How much horsepower does the a/c rob?

Messages
172
Location
Laurel, MD
My truck is a real dog when the a/c is running. Does anybody know how much horsepower it takes to run it? My seat of the pants dyno says about 40 HP. [Cheers!]
 
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1,979
Location
Houston
I think it's probably proportional to the engine, since larger engines in larger vehicles usually have larger compressors....but I've heard anywhere between 20-30%. Some cars have a vacuum switch that cuts the compressor off when the vacuum drops from stompin' on the juice. I kludged one into a Mazda Protege, it did make a significant difference.
 
Messages
432
Location
central Georgia
I seriously doubt it's as much as 40 hp. Good related story: On a lark, an acquaintance of mine decided to join me at the drag strip, bringing his '98 Jaguar XJ8 along to race. He had been running 1/4 miles in the mid 15 second range, trapping in the low 90's. He lines up for his third pass, and the tech in the staging lane starts yelling at him-- he had been running the car with the climate control on [a/c dripping water on the track is a BIG no-no]. After that, he ran 15.2's for the rest of the evening in the mid 90's... everyone was looking at him as if he were insane, putting that Jag on the track. FWIW, most ECU's on cars nowadays shut off the a/c in high acceleration/high rpm situations.
 
Messages
1,357
Location
California, USA
4 is the figure I've seen. Most cars and trucks hardly notice. We once had an airconditioned VW Squareback with a 65hp engine and the A/C did not drag that engine down too much. If it feels like a big drag, something is probably wrong with it.
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by C4Dave: About 5 hp.
It takes about 30 hp to move an Accord sized car at 60 mph. Under those conditions one will easily get 30 mpg. If the AC were taking 5 hp, then running the AC would cause as 5/30 = 17% hit on fuel economy, or a resulting 25 mpg. Differences I have observed have been more like 0 to 1 mpg.
 
Messages
9,283
Location
Fayetteville, NC
I notice it well on my wife's 4-banger '03 Elantra, and I do shut a/c off to pass, especially uphill(adviced her to do the same) with the 4.6L T-bird, I don't notice it much in the performance, but the brake pedal makes a squeaky noise when the a/c is on full blast (robs power from power brakes??). On my brother's 1987 Accord (in Sri Lanka), the engine bucks going slowly up a steep hill if a/c is on. However, I think that car has other fuel-problems that bogs the engine down anyway.
 
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2,095
Location
IL
I'd be surprised if it is that simple XS650. Especially since gasoline engines are least efficient in cruising w/ low throttle openings. So you think AC takes about 1/2hp? That is about 370 Watts. Considering the 12000BTU window unit I have consumes about 1200Watts, I can't see 370Watts keeping an uninsulated glass box very cool. I'd bet the 5hp is a fairly close estimate.
 
Messages
1,979
Location
Houston
Here's a link to a guy that at least knows how to do math. Buried way down there is a statement re: a/c of 6hp loss. But i will insist that it does depend on the compressor type and size as well, altho it may mean only a bit of difference. The old GM piston pump compressors (long and round), were great compressors but true power hogs. Modern rotaties are more efficient. http://mb-soft.com/public/headlite.html I just may use my AC in the car more often.....
 
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13,132
Location
By Detroit
Considering I can run the roof air conditioner on my 27-foot motorhome using a 5 hp generator, I would have to guess the automobile air is pulling no more than that off the engine, since 1) it will have much less space to cool, and 2) it does not have losses from conversion from mechanical to electric and back to mechanical.
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by kenw: Here's a link to a guy that at least knows how to do math. Buried way down there is a statement re: a/c of 6hp loss. But i will insist that it does depend on the compressor type and size as well, altho it may mean only a bit of difference. The old GM piston pump compressors (long and round), were great compressors but true power hogs. Modern rotaties are more efficient. http://mb-soft.com/public/headlite.html I just may use my AC in the car more often.....
He also has himself convinced that it takes about 1.5 hp to run the lights on a car at night. His power consumption figures are high. Car bulbs are power rated at nominal operting voltage of 14.? volts not 12 volts, so his increase of power calculations based on operating voltage added a power consumprion boost that doesn't exist. Also automotive alternators are about 60% efficient and belt drives are 95% efficient or battery. Power consumption to run a cars lights is more like 150/746 = 0.2 hp. Crank in about 55% system efficiency and it takes .36 hp, not 1.5.
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by Virtuoso: I would imagine the A/C system on my Saturn takes at least 5, and more likely 10 HP.
That's easy to verify. If it's taking 10 hp, then your highway gas mileage would drop from around 30 mpg to 20 mpg.
 
Messages
2,837
Location
MO
quote:
Originally posted by XS650:
quote:
Originally posted by Virtuoso: I would imagine the A/C system on my Saturn takes at least 5, and more likely 10 HP.
That's easy to verify. If it's taking 10 hp, then your highway gas mileage would drop from around 30 mpg to 20 mpg.

????? [ July 30, 2005, 12:08 AM: Message edited by: 59 Vetteman ]
 
Messages
10,833
Location
Nokesville, VA
A 2 ton AC unit (which is either a large window unit or a small central air unit) takes about 20 amps at 240 volts. That's 4800 watts. A horsepower is 746 watts. 4800 watts is equal to 6.7HP which gives some idea of how much horsepower an AC compressor might rob if it's got a 2-ton capacity. Oooh, one thing I'm forgetting to include is the fact that the 2 ton AC unit I'm thinking of is a 10 SEER with a large condensing coil, much larger than anything you'd find on a car. (It has been my observation that as the SEER rating goes up, so does the size of the condensing coil). Therefore, a car AC unit would likely need even more horsepower to deliver the same cooling capacity.
 
Messages
11,284
Location
Spring HIll
My old Saturn DOHC is pathetically slow with the A/C on. I used it 100% of the last tankful and only got 30MPG. I'm thinking it's 25% of the total HP is drained with the A/C on....
 
Messages
4,468
Location
Central Arkansastan
Seems to me that 5 hp is at idle speed, and goes up with RPM. I know at idle most AC units will not go to full pressure, this can be verified with guages, and also in manuals that specify 1500 or so RPM to accurately get pressures. by 'seat of teh pants' method, seem if I pull one plug wire off my 4 cyl engine, the resulting power loss is similar to the AC being on.
 
Messages
10,833
Location
Nokesville, VA
quote:
Originally posted by Jason Troxell: So you think AC takes about 1/2hp? That is about 370 Watts. Considering the 12000BTU window unit I have consumes about 1200Watts, I can't see 370Watts keeping an uninsulated glass box very cool. I'd bet the 5hp is a fairly close estimate.
Old (from the 70s) figures I have seen put the cooling capacity of the average automotive AC unit at 3 to 5 tons, or 36,000 to 60,000 BTUs.
 
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