Dominic Sorresso

Working w someone remotely right now on tuning his 90 ZR-1. when we first got started his IAC counts were in good shape, ie 11-15, for idle. Now we are about to finalize his tune, but the IAC count is now showing 0.
I tuned carbs back in the day and the objective was to get the highest vacuum reading w the idle mixture screw adjustment. Later on my 84 Xfire, the objective was the same but this was using the throttle blade opening and checking the IAC counts. Also used a vacuum gauge on a port available for that.
I recall when Bruce (Grumpy) first started modifying the calibration, he used basically a tachometer and vacuum gauge. He would make a change, if vacuum dropped, he'd go back. If vacuum increased he would keep moving changing in the same direction. So my question is "If the tune is getting better, we would see the motor pulling higher vacuum (lower MAP) for the same rpm. Does that mean I should see the IAC counts drop given that the motor is pulling more air thru the venturi? And will I be re-setting the Min Air?

ULTM8Z

Yep, I remember the carb days... and yeah, I used to the same thing.

Keep in mind though, IAC counts are really indicative of the engine rpm. Increased idle speed will give you increased vacuum (engine is trying to pull more air against the same closed throttle blades). So the IAC counts will go down.

But if you're seeing higher vacuum for the same rpm, then yeah, it's probably indicative of a real improvement IMO.

Dominic Sorresso

Fast355

iTrader: (2)

I agree with that. I just like that you can do it without poping the hood and getting dirty hands or breaking a sweat.

When I am real time tuning I watch the MAP reading and aim for the lowest map reading. I discovered through real time tuning the 5.7 Vortec in my Express van likes as much as 52* of timing in some places and loads. It runs very smooth at 52* of timing at 2,600 rpm @ 30 kpa for example.

ULTM8Z

Hah... yep probably some amount truth to that. [flame suit on]... Seemed we were "closer" to the engine in terms of knowing the fundamentals of how it worked back then... having to get under the hood to make adjustments.

Though, I know a lot of carb guys have a difficulty grasping EFI, but for those of us that made the leap, I agree the carb background does help.

Orr89RocZ

iTrader: (20)

I tend to ignore iac and set it later on once my idle vacuum is good and rpm is stable. I then reset iac so that it is like 10 or so counts at idle. You want some room for it to adjust.

But i guess if no other changes, and engine idles same rpm with higher vacuum its probably getting more air thru the throttlebody since greater pressure delta, so less iac openig needed

Dominic Sorresso

Ult,

Yes absolutely true. Some old school guys have never made the transition. Some are afraid of it. I have to thank people on this forum like Grumpy and RBob that helped me make it.

Orr,

I agree completely. First get the tune right, then finalize the Min Air to top it off. My question had to do with the observation that as the tine got closer, there was less of a need for the IAC to interfere. But of course we want some involvement of the IAC as a backstop.

InTech

Yes, it's very similar to using a vacuum gauge when "tweaking" a carburetor and distributor at idle. The IAC counts are a very good representation of what the engine likes. The IAC counts get lower because the engine is happier with whatever fuel or timing changes you did as long as you didn't change desired idle speed, and because the engine is happier, the computer isn't raising the controlled electronic vacuum leak (IAC).

The fun can happen when you deal with huge camshafts, the vacuum is extremely low at idle so you have to crack the throttle open higher so the engine gets the mass of air it needs to idle. If you don't have an adjustable TPS now you need to drill a larger hole in the throttle body so the TPS voltage stays in range. That all works fine for steady state idle but we all know as the rps rises the vacuum with the big cam does as well, creating a chase situation and won't come back down to idle till you turn the car off. In those situations you have to decrease timing as vacuum increases(map decreases numerically) to make it less efficient there so it will drop rpm and come back to steady state idle.
Sorry to blab.

Dominic Sorresso

Thanks guys. i simply wanted some confirmation of my thinking. Which is, a better tune may lead you to reset the Min Air since the motor has a reduced need for the IAC to maintain idle.