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Old 05-05-2017, 08:40 PM   #31 (permalink)
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I ran another tank, trying to use the A/C about the same amount as the last one. MPG ended up at 13.92, compared to 13.79 the last week. This is substantially lower than before the 12 degree mod @ 14.31/14.44 for 2 ~32 gallon tankfuls. So now that I'm fairly confident of ~13.85mpg with the mod and A/C, I'm running the next tank without A/C. Hopefully that'll tell me if the aero mod is a net negative or positive.

I hadn't thought about replacing the A/C system, but I don't really use it all that much. That's either a good argument for not bothering to fix it, or that replacing it with a smaller unit would be perfectly acceptable. Conventional wisdom is that the AC compressor uses around 8hp, and is about the equivalent of a 1.5-2 ton system. At around 20,000BTU/hr that's an electric equivalent of 1500-2500 watts, or 2-3.3hp. I could probably get away with an effective A/C about 1/4 that size, since I rarely put it over "low" on the fan except in the middle of summer. Even at 100% efficiency, an electric one would need to be good for 375-625 watts, or up to ~45A. The 12V car compressors are good for around 300-400BTU/hr, which is 1/50th as big as the stock car system. Unless I'm missing something, I don't see how this could work.

Last edited by Merlyn2220; 05-06-2017 at 09:23 AM..
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Old 05-06-2017, 07:44 PM   #32 (permalink)
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The average power drawn by the AC compressor is proportional to how much cooling you draw from the system. The compressor is clutched on and off to maintain the high side pressure. The compressor clutch is directly controlled by a pressure switch.

The compressor is sucking a lot of power when it is running, while the mileage hit is proportional to the percent of time it is running.

The more air you push through the system, the more the compressor runs. The slower the heater blower speed, the less cooling and less average power to the compressor. If the slowest setting on the heater blower is more cooling than you need, then add a resistor to slow the blower even more.

Look for ways to use less cooling. Sun shades when parked to keep the interior cooler. Interior as warm as you can comfortably tolerate. And pull the compressor fuse when running the defroster.
The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 10,588.6 miles in 2016 at 35.0 MPG.
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Old 05-07-2017, 10:43 PM   #33 (permalink)
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I added a lighted toggle switch to control the compressor on a Suburban I had years ago. It let me run defrost functions without the compressor running.

84 Gmc 6.5 na diesel K30 4x4
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