This question is for one of my project cars but figure I might get a wider audience here.
I've got an old fiat x1/9 running an ITB setup that has no idle control air valve. It has an old electromotive standalone that has some idle control logic, but most of it is setup for controlling an air valve. There is a provision for using spark advance that I think I can use. But also at the end of the day this is mostly 'racecar' use case so I can live with a highish idle when warm if necessary.
The car does have a physical throttle stop screw, but of course a good idle spot on a cold engine is not the same as a good idle spot on a warm engine.
The question is, any idea how much effect I can expect to have with just spark advance? And is the general best practice to set the idle screw when warm and advance the ignition to speed up the idle when cold or to set the idle screw cold and retard the timing to slow the engine speed when warm?
I also have not attempted to 'balance' the cylinders physically yet via a vacuum signal or similar, I have just looked to make sure all throttle plates look lined up...
My megasquirt has a closed loop idle spark advance feature that will change from the base advance in order to maintain a specific idle RPM. I think it is mainly for cars that have AC and the idle will drop when the compressor kicks on. Like anything using an electronic PID closed loop control system the trick it to tune the P, I and D variables in order to get the loop to operate accurately and quickly enough to be useful and not cause more problems than it solves. Lots of trial and error usually in this sort of application. I don't know if whatever it is that is controlling the timing has the capability to have it's PID terms tweeked, but if so it can be an effective way to control the idle once properly dialed in.
Plugging the vacuum hose into the vacuum advance pot on an old GM product would net you 200 or so rpm. Volvo used the vacuum advance for a/c idle compensation back in the olden days.
It is certainly a usable theory.
That was the ticket to LS Microsquirts for awhile before the IAC add-ons were cheap and easy to use. It is also one of the ways to get the big chop chop noises that cammed LS engines are so well known for.
I have no idle air controller in my 4AGE/ITB locost, so I rely on a healthy amount of advance below idle speed to keep it from stalling when cold. If the idle speed drops, I get more advance, which brings the idle speed back up. It works so well, I never did sort out any kind of IAC.
This is all very encouraging info so far, thank you!
Fwiw, oem's have been spark controlling idle for as long as my career.
There is a gross airflow adjustment, but the high speed control was all spark.
I used to have a Digifant equipped VW Golf that had nothing for idle control but ignition timing.
Mind you, it HAD (past tense) an IAC, but it was problematic, and the idle was a lot more stable after deleting it. Car was driven at +100F to -11F and with air conditioning, never had any weird high or low idles.
Yep, it can work great. Got frustrated with the PID controller in megasquirt for one of the E30s - the idle would go nuts whenever the fan kicked on or off because of alternator load, and I'd get it all dialed in and 6 months later the weather would change and I'd have a problem again. Ended up using the IAC controller as a gross airflow adjustment and used ignition control to do the rest and never had an issue again.
Now that I've spent more time in EFILive, this appears to be (generally speaking) how GM has been controlling idle for a long time.
In reply to gearheadE30 :
Most OEMs use IAC for gross changes and ignition timing for more rapid corrections. Gotta get that table smooth idle 
Another thing they do is model how much torque the engine requires to turn various things like the A/C compressor at different high side pressures, engine friction compared to oil temperature (modeled or measured), and so forth, so that idle correction is more proactive instead of reactive. I know the MS3 firmware has an automatic idle up for A/C engagement, I cannot remember if there is one for the fan as well.
I have managed to get really good idle control without using ignition tricks with the MS3Pro systems, but it took a while.
Depends on the engine. I just tuned one without yesterday and it was 100 rpm bwtween 0deg and 34deg
I was playing with it a bit last night, and I'm not sure the ECU is getting into the "idle strategy". Maybe have to play with TPS voltage settings.
But, I did show that I can slow from about 1375 rpm to 1175 rpm by reducing about 10 degrees of advance. The engine is significantly rougher that far retarded though.
I guess I should also get to that physical balancing procedure before I go too much further though.
Also make sure the ignition timing in the standalone is synced to real life before moving ahead. Depending on the can setup the idle will be better w more advance
Robbie (Forum Supporter) said:
I was playing with it a bit last night, and I'm not sure the ECU is getting into the "idle strategy". Maybe have to play with TPS voltage settings.
