WOT to put those extra 3-5 PS in motion as well.

So I do believe that those who believe they feel a difference (especially on a 300+ PS performance car) are having a placebo. The only possible reason for some negligible performance impact might be the extra generated heat, as dereksM3 suggested.

Since I had problems with the AC compressor on my Audi (which I only used when needed in the summer, otherwise off), I was advised to drive with AC on all the time (Including winter), as this is ensuring continuous lubrication of the compressor. The compressor is anyway automatically disengaged below certain temperatures (+5C on the Audi if not mistaken).
On the BMW I have been so far following the same advise and not messing with it. Have it constantly on. How do you use it?
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      01-18-2019, 09:32 AM   #10
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I turn it on and off and didn't have problems with it breaking. Did it on my Honda as well. On my Toyota for some reason I didn't notice any parasitic loss while driving and this is on multiple generations of Toyota's I've daily driven so not sure what gives. I've thought it would make sense to go with an electric compressor instead with a 40 volt system like how they must be doing it in a Tesla.
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      01-18-2019, 03:18 PM   #11
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Quote:
Originally Posted by bimmer456 View Post
I turn it on and off and didn't have problems with it breaking. Did it on my Honda as well. On my Toyota for some reason I didn't notice any parasitic loss while driving and this is on multiple generations of Toyota's I've daily driven so not sure what gives. I've thought it would make sense to go with an electric compressor instead with a 40 volt system like how they must be doing it in a Tesla.
You’re wasting your time basing all this thought on what is very clearly a placebo effect.
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      01-18-2019, 08:01 PM   #12
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Quote:
Originally Posted by bimmer456 View Post
I turn it on and off and didn't have problems with it breaking. Did it on my Honda as well. On my Toyota for some reason I didn't notice any parasitic loss while driving and this is on multiple generations of Toyota's I've daily driven so not sure what gives. I've thought it would make sense to go with an electric compressor instead with a 40 volt system like how they must be doing it in a Tesla.
It has an electric clutch (they've been doing this since AC was added to the cars at the dawn of time). It's more efficient to cycle with a clutch than convert gasoline to electricity to run an electric motor to run the compressor.

I'm going to re-iterate it's more an increase in air box temperatures, and nothing with the actual A/C compressor. The compressor is only taking 1.6% of max power. You snap the throttle, that's just "noise" in the dyno (butt or actual).
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      01-18-2019, 09:31 PM   #13
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The alternator has parasitic loss less than the AC and that is disengaged when it isn't needed so I believe the parasitic loss is significant. The alternator only engages when applying the brake.
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      01-19-2019, 12:47 PM   #14
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Everything has a parasitic loss which effects mileage to some degree. That's why the alternator has a clutch, and why the steering rack is no longer run from a hydraulic pump, and partially why the push to LED lights. The difference in mileage between my '08 335 and my '16 340 was a 20% reduction in fuel (which is huge).

So circling back to your original question, are you talking cruise (where boost is non-existent, so the point is moot), or under decent throttle (which the ECU will target torque based on fuel delivery)? When you stab the throttle, the ECU will try to hit that 300ish ft/lb torque target instantly, and detecting those losses would be near impossible, even on a calibrated dyno. You don't get a fixed x PSI boost, it varies based on elevation, intake temps, air density, etc. This is why a 1/2 throttle stab feels like it pulls almost the same as a full throttle one.
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