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AC power drain

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dkrtgood View Drop Down
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    Posted: Oct/04/2016 at 10:06am
Ok we drove to Atlanta last weekend for SE AMO show and I get 12.00 mpg with air on and 16 with air off.
Does this seem odd? here is my spec

390 with cam, t5 tranny cruising 80 mph or so turning 2500 rpm and ac worked great all day long for about 6 hours straight.

Factory ac box under dash, sanden compressor with vintage air dryer and condenser. I took to ac shop this am and he said low and high side readings were perfect and said everything is fine.

Any input on this variance in mpg?
Dave Goodwin
President of Tampa Bay AMC

70 Mark Donohue BBO 390 5-speed
77 Matador Coupe AKA "The Beave"
70 Javelin 360 / Auto - BBG


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72AMX Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2016 at 10:39am
Was your compressor cycling properly or staying on 100% of the time?  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dkrtgood Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2016 at 10:43am
Originally posted by 72AMX 72AMX wrote:

Was your compressor cycling properly or staying on 100% of the time?  

With the expansion valve std setup it runs 100% of the time unlike new cars...unless i'm wrong? I do not know much about AC systems except that I like them :-)

Can someone debunk and tell me how the old style is supposed to run? 100% or kick off when it's not needed? Obviously I mean 100% when I turn it on...not 100% of the time the car is running just be be clear.
Dave Goodwin
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70 Mark Donohue BBO 390 5-speed
77 Matador Coupe AKA "The Beave"
70 Javelin 360 / Auto - BBG


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304-dude View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2016 at 11:17am
I thought the thermostat controls when the AC cycles. My 71 has such a device to control when the AC compressor kicks in.

71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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mixed up View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mixed up Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2016 at 12:37pm
I think most of these new system are climate controlled so they cycle on and off not like the old systems were you use the mix lever to control how much outside air you let in
69 amx 290 auto
65 220 290 4spd
80 ford fairmont
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6768rogues Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2016 at 4:16pm
New systems are clutch cycling orifice tube and they cycle the compressor clutch as the name says.
Older systems are expansion valve systems and the compressor runs all the time unless the thermostat in the evaporator gets too cold, then the compressor will shut off for a short time to avoid freezing the evaporator. Also, if the temperature selector is not turned up all the way, when the temperature selected is reached the compressor will shut off. If it was really hot outside, the compressor would probably run all the time. At least that is how mine work and they cool well.
With a Sanden type compressor and everything else being equal, the drop in mileage is more than I would expect. I have one on a 6 cylinder and I can barely notice the difference whether it is on or off. The only thing I can think of is that something is not right with the compressor and it is turning harder than it should.
The topography of where you are driving will matter, too. On my recent return from Florida towing a trailer, I got 13 MPG through West Virginia, but when I got north of Pittsburgh and into New York where it is flat my mileage went up to 14.8.


Edited by 6768rogues - Oct/04/2016 at 4:21pm
Content intended for mature audiences. If you experience nausea or diarrhea, stop reading and seek medical attention.

Located usually near Rochester, NY and sometimes central FL.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dkrtgood Quote  Post ReplyReply Direct Link To This Post Posted: Oct/05/2016 at 7:44am
Originally posted by 6768rogues 6768rogues wrote:

New systems are clutch cycling orifice tube and they cycle the compressor clutch as the name says.
Older systems are expansion valve systems and the compressor runs all the time unless the thermostat in the evaporator gets too cold, then the compressor will shut off for a short time to avoid freezing the evaporator. Also, if the temperature selector is not turned up all the way, when the temperature selected is reached the compressor will shut off. If it was really hot outside, the compressor would probably run all the time. At least that is how mine work and they cool well.
With a Sanden type compressor and everything else being equal, the drop in mileage is more than I would expect. I have one on a 6 cylinder and I can barely notice the difference whether it is on or off. The only thing I can think of is that something is not right with the compressor and it is turning harder than it should.
The topography of where you are driving will matter, too. On my recent return from Florida towing a trailer, I got 13 MPG through West Virginia, but when I got north of Pittsburgh and into New York where it is flat my mileage went up to 14.8.

That's what I was thinking about compressor cycle....I took to shop and they said low and high side were perfect...but I too am suspect of a tight compressor. I'm going to try a test...take belt of car energize clutch and see how hard it is to turn and then do the same with my matador which also has a sanden compressor and see if there is a noticable difference. I can say my matador with stock 304 has basically no affect on idle when engaged but my 390 in jav drops a good 2-300 rpm...so something tells me it's struggling. Will do my comparison test Saturday. Thanks for the input
Dave Goodwin
President of Tampa Bay AMC

70 Mark Donohue BBO 390 5-speed
77 Matador Coupe AKA "The Beave"
70 Javelin 360 / Auto - BBG


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Oct/05/2016 at 8:04am
Could be a timing, a fuel mixture thing, a vacuum leak, or the lack of A/C soliniod for the drop in RPM at Idle.

Comparing Sanden to York is apples to oranges. Sanden pumps have less drag. Note the big arsed pulley on the York, over Sanden. The bigger the pulley the more it compensates for torque to make it spin.

71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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dkrtgood View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dkrtgood Quote  Post ReplyReply Direct Link To This Post Posted: Oct/05/2016 at 8:09am
Originally posted by 304-dude 304-dude wrote:

Could be a timing, a fuel mixture thing, or the lack of A/C soliniod for the drop in RPM at Idle.

Comparing Sanden to York is apples to oranges. Sanden pumps have less drag. Note the big arsed pulley on the York, over Sanden. The bigger the pulley the more it compensates for torque to make it spin.


Timing and air mix is good, car is running great....16 mpg from a 390 with big cam tells me it's running good. till ac comes into play. no solenoid setup right now to kick up idle...working on that now. but my matador which also does not have a kick up solenoid works fine and you barely notice rpm change. I'm thinking more like a tight compressor. May swap compressors just to be sure saturday..they are the same
Yorks are tanks, that's why I went to Sanden style to lessen the load and for sure was not expecting this issue.
Dave Goodwin
President of Tampa Bay AMC

70 Mark Donohue BBO 390 5-speed
77 Matador Coupe AKA "The Beave"
70 Javelin 360 / Auto - BBG


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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Oct/05/2016 at 8:22am
Not sure, but there is a photo somewhere where someone pulled a late model Jeep engine with a York and It had one of the biggest compressor pulleys I have seen on a York. I think there may be a few variances in how they are setup, since late 70s all manufacturers did little things to Improve milage. Maybe the big pulley helped with that, as York compressors operate on pumping force during rotation, while Sanden compresses by rotation speed.



Edited by 304-dude - Oct/05/2016 at 8:25am
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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