Quick IAC question
 

What should the IAC counts be at idle? I'm showing around 12, is that too high or low?

Thanks!

That sounds pretty good to me. You want enough so that the ECM is in control. 5 or more is probably pretty acceptable. Is that with the car fully warmed up?

Shawn,

Twelve is fine. I've always tried for less, say 5-8. You can crack open the TB a bit to bring the IAC counts down. The bigger the TB, the more you want air to go thru the throttle blades and not bypass via the IAC opening. Keeps up the air velocity. :thumbs:

Why would the air have any more or less velocity once in the plenum? Also, is velocity relevant at idle?

Maintaining good velocity through the throttle bores is really only a priority for TBI systems, where the fuel is added at the TB. It helps prevent puddling and helps atomization.

The benefit of low IAC counts for a port injected engine is more about how responsive the engine is to transients and coming back to idle. I also tune for high single digit IAC counts at idle, which takes most of the air control away from the IAC program. With the motor able to hold a good idle on its own without relying on the bypass air, then I can get more aggressive with things like the throttle follower action and decay rates. The result is an engine that has a quick response to throttle tip, and a very fast recovery to idle speed.

Todd

i have a? on both my 90s my Iac when cold is around 29 when they are fully warm they are 0. both cars run really good but am IO missing a problem

Possibly a vacuum leak preventing the engine from reaching the warm idle speed. The IAC is fully closed (zero steps), but the idle speed is still above the calibrated value. What is your observed warm idle rpm?

Also possible, but less likely, is that the throttle blades could be misadjusted, allowing too much air to enter when closed.

Todd

My 91 shows 20 stock chip 675rpms and 30 with SGC chip 900rpms

Wow, great information guys! Thanks!

Steve, I don't know if I've ever seen counts that high, of course, I've only scanned one other ZR-1! :lol: I'm by no means an expert here! :nono:

One question: Why don't I see an IAC follower table in TunerPro RT? Does it exist for a '91?

I have never checked any others.. My idle is rock solid no vac. leaks.:thumbs:

the black car tuned by corey has an idle speed that suppose to be 800 but runs 875 to 900 . red car z industries ship 675 and runs 850 .. so would i look for a vaccume leak first i have tried the propane method on both cars and havent found a leak. how does one set the throttle blades ? thankyou for any help.

Its there in the cal. Nobody has created the item in the .xdf. There are quite a few items that are in the cal but aren't found in either the TC or TP tdf/xdf. With a hack of the code, these could be easily added.

Todd,

My TBI's were 2.13". I didn't need extra flow coming from the IACs.
I minimized IAC steps to:

1. Maintain a smoother idle even while using a cam with a 108d LSA. The Xfire motor uses "diffusers" below the TB to maintain swirl keeping fuel in suspension.
2. At part and full throttle, throttle response was significantly improved in much the same way as a "dry manifold" design.

I agree that a goal of tuning is to have the ECM "intervene" as little as possible. The better the "mechanical" tune of the motor, the better the overall tune is. The CW that "the ECM will take care of it" is bogus.

...sooo, who wants to hack the code? :bigears: :)

:bigears

waht are you using to read the IAC?

DIACOM.

Hmmm, so nobody wants to delve deeper into the code eh?

Well, I guess I can go without sleep for the next few weeks then... :) Time to dust off the binary side of the brain.

Sounds like its all you..:thumbs:

What are you talking about, all me? Get your butt up here and help me! :smash:

:lol:

Did you try posting your question here?

http://www.zr1netregistry.com/forum/index.php

Hows this weekend looking???? give me a call..

:bb Ummm, no.... I might though. :) Honestly, I haven't seen much traffic over there especially regarding tuning the LT5.

Shawn,

3rd Gen is really the place to look at for tuning expertise. Some seriously talented people there.

If you're serious about this, here are few tips.
(1) Grab a copy of the ANHT ($8D) listing, which is widely available on the 'net.
(2) Work through the calibration section byte at a time, comparing the LT5 listing with the ANHT. They are somewhat similar. To fully determine what a particular calibration byte does, you'll have to trace the calls to it in the program. If they are called in a similar-looking segment of code, you can be pretty sure it is the same variable.
(3) Fill in the major items, by looking at the addresses from "public" definitions out there, TunerCat, Tuner Pro.
(4) Once you feel reasonably confident you've identified 90% of the variables, you could always send it off to someone who knows and have them check it for you.

I hope you are patient. That is basically how I did it back in 2002-3. It can take literally hundreds of hours doing it alone. Once you've finished the calibration section, then you can move on the program code, and understand not only "what" is there, but "how" it's used. The nice thing is, GM used a library of very similar code routines in many of the P4 ECMs, so there are definitely similarities with other well-hacked calibrations of that era.

You'll find the LT5 cal has nearly 600 constants, and over 70 tables.

Todd

Todd,

Where can we find the ZR-1 listing and which Mask ID is it of? Or does it matter? Of the 600 constants and 70 tables, how many did you find were really relevant to tuning beyond the basic SA and VE stuff? As you and I have discussed previously, I'm interested in PID controls, IAC/TF, sensor voltages and filter coefficients.
Thanks for the help. :bigears

Dom,
I doubt you will find a complete ZR-1 listing in public domain. My post above was to give tips on how one might go about *creating* their own ZR-1 listing from a binary file. Believe me, it takes a lot of work, and I have not seen anyone else show the initiative to attempt it on their own.

There are two official documents in private hands...1) an '89 pre-production calibration listing, maybe 100 pages. This one has made the rounds throughout the early tuning community, several have bootleg copies.

2) the motherload...a complete 750-page GM Engineering printout of the 1990 Level 1 development release calibration. The whole program. This is rarer than a golden unicorn, and there aren't copies runnning around out there. I am grateful to a good friend for even letting me put eyes on it.

Unfortunately (for me), neither of these documents were available to me when I started disassembling the LT5 code. I did it the old-fashioned way -- lots of hours slogging through the code. Had 99% of it nailed myself, then confirmed and corrected my errors with this documentation.

As far as relevance to tuning, it all depends on what youre trying to do. But I've done cals with 300 byte changes, or just a handful. Just depends on what I was working on.

Todd