AC Power Hog?

asc08 · May 30, 2012

kdixer, I agree with the personal sensitivity to the operation of a truck. I can tell when the compressor kicks in, but I would not say it bogs down my truck. I had a car with a bad cat converter, and to me, that is what I would describe as 'bogging down'.

My point in my earlier posts was that if it is indeed causing that much of a power loss, there may be something wrong with the truck that should be looked into.

kdixer · May 30, 2012

kdixer, I agree with the personal sensitivity to the operation of a truck. I can tell when the compressor kicks in, but I would not say it bogs down my truck. I had a car with a bad cat converter, and to me, that is what I would describe as 'bogging down'.

My point in my earlier posts was that if it is indeed causing that much of a power loss, there may be something wrong with the truck that should be looked into.

The fact that I can feel the "release" and increase in power when the AC disengages at high rpm's tells me there is a significant power loss related to the AC compressor. It is typically not noticed because drivers are not working the truck hard under normal driving conditions. That is not to say that anything is wrong if the difference is felt. The parasitic loss is completely normal. And a description of the power loss is also very personal. To some people, any sense of power loss is not acceptable or tolerable and may cause concern even though it is completely normal.

I have been driving and felt like a tire was flat, but upon inspection, it was not flat at all. Perception can be a funny thing.

Doug_Scott · May 30, 2012

06Sierra,

My comment was based on a commercial 1 ton ac compressor, typically requires 1 horsepower at full load. A vehicle compressor should be about the same power requirement, and while I don't have the capacity specs on the silverado ac systems, 1 ton of cooling is an awful lot.

My point is that even if it takes thee times as much power, say 3 HP, on a 300 HP engine, you should hardly notice it.

True but, that 300 HP is at a certain RPM range, the 5.3 is what 5k RPM? When you are first taking off froma a stop the AC load (electrical and mechanical) is being put on an engine that is at idle, so your engine isn't going to be producing no where near 300HP. That's why it is noticeable on these engines. Also as stated some vehicles are also designed to cut out AC during heavy or WOT acceleration for the purpose of cutting the "drag" on the engine. Now once your engine's "power band", absolutely shouldn't be feeling "drag". This is exactly how my truck acts, noticeable "drag" on the engine until ~2500-3k RPM, then truck will take off like a scalded cat. Not trying to start an argument, just throwing out information.

Cutting the a/c on hard accleration is also done to negate the effect of no engine vacuum to hold blend and mode doors in place. WOT will drop vacuum to about an inch of vacuum. Any a/c door that is spring loaded will move on WOT, unless you put a tomato juice can in as a resevoir like full sized Fords in the 70's did. Used to have a 68 AMX that had vacuum wipers, and no resevoir, a real treat to try to pass a transport in heavy rain on a 2 lane road.

As for the power required to run the a/c, my 2 story 1600sqft house has a 2 ton central air system. The house was measured, windows measured, direction of windows, all that crap they do to properly size the a/c unit was done before they suggested the 2 ton unit. If you multiply the sqft by 8 feet (height of all ceilings) you get 12800 cubic feet. If a 2 ton unit can cool the house, the a/c in the truck has to cool about 120 cubic feet. It is about 1% of the size of the house. The truck numbers are not exact, and not sure if you should even multiply to get cubic feet, but, it must be a lot smaller of an a/c unit to cool the interior of a truck. I am beginning to think that it is no where near 1hp on the truck. The motor on the home a/c unit is less than 5hp, and the nice thing about scaling something is that the scale is applied to every part.

One more thing to keep in mind, running an oversize a/c unit will actually cool less effectively as compared to a proper sized unit.

&CDFiXER% · May 30, 2012

I rarely noticed the difference with my 99' with the 4.8, except during hard acceleration. If it matters, the 99' got 18mpg. All the time. 18 on the highway, 18 in the city, 18 with windows up, 18 with the windows down, 18 with the AC/defroster on, 18 with em off, pretty much 18 mpg was its story, I have never owned a vehicle with such rock solid gas mileage (pre-DIC IIRC, I calculated @ each fill-up from the same pump). Until you put it in 4 wheel drive. Then you could almost see the gas gauge needle move!