Tuning the A/C cutoff

[Shdwdrgn]

Has anyone with a programmer played with the A/C disable settings? I was hoping to find something based on TPS (my real problem is when trying to take off fast from a stoplight, but not WOT), but the only thing I can find is an RPM disable, which doesn't seem to actually be used. My stock settings are to cut off the A/C at 8000rpm, and re-enable at 7500. Does anyone have thoughts on more practical settings? Google found me some suggestions for the Silverado, using 4000/3000 but that seems like a pretty wide range at that low of rpm. Maybe 4000/3500 would be better?

FYI this is a daily-driver, and most of the time the A/C is used is either in town, or in rush-hour traffic, although obviously I don't want it to kick off at normal highway speeds. I guess the easiest solution might be to watch my rpm's and see what I would consider the high range for a cutoff, but I was hoping someone else may have already played with this on our platform.

 

[IllogicTC]

"When taking off fast from a stoplight" - It'd be best to turn the A/C off for the duration of this, then. It sucks some valuable power out. I'm pretty sure it's pretty-well WYSIWYG, meaning that RPM-disable is the only functionality offered for adjustment.

"Normal highway speeds," with a 3.73 rear I sit at ~2100 RPM or so at 65-70. Kicking down to third brings it up a bit, but as long as it doesn't try jumping down to 2nd it should definitely sit below 4k.

 

[Shdwdrgn]

Yeah I seem to remember sitting at about 23-2400 at highway speeds, so the 4000rpm mark sounded pretty reasonable, although in practice I might even consider lower. Oh well, guess I'll just have to play with it and see what I get.

 

[bobdec]

Attached file is for a 4l60E w/3.42 rear and 245/65 R17 tires. At 3K your only highway exposure would be if towing in 3rd over 77 MPH. What tuning SW are you using, can you do a log of a WOT run and see what the actual RPMs look line while running the gears. Or just log a peppy start from a light to see what the RPM's look like in 1st through 4th. Can you log the AC clutch status ? . Personally don't like being tied to RPM, if you chose 3K then during street driving the compressor would be on in 1st gear all the way to 25 MPH and cut back on when you hit 2nd till 44 MPH and again back on till you hit 77 in 3rd. Basically on during the 1500-3000 ramp-up of torque. A log of the clutch status would tell the rear story. Now in WOT if you hold TPS above 75%-100% then the you will be shifting in the 4-5200 RPM range and the AC will stay off through the complete run. It's street driving that could possibly see the AC oscillation.

I can tell you the '06/07 (p12 PCM) has a RPM and TPS cutoff in the SW. RPM is same as your '04 7-8K and TPS is set at 100% so basically cutoff is disabled unless throttle is to the floor. Wish I knew this before I got my tune, I would of had Jeremy drop the TPS% cutoff. I run 40% TPS cutoff in my Z28 w/o any problems.

 

[Shdwdrgn]

I'm running 4.10 gears with 30.6" tires, so I'll be running higher RPMs. My software is HPTuners, so logging is certainly possible. As noted, I have a 2004 so unfortunately there doesn't appear to be a TPS setting. If I'm reading this right, I have RPM settings for in-gear, and park/neutral. I agree that a TPS setting would be much more useful, but unless it is hidden in another menu, I think I'm out of luck. I haven't even noticed a cutout when I hit WOT.

 

[Sparky]

I've noticed the drag that AC has when starting from a dead stop, so I've always just manually kicked the AC off when doing a fast takeoff (like busy highway merge with a short ramp). I would think that even with a lower RPM AC cutoff around 4000 it would still affect your get up and go. At least on mine the biggest impact is the initial start takes more time to get the RPM coming up. Once the RPM us up that high the drag isn't really all that noticeable. TPS would certainly be more helpful in that case.

Since it sounds like that option isn't there you may be stuck doing it manually.

 

[Shdwdrgn]

Yeah but when you're in stop&go traffic, that's not really an easy option. At the very least I'd like to get the rpm settings changed to something more useful. Can the compressor even be run at 8000rpm without blowing up?