But, I did show that I can slow from about 1375 rpm to 1175 rpm by reducing about 10 degrees of advance. The engine is significantly rougher that far retarded though.
I guess I should also get to that physical balancing procedure before I go too much further though.
A good spark table is far more important than any additional strategies such as adaptive spark control. I always recommend getting as far as you can on just the spark table before you bring in more advanced options. Adding these in when the underlying spark table needs more tuning will make a bad idle worse.
I'm going to be putting together some videos on idle tuning without an IAC soon - just need to make sure I'm not leaking any more oil first.
In reply to MadScientistMatt :
I can't wait to see it!
My strategy has always been to first mimic a distributor with flat timing below a few hundred RPM over idle speed and only then playing with it. It is easy to get a weird situation where high vacuum on decel to idle, or a timing curve that ramps up at speeds just off idle, causes an idle hang that the IAC strategy tries to compensate for, and then when idle does come down off of the timing related idle up it starves for air and the IAC has to react more quickly than it can.
This is also a concern with a carburetor but adding closed loop idle into the mix compounds the problems rather than bandaids over them.
Yes lock timing from the bottom of your desired idle zone and up to 1200RPM to slow down idle hunting especially with a big cam. That way you don't have it moving up into a higher advance which speeds the motor up and then it has to drastically drop timing to get the RPM down. You aren't going to miss a little advance at 1200RPM.
But not 10 degrees -- that's too too much. That's indicative of something else wrong.
In reply to MadScientistMatt :
Nice! if you wouldnt mind posting the video here when you do it I would love to see it.
From a baseline point of view I have had luck getting the setup warm, set timing low (0-10deg cam dependent) and then set hot idle speed mechanically. Then go back and map out the colder idle timing to get closer to target and build a clt/timing curve for idle. There are a few ways to implement it, depending on the system and it works decently open loop to get a car warmed up without the ISV. Making it instantly driveable without it is a bit trickier and closed loop strategies can help.
In reply to Paul_VR6 (Forum Supporter) :
that's useful, thank you.
here's another question for ITB folks - if I were to try and 'balance' the throttles, should I just aim to make sure they all have the same vacuum at idle? I'm a bit worried that the pulses will really make that difficult.
Yes, shoot for same idle vacuum. You can buy gauges to do multiple cyls at a time so you can make those comparisons easily. I have one gauge for a 4cyl that I use to sync and then use at least ref cylinder from the balanced set to sync the rest slowly moving around.
Robbie, it looks like you are headed down the path of using timing to do your idle speed control.
Take this next bit with the understanding that it was used on race engines (mid-2000's IRL engine).
We used spark to get idle speed own to reasonable (3000 rpm) after finding that the throttle plates were deflecting on decel and wedging in the throttle bores. To keep the wedging from happening, the throttles were set more open than we wanted for a 3000rpm idle. To get the idle back down we actually ran the ignition timing below advance and into retard (negative advance). This resulted in the inconel exhaust pipes getting hot enough it looked like you could probably push a hole in them if you really wanted to. A quick blip of throttle would cool them down again for a sew seconds.
This solution was short lived. and was replaced by adding air bleeds below the throttle plates, which allowed the throttles to be closed enough to keep idle happy and not wedge on decel.
My suggestion to you is: set you throttle stop for hot running and use the igition timing to help keep the engine running close to your preferred idle when cold. The retard option worked, but you will not want to change exhaust as frequently as we were doing. If you needed to, you can find an in-line needle valve to bypass and leave the throttle stop in one place.
hmm, looks like I may only be able to adjust the balance between "left 2" and "right 2". Maybe there is something internal to the carb bodies that can adjust between the 2 plates, but I doubt it. I may just have to balance by getting the engine warm, and then working toward the smoothest idle.
In reply to stafford1500 :
that's hilarious. They lightened the throttle plates enough that they weren't stiff enough anymore!
but also makes sense about using advance to speed up the cold idle vs retard to slow the fast idle.
Paired throttles like that shouldn't need to be synced between the plates of each body, they will be pretty damned good as is just due to how they are made.
