[AW128i]

Having owned many manual transmission cars I can say that they all required a tad more finesse to drive perfectly smooth when the A/C was on. I've also owned a number of turbocharged cars and, in my opinion, running the A/C on those cars was even more noticeable, mainly because the engine was more sluggish than usual off-boost and when boost did hit it was more noticeable than with the A/C off. Plus, turbocharged cars don't like heat and chances are, if you're running the A/C, it's hot out. So, the combined higher ambient temps and the higher engine load caused by the A/C conspire to produce more lag and more noticeable boost onset.

P.S. BMWs have variable displacement A/C compressors. So, unlike many cars that cycle the A/C compressor on and off, the BMW compressors run all the time when the A/C is switched on, varying its displacement to compress lower or higher volumes of refrigerant through the system. This means that the load on the engine won't be consistent; i.e., higher when first turning on the A/C and lower as the cabin temp/selected temp start to equalize. Actually, many European cars operate this way and it's something I've always appreciated about European cars. I hate driving cars where the A/C compressor cycles on and off because you're constantly having to adjust the throttle position because engine power is constantly fluctuating.