 

[bobdec]

Since the '04 P10 PCM does not have TPS as an AC cutoff option, I love playing with theoretical solutions....an avid DIY'er could use an adjustable vacuum switch (one w/ 0-100 Kpa range) that controls a relay inserted in series with the clutch pick line. Move the clutch status wire (+12v on clutch) to the new relay upstream (common) contact (this prevents the clutch status DTC). Now when vacuum drops to say 85 Kpa , the relay picks, that opens the new relay's NC points between stock AC relay to clutch and compressor stops. When vacuum raises and Kpa drops below 85 relay drops and compressor starts back up. Not sure it's all worth the 5 HP gained, but people have paid much more for a few HP. I estimate parts would run about $50.00. You could play with the Kpa setting, cruising in the 40's then hitting throttle to get up to the 50's may get MAP up to 80-90 Kpa. And that's when you would want to kill the AC for a few seconds, I'm guess AC compressor has to be off probably near 15 secs to feel it. Not as bad as the windshield wipers on the pre mid 50's vintage cars that stopped whenever you went up a hill...

 

[Shdwdrgn]

If the compressor only pulled 5hp we would never feel it... but there is a very definite and significant power drain while the AC is on. I'm really surprised they left out a throttle position cutoff setting, you would think that would always be one of the basic fields.

 

[IllogicTC]

I have an idea on TPS-based cutoff. The TPS has two sensors for determining position, and it may be possible to "patch" into one of them. Not directly as I would think that wiring is pretty fine-tuned. But with an induction coil around one of the sensor wires, we may have something.

Let's say we picked the sensor that increases in voltage as the blade opens more. We'd put the induction coil on the return wire from that, which would output based on the current amperage running through that wire. Then, find out how much current flows at some decided throttle setting (say, 75% throttle), and have the induction coil output to something which could measure its current output, and make a determination on whether to engage or disengage a relay which would control the compressor.

I'm not sure if an Arduino could be rigged up to do this or not, it seems pretty fancy and would definitely not be a $20 project but it may be what you're looking for. While you're at it, could also have an LED output from the logic board running into the cabin somewhere to let you know if the A/C TPS cutoff has been enabled or not, if you want to really flesh the project out. Or maybe there's something that's not necessarily a fancy programmable logic board that could engage/disengage a relay based on the input from an induction coil.

 

[Shdwdrgn]

If you already have an arduino, why not use it with a display for some cheap stats?

http://arduinodev.com/hardware/obd-kit/

Add the code to read the TPS and you resolve having to tap into the line. You *might* even be able to send the ODB-II signal to directly shut off the compressor when you want to. If you have a display on it, you could also read other other stats such as transmission temps...

 

[Sparky]

If the compressor only pulled 5hp we would never feel it... but there is a very definite and significant power drain while the AC is on. I'm really surprised they left out a throttle position cutoff setting, you would think that would always be one of the basic fields.

At full or even half throttle you don't feel it. But at low RPM when we are at our lowest power output that 5HP makes a big difference. Really not a whole lot of power being made at only 1000 RPM.

 

[IllogicTC]

At full or even half throttle you don't feel it. But at low RPM when we are at our lowest power output that 5HP makes a big difference. Really not a whole lot of power being made at only 1000 RPM.

Are you implying you idle at 1000?

I wasn't aware an OBD kit was out for Arduino, there you go then!

 

[bspurloc]

how much HP at full high does this compressor draw?
I see guesses of 2.5 to 10hp.
are these 18,000BTU?? or higher?

People never believe the BTU's of a cars AC.

 

[Shdwdrgn]

Well either the load is a lot higher that people imagine, or my engine is just really affected by it... Either way, I wouldn't worry about it except that there is a pretty significant drop in HP when I'm driving home in rush-hour traffic. Yes, I have a lead foot. So does the guy in front of me. When I am on the interstate maintaining speed up and down hills, the effects are barely noticeable -- however when I'm in stop&go traffic, the load from the compressor is pretty obvious.

 

[BlazingTrails]

I can feel it big time when my ac kicks in. According to my dash command the power out put at idle jump up by 21hp when the ac kicks on. I think that is legit because it runs a lot different with the ac on.

 

[IllogicTC]

I can feel it big time when my ac kicks in. According to my dash command the power out put at idle jump up by 21hp when the ac kicks on. I think that is legit because it runs a lot different with the ac on.

For simpler live readouts that show the calculated engine load (a PCM-sourced figure) there is a pretty noticeable jump up.