Tuning approach to stalling with AC

[MartinS]

Hi, Subaru H6, running well with good drivability overall.  Only issue is stalling with AC on the overrun.  When pushing in clutch to downshift (up shifting am back on the throttle before the revs drop much) the engine always slows to the point the oil light comes on, initially was stalling 2/3 times before recovering initially, with my tuning I've got it to about 1/5 times it stalls.

I've tried several things: I've increased the baseline throttle position from 1.5% to 2.0% (higher and the idle is too high without AC) with a bump to 4% below 500 rpm.  This has helped.

I have played with idle mixture but in an AFR range from 12-15, it really makes very little difference, overall happiest in the 14.4-14.8 range.  AC 'on' injector time increase is 0.5 ms.

I've adjusted the ignition idle control to be much more aggressive with below target RPM: base idle timing is 10 degrees, I now have the + side of the idle control table set 10 degrees at idle, 16 degrees at 100 rpm below target, then 18, 20, 22 for each of the next 100 rpm steps below idle target.  More aggressive advance seems to make little difference (up to about 25 degrees anyway).

As noted, this leaves me with the oil light always flickering when pushing in clutch from any RPM over about 1800 and stalling about 1/5 times.  

What more can I do to stop the stalling? 

[Adamw]

Have you experimented with the idle control AC step?  This table specifies how much extra the throttle is opened when the AC Clutch is activated.  There is also a short hard-coded delay so the throttle opens slightly before the clutch engages. 

[MartinS]

I saw that table but when I read the help file it says I need an auxiliary out controlling AC compressor clutch. I didn't wire that way instead splitting the 12V from the AC clutch control relay: one limb to the clutch, the other to a digital input.  Because of that I thought I couldn't use this table without a re-wire and I'm currently away from home with the vehicle and don't really have the tools/facilities to do the rewire until I get it home..

However, just thinking about it, is this where I could use a virtual Aux out assigned to AC clutch or could I just assign an unused Aux to the AC clutch but not actually hook it up at this point?  Would this allow me to use this function?

 

[Adamw]

Yes you could use a virtual aux or spare unconnected aux as a dummy to trigger the AC step, with the clutch wired direct.  However, be aware this means there is no delay between the AC request switch and the AC clutch activating so it is not going to work as well as it should - you really need a small delay as the throttle takes a little time to open, then the manifold takes a little time to fill to the higher pressure, then the extra air takes a little time to turn into an actual torque increase. 

You could also try putting the AC request DI on one axis of your idle ign table so it instantly gets say another 15deg advance when you switch the AC on.  

 

[MartinS]

That's great!  I'll try with a virtual Aux for now and re-wire when I'm able.

The idle ignition table idea is excellent! Nothing like that had even crossed my mind. I'd been trying to accomplish this by making the +side very aggressive but it only seemed to work to a certain point as the crossover in the '0' column was problematic with the big jump from 0-100 not being a very stable part of the curve.

Thanks Adam, I'll try these today.

 

[MartinS]

That worked beautifully.  

Using the virtual Aux as compressor control and then bumping the TP from 1.5 to 2.5% with AC on made the biggest difference.  With that, the overrun went from dropping to a 450 rpm near stall to 700 rpm at the lowest.  I'd tried a 2.5% baseline previously but then idle was too high with AC-off.  I will rewire to take advantage of the compressor delay but, honestly, this setup has a barely perceptible rpm waver when the AC is turned on so it's already very good.

With the idle ignition table tweak, it pretty much drops to the set 850 rpm AC-on idle on the overrun and doesn't budge.  Maybe 800 for a fraction of a second.  Aggressive slope for AC-on, mild slope for AC-off works perfectly.

Thanks Adam!