2005 Honda Civic Sdn - Does AC drag on a car engine?

A friend of mine leaves his air-conditioning on all the time whether he needs it or not and even if he’s going up a mountain punching the gas and wondering why he doesn’t have much acceleration I told him I think putting the a/c drags on the engine a little bit is it true or not?

Yes. It is true that AC reduces power somewhat in almost all vehicles. The amount one notices it varies pretty widely from model to model.


Yes it does and you will notice it a lot more in smaller 4 cylinder engines like Civic uses. It is a lot less noticeable in V6 and V8 engines .

An AC compressor draws about 5 H.P. from the engine.


In the olden days, I remember a comedian saying, the brakes are so bad, and the engines so under powered if you need to make an emergency stop, turn on the AC. I have not personally experienced any issue with AC negatively affecting vehicle performance to the point I was concerned.

I can feel it, subtly, kick in and out on my 1999 Civic, especially when giving it a lot of throttle. I think when you floor it the computer prevents the compressor from cycling on, so you don’t have that HP loss when you are calling for max power.

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Back in the day I added a vacuum actuated switch to my 1981 Accord that would disconnect the AC compressor under heavy throttle.
With 76 HP there was little to spare on the highway.

Many smaller cars back in the day had switches and so on that would cut the power to the compressor when the throttle was mashed hard.
The first SAAB I owned was an absolute dog with an automatic trans and naturally aspirated. In the CO mountains it really struggled even with the A/C off. A 116 Horsepower on a near 2 ton car in altitude could have used an hourglass instead of a speedo.

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I read 8hp years ago in a car magazine. 5hp sounds about right for newer vehicles. Since the base engine in a 2005 Civic produces 115hp the difference is probably noticeable.

Chrysler products for years automatically disconnected the A/C at full throttle only letting it function again when the vacuum came back up.

Don’t know if they still do. The newest Chrysler product I owned was an 04.

And less than half of that horsepower @ 3000 RPMs in high gear. Twenty-five years ago I used to see Hondas crawling along in the truck lane while going up mountain grades. I had a 1988 four cylinder Dodge with a 100 horsepower engine but with the larger displacement engine I was able to climb those grades @ 60 MPH with the A/C off.

The PCM disengages the clutch at 80% throttle or higher, nothing to do with vacuum.

Chrysler cars were disabling the A/C long before the cars had PCMs.

I remember those commercials, as well

but as I recall, they were made in a time when junky econoboxes had barely 60hp, if that

Carbureted cars of the late 1970’s, early 1980’s had a full throttle cut-out switch, that was for a short period of time, fuel injection was phased in from 1983 to 1990 on Chrysler vehicles.

Simple test for the OP’s friend: next time they’re going up a mountain pass and feel a lack of power, turn off the a/c and see if it makes a significant difference. Could just be a combination of low hp and altitude causing the poor performance.

Imagine the horsepower one of those old GM A6 compressors would eat up.

Many years ago I was reading a Hot Rod mag or something similar and they did a dyno test on a Chevy 350. They said the engine gained 7 horses by simply removing the mechanical fan blade.

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How did the chrtsler cars of the mid 50s to 1970 handle it?

There was a switch on the throttle linkage, IIRC.

I own a 2005 Ford Explorer with the 4.0L V6. With a new belt, pulley, and tensioner, the air conditioning increases the engine load at idle by approxmiately 15%. I had wondered about this for a while and performed some basic and unscientific tests in my driveway in the fall using a wifi-based OBDII dongle. These values will vary based on vehicle, environmental conditions, and a variety of other factors which I cannot begin to scratch the surface of.

Ballpark value I can give you is 15% based on my experience. The number is likely to be higher (by how much, I have no idea) with your vehicle.

Possibly less than that. My '09 Corolla ticks over at about 2400rpm at 60mph on level pavement. Let’s keep it simple and say our theoretical '05 Civic is putting out 45hp at 60mph. Subtract the 5hp drag from the A/C and you’ve lost 11% of available power. That would be noticeable.