Holley Sniper - IAC Rampdown and RPM

[CV355]

I've tried asking this over on the Holley forums but they won't touch the question unless I post datalogging results (can't do that at the moment, my laptop bricked during one of Microsoft's unnecessary hourly updates).  I'm hoping someone might be able to provide me with an explanation on effects of IAC settings:

IAC Hold Position
Ramp Decay Time
RPM Above Idle to Start Ramp
RPM Above Idle to Start Idle Control

https://www.manualslib.com/manual/1379721/Holley-550-407.html?page=48

These are self-explanatory in function, but my question is- what are the negative effects of having these parameters adjusted incorrectly?  I am facing an intermittent RPM hanging and surging issue.  There were multiple factors to it (BOV adjustment and vac leak, needed stronger springs on the TB as it was sticking open at 5%, and AC kick was on for some reason), but I have solved a few and narrowed it down to just IAC tuning at this point.  If I let off the throttle with the engine up to temp, the IAC% hangs at the hold position and refuses to ramp down unless I blip the throttle (start ramp rpm issue, right?).  Really fun when you're approaching a busy intersection and the engine is fighting the brakes.

I adjusted the IAC hold from 20% down to 10%, Ramp from 4s to 2s, and RPM start ramp from 1000 to 1250.  This reduced the issue but didn't solve it.  I've been told that the RPM start ramp needs to be higher, but I don't fully understand what will occur if this is set too high or other values too aggressive.  The rampdown itself makes sense- too aggressive and it'll make decel jerky, too gentle and RPMs hang.  I don't want to overcompensate for one issue and create new issues.

[My427stang]

IAC Hold is where the IAC parks once you give it throttle, if it's too high it won't idle down easily

Ramp Decay is the time it takes to relax from the hold point once the throttle returns

RPM above idle is where it starts to settle

RPM above idle to start idle control is when the IAC takes over actively, not the ramp down

I think your idle speed to start ramp was high, and you went higher. That will essentially ease the idle starting at that point

However, logs are your friend, going to need to get that working again to really figure it out

[66FAIRLANE]

Without going out to the shed and looking I believe this is what I use
IAC Hold Position 10%
Ramp Decay Time 2s
RPM Above Idle to Start Ramp 1700rpm (I believe Holley recommend starting at 1000 above idle in the manual)
RPM Above Idle to Start Idle Control (150rpm)

[CV355]

IAC Hold is where the IAC parks once you give it throttle, if it's too high it won't idle down easily

Ramp Decay is the time it takes to relax from the hold point once the throttle returns

RPM above idle is where it starts to settle

RPM above idle to start idle control is when the IAC takes over actively, not the ramp down

I think your idle speed to start ramp was high, and you went higher. That will essentially ease the idle starting at that point

However, logs are your friend, going to need to get that working again to really figure it out

I did test the idle speed to start ramp parameter down to 500 to see the effect (1325 total).  It resulted in an angry, erratic idle of about 1400rpms, like the engine was loading up to launch.  So, I changed it in the other direction to 1250 (2075 total) and it settled down immediately as IAC dropped to 0%-2%.  My idle is set at 825 in neutral, about 7xx in gear.  Any higher and it diesels at shut down.  Any lower and it shakes the car apart and feels like it's going to stall out at red lights.   

I agree, I need to get that laptop fixed so I can datalog.  Microsoft has been awful lately with pointless and damaging updates (practically every phone call I get on for work has someone mentioning how recent updates wreaked havoc, but I digress...)

What would the negative effect be if the idle speed to start ramp is set far too high? 

[My427stang]

I have not tried it, but thinking this through

- If the throttle is returning and the IAC is at the hold point (parked), the "ramp" for when the IAC takes over is a function of time and start and stop points.  Should literally look like a ramp with RPM on the Y axis and Time on the X axis.
- The RPM points on that graph are RPM Above Idle to Start Ramp (high limit) and RPM Above Idle to Start Idle Control (low limit)
- The time or rate it drops is ramp decay, or how fast to get across those two points above (seconds)

If RPM Above Idle to start Idle control was too high, I think your IAC would have to unscrew really quickly then screw back in very quickly (it's a small motor that drives a vacuum leak)

If I misunderstood your question and you were asking if RPM Above Idle to Start Ramp was too high.  I would think it would make the ramp less steep and slow throttle return. 

I have not played with these values enough to know how much matters.  My hunch is you had something with the low start ramp causing the IAC to cycle back and forth into the ramp, but it is just a WAG

Without going out to the shed and looking I believe this is what I use
IAC Hold Position 10%
Ramp Decay Time 2s
RPM Above Idle to Start Ramp 1700rpm (I believe Holley recommend starting at 1000 above idle in the manual)
RPM Above Idle to Start Idle Control (150rpm)

These settings seem very logical to me, start the ramp 1000 above idle, end it at just above idle, which ends up to be very close to where the IAC is parked (10%) and it takes over

[CV355]

I updated the settings as recommended above, and the issue is now completely resolved.  Zero RPM hang, IAC is operating properly, and the drive quality was far better.  Thanks guys!