Your cars hidden Turbo Boost Button

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We find out how many horses it takes to cool a Z4: How much power does A/C really sap?

I'm sitting at a red light in the right lane - a lane that I know ends on the other side of the intersection. I look to my left and realize that the driver of the car next to me is also aware of this, as he's glaring at me.

I reach down and switch off the air conditioning. Just as my passenger is asking me why I did it, the light turns green and I floor it. Despite the best efforts of the car next to me, my passenger and I emerge victorious-we merge safely over with loser-boy a few car lengths behind.

Turning off the A/C is, as I explained to my passenger, like having KITT's Turbo Boost button.; Well, same idea, different intensity. It's common knowledge to the enthusiast that running the air conditioner robs power from the engine. What seems to be anything but common knowledge is how much power it actually takes.

It must be significant, because some cars' computers will automatically disengage the A/C at full throttle. New stuff like the Mazda 5 does it, as did antique cars like my dearly departed 1979 Mercedes 6.9.

But how much power does air conditioning really take to run? Every car is different, but to get one example, I took a BMW Z4 Coupe 3.0si to the dyno shop, and performed two tests - one with the air conditioning running, and one without. The Z4's 3.0-liter six-cylinder put 232 horsepower to the wheels without the A/C running, but this number dropped to 221 while blowing cold air.; This equates to about a five percent drop, which doesn't sound like much, but remember, this is a 255-horsepower engine.

The drop in power would likely be similar on a much smaller engine, say, like a 103-hp Chevy Aveo.; An eleven horsepower drop on an engine that put only 80 horsepower to the wheels is a huge difference - and one that you'd definitely notice driving.

One thing to note - the BMW's A/C compressor didn't remove a steady 11 horsepower - the loss in horsepower increased with engine speed, which is what we'd expect.; After all, the faster the engine is turning, the faster the A/C compressor is.; The loss is consistent on the torque curve - eight to nine pounds-feet of torque throughout the engine's rev range. I've seen people spend hundreds of dollars on their engines to get that much extra thrust, and then drag race their friends with their air conditioning on.

Obviously, every car will perform differently - but the results should be similar.; So, the next time you're participating in a stoplight drag race, don't forget to activate your turbo boost button - turn off the air conditioning.

Not, of course, that we advocate drag racing. Click the image for a larger version.

And CLICK HERE to hear what the Z4 sounds like -- we did a through-the-gears run on the dyno just for your listening pleasure.

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