How much horsepower loss when A/C on?
- Thread starter Rubiconned
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I see you have an '04. Is it the 2.4? I can certainly feel the difference in my 2.5.
I don't know how much HP is lost to the AC, but I'll watch to see if others know. If I'm ever on a dyno, I'll see what happens when I flip it on.
I don't know how much HP is lost to the AC, but I'll watch to see if others know. If I'm ever on a dyno, I'll see what happens when I flip it on.
Back in the '70s I heard numbers like "about 5 to 8 HP" to run the A/C pump (from backyard mechanics, FWIW). Then in the 2000s I heard "almost 0 HP because modern pumps are so much more efficient". That sounds completely wrong, since they still have to pump refrigerant at the same volume and pressure as ever.
I once had a car which would turn off the A/C pump above about 3/4 throttle. Then later, had a Honda 4-banger which really needed that feature, since the A/C made it accelerate like a snail. It should be possible to add that to our TJs.
I'm curious to see if anyone has hard numbers on this question.
I once had a car which would turn off the A/C pump above about 3/4 throttle. Then later, had a Honda 4-banger which really needed that feature, since the A/C made it accelerate like a snail. It should be possible to add that to our TJs.
I'm curious to see if anyone has hard numbers on this question.
You can look up the power requirements of a full electric system and convert that to hp and that will give you a much more accurate idea. Based on what I've seen in EVs, it's about 3kw, about 4 hp, depending on the BTU rating. I've also been told that belt driven are closer to 5kw, and that conversion is just under 7hp.
A few other factors for the more efficient modern a/c: most newer cars have a much reduced greenhouse area-smaller windows with heat-reducing glass or tinting, leading to much less heat gain. Another factor is the much improved sound and heat insulation, air tightness, and drivetrain heat management, which also reduces heat gain. Increase the heat transfer capacity of the condenser and evaporator coils by about 11%, use an electronically controlled system, and you can then use a smaller hp or BTU rated compressor. Contrast all of that to the TJ and you see why our compressors are likely on the higher end of BTU rating-higher HP requirement.
I have no noticeable power loss or temp gain while using my AC on my 05 4.0. Even at idle I see no change when the AC kicks on. I read an article that stated that driving with the windows down lowers mpg more than using the AC.
I remember Engine Masters doing an episode or two, "If Accessories Could Kill", covering power requirements of typical accessories, but an A/C compressor wasn't one. I don't recall specifics, but I think a water and/or oil pump needs 2-3 hp, an alternator needs around 5 hp with a load on it, and mechanical cooling fans ate up 10-30+ hp. I'd expect an A/C compressor to be similar to an alternator. I can feel my compressor cycle on and off, but it's just enough to be noticeable, not load the engine. Older compressors require more power.
The MythBusters covered that about 20 years ago. I think they determined windows down below 50 mph was most efficient, windows up A/C on is more efficient above 50.
A few '90s Hondas had a feature that would kill the A/C at a set RPM, usually near the VTEC crossover, so somewhere around 5,000-5,500. I'd bet many cars have similar functions today.
I don't know about the HP numbers, but I lose 2 MPG with the AC on in my Jeep.
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My 69 Camaro 300 h.p. 350 would actually chip the rear tires if you engaged the A/C at 50 m.p.h .
A Frigidaire A - 6 compressor and R -12 could turn that Camaro into a freaking freezer ! If you used max A/C ( recirc. in non GM speak )
at 4000 compressor r.p.m it puts out 42,000 btus. That's 3.5 tons of cooling.
That will cool a average size house.
You shouldn't notice a power difference if the AC is on - engaged, it will sap a few hp, but you shouldn't be needing max hp unless you're flooring the throttle.
I don't know if the TJ has a clutch that will disengage the AC at full throttle, but that is common on some cars too.
Maybe start by testing the pulley to see if it rotates freely or is dragging?
I don't know if the TJ has a clutch that will disengage the AC at full throttle, but that is common on some cars too.
Maybe start by testing the pulley to see if it rotates freely or is dragging?
I installed a A/c deactivation switch- mainly so I can run the blend door to both the foot well and to the vents for winter heat. But it doubles as a press to pass button when you want the full Monty.
My rig puts out a good number of ponies, and I definitely notice the power loss when the A/C is on. I believe a stock Hemi setup disengages the A/C compressor at full throttle, but I’m not sure if mine does so or not. But even flooring it, I still notice the drop.
What are you running for engine management? I'm curious to know if an A/C kill threshold is common now.
I've driven my share of cars with those old R12 setups, and you can 100% feel it engage. I can't say that I’ve had it chirp tires, but definitely slow the vehicle.
‘05 Dodge ECM, tuned. I don’t run the A/C much at all, so I’m not sure. It’s never been a big deal, but now I’m a bit curious.
I'm curious too. I have an '05 HEMI truck, probably something I should know for some reason. We have some diggin to do.
